This Ecma Standard defines the ECMAScript 2016 Language. It is the seventh edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The Fifth Edition was adopted by the Ecma General Assembly of December 2009.
The fifth Edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this addition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplemental characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.
This ECMAScript specification is the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. This specification also includes support for a new exponentiation operator and adds a new method to Array.prototype called includes.
Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, 6th Edition Project Editor
Brian Terlson
ECMA-262, 7th Edition Project Editor
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text input in conformance with the Unicode Standard, Version 8.0.0 or later and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application programming interface that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.
A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of the “future reserved words” listed in subclause 11.6.2.2 of this specification.
A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause 16.2.
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 10646:2003: Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety of host environments. Therefore the core language is specified in this document apart from any particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so has the features and facilities it provides. ECMAScript is now a fully featured general propose programming language.
Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular C, Java™, Self, and Scheme as described in:
ISO/IEC 9899:1996, Programming Languages – C.
Gosling, James, Bill Joy and Guy Steele. The Java™ Language Specification. Addison Wesley Publishing Co., 1996.
Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp. 227-241, Orlando, FL, October 1987.
IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.
A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction and there is no need for a main program.
A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.
The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of zero or more properties each with attributes that determine how each property can be used—for example, when the Writable attribute for a property is set to false, any attempt by executed ECMAScript code to assign a different value to the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object; objects that are fundamental to the runtime semantics of the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and, reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named "prototype" that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009,11) creates a new Date object. Invoking a constructor without using new has consequences that depend on the constructor. For example, Date() produces a string representation of the current date and time rather than an object.
Every object created by a constructor has an implicit reference (called the object's prototype) to the value of its constructor's "prototype" property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.
In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named q1 and q2. The dashed lines represent the implicit prototype relationship; so, for example, cf3's prototype is CFp. The constructor, CF, has two properties itself, named P1 and P2, which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named CFP1 in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp's implicit prototype chain that are not named q1, q2, or CFP1. Notice that there is no implicit prototype link between CF and CFp.
Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.
The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.
In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.
object that provides shared properties for other objects
Note
When a constructor creates an object, that object implicitly references the constructor's prototype property for the purpose of resolving property references. The constructor's prototype property can be referenced by the program expression constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.
object specified and supplied by an ECMAScript implementation
Note
Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects. A built-in constructor is a built-in object that is also a constructor.
member of the Object type that is an instance of the standard built-in Boolean constructor
Note
A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer
Note
A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.
member of the Object type that is an instance of the standard built-in String constructor
Note
A String object is created by using the String constructor in a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (21.1.1.1).
member of the Object type that is an instance of the standard built-in Number constructor
Note
A Number object is created by using the Number constructor in a new expression, supplying a number value as an argument. The resulting object has an internal slot whose value is the number value. A Number object can be coerced to a number value by calling the Number constructor as a function (20.1.1.1).
member of the Object type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.
Examples of built-in functions include parseInt and Math.exp. An implementation may provide implementation-dependent built-in functions that are not described in this specification.
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.
Standard built-in methods are defined in this specification, and an ECMAScript implementation may specify and provide other additional built-in methods.
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used throughout the specification.
Clauses 6-9 define the execution environment within which ECMAScript programs operate.
Clauses 10-16 define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.
Clauses 17-26 define the ECMAScript standard library. It includes the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.
A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.
A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.
A lexical grammar for ECMAScript is given in clause 11. This grammar has as its terminal symbols Unicode code points that conform to the rules for SourceCharacter defined in 10.1. It defines a set of productions, starting from the goal symbol InputElementDiv, InputElementTemplateTail, or InputElementRegExp, or InputElementRegExpOrTemplateTail, that describe how sequences of such code points are translated into a sequence of input elements.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (11.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form /*…*/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in 21.2.1. This grammar also has as its terminal symbols the code points as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of code points are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.
Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbols SourceCharacter. This grammar appears in 7.1.3.1.
Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.
The syntactic grammar for ECMAScript is given in clauses 11, 12, 13, 14, and 15. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative goal symbols Script and Module, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (Script or Module), with no tokens left over.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses 12, 13, 14 and 15 is not a complete account of which token sequences are accepted as a correct ECMAScript Script or Module. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.
In certain cases in order to avoid ambiguities the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. In certain contexts, when explicitly specified, the input elements corresponding to such a production are parsed again using a goal symbol of a supplemental grammar. The input stream is syntactically in error if the tokens in the stream of input elements parsed by a cover grammar cannot be parsed as a single instance of the corresponding supplemental goal symbol, with no tokens left over.
Terminal symbols of the lexical, RegExp, and numeric string grammars are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin range, as opposed to any similar-looking code points from other Unicode ranges.
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:
states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:
states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:
so, in this example, the nonterminal IterationStatement actually has four alternative right-hand sides.
A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.
If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token sequence is a member of the given set. The set can be written as a comma separated list of one or two element terminal sequences enclosed in curly brackets. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. If the set consists of a single terminal the phrase “[lookahead ≠ terminal]” may be used.
matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.
If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:
indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.
Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.
When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-code point token, it represents the sequence of code points that would make up such a token.
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:
means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points could not replace ReservedWord.
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.
Algorithms may be explicitly parameterized, in which case the names and usage of the parameters must be provided as part of the algorithm's definition. In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as operationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.operationName(arg1, arg2).
Calls to abstract operations return Completion Records. Abstract operations referenced using the functional application style and the method application style that are prefixed by ? indicate that ReturnIfAbrupt should be applied to the resulting Completion Record. For example, ? operationName() is equivalent to ReturnIfAbrupt(operationName()). Similarly, ? someValue.operationName() is equivalent to ReturnIfAbrupt(someValue.operationName()).
The prefix ! is used to indicate that an abstract operation will never return an abrupt completion and that the resulting Completion Record's value field should be used in place of the return value of the operation. For example, “Let val be ! operationName()” is equivalent to the following algorithm steps:
Algorithms may be associated with productions of one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text.
When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.
Unless explicitly specified otherwise, all chain productions have an implicit definition for every algorithm that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same algorithm name with the same parameters, if any, to the chain production's sole right-hand side nonterminal and then returns the result. For example, assume there is a production:
but there is no corresponding Evaluation algorithm that is explicitly specified for that production. If in some algorithm there is a statement of the form: “Return the result of evaluating Block” it is implicit that an Evaluation algorithm exists of the form:
For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.
Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which unless otherwise noted do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point number must be finite, and if it is +0 or -0 then the corresponding mathematical value is simply 0.
The mathematical function abs(x) produces the absolute value of x, which is -x if x is negative (less than zero) and otherwise is x itself.
The mathematical function min(x1, x2, ..., xN) produces the mathematically smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN) produces the mathematically largest of x1 through xN. The domain and range of these mathematical functions include +∞ and -∞.
The notation “xmoduloy” (y must be finite and nonzero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x-k = q × y for some integer q.
The mathematical function floor(x) produces the largest integer (closest to positive infinity) that is not larger than x.
Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScript Script or Module that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.
Unless otherwise specified every grammar production alternative in this specification implicitly has a definition for a static semantic rule named Contains which takes an argument named symbol whose value is a terminal or nonterminal of the grammar that includes the associated production. The default definition of Contains is:
For each terminal and nonterminal grammar symbol, sym, in the definition of this production do
If sym is the same grammar symbol as symbol, return true.
If sym is a nonterminal, then
Let contained be the result of sym Contains symbol.
If contained is true, return true.
Return false.
The above definition is explicitly over-ridden for specific productions.
A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 16) that are associated with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.
Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.
Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause. When the term “empty” is used as if it was naming a value, it is equivalent to saying “no value of any type”.
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.
The String type is the set of all ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 253-1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.
Where ECMAScript operations interpret String values, each element is interpreted as a single UTF-16 code unit. However, ECMAScript does not place any restrictions or requirements on the sequence of code units in a String value, so they may be ill-formed when interpreted as UTF-16 code unit sequences. Operations that do not interpret String contents treat them as sequences of undifferentiated 16-bit unsigned integers. The function String.prototype.normalize (see 21.1.3.12) can be used to explicitly normalize a String value. String.prototype.localeCompare (see 21.1.3.10) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results.
Note
The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.
Some operations interpret String contents as UTF-16 encoded Unicode code points. In that case the interpretation is:
A code unit in the range 0 to 0xD7FF or in the range 0xE000 to 0xFFFF is interpreted as a code point with the same value.
A sequence of two code units, where the first code unit c1 is in the range 0xD800 to 0xDBFF and the second code unit c2 is in the range 0xDC00 to 0xDFFF, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 10.1.2)
A code unit that is in the range 0xD800 to 0xDFFF, but is not part of a surrogate pair, is interpreted as a code point with the same value.
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (8.2).
Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 1.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
@@hasInstance
"Symbol.hasInstance"
A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the instanceof operator.
@@isConcatSpreadable
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should be flattened to its array elements by Array.prototype.concat.
@@iterator
"Symbol.iterator"
A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match
"Symbol.match"
A regular expression method that matches the regular expression against a string. Called by the String.prototype.match method.
@@replace
"Symbol.replace"
A regular expression method that replaces matched substrings of a string. Called by the String.prototype.replace method.
@@search
"Symbol.search"
A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.
@@species
"Symbol.species"
A function valued property that is the constructor function that is used to create derived objects.
@@split
"Symbol.split"
A regular expression method that splits a string at the indices that match the regular expression. Called by the String.prototype.split method.
@@toPrimitive
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag
"Symbol.toStringTag"
A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method Object.prototype.toString.
@@unscopables
"Symbol.unscopables"
An object valued property whose own and inherited property names are property names that are excluded from the with environment bindings of the associated object.
The Number type has exactly 18437736874454810627 (that is, 264-253+3) values, representing the double-precision 64-bit format IEEE 754-2008 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 253-2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see 24.1) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.
There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞ and -∞, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)
The other 18437736874454810624 (that is, 264-253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.
Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols +0 and -0, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)
The 18437736874454810622 (that is, 264-253-2) finite nonzero values are of two kinds:
18428729675200069632 (that is, 264-254) of them are normalized, having the form
s × m × 2e
where s is +1 or -1, m is a positive integer less than 253 but not less than 252, and e is an integer ranging from -1074 to 971, inclusive.
The remaining 9007199254740990 (that is, 253-2) values are denormalized, having the form
s × m × 2e
where s is +1 or -1, m is a positive integer less than 252, and e is -1074.
Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type (indeed, the integer 0 has two representations, +0 and -0).
A finite number has an odd significand if it is nonzero and the integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x represents an exact nonzero real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is +1 × 253 × 2971) and -21024 (which is -1 × 253 × 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and -21024 are considered to have even significands. Finally, if 21024 was chosen, replace it with +∞; if -21024 was chosen, replace it with -∞; if +0 was chosen, replace it with -0 if and only if x is less than zero; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754-2008 “round to nearest, ties to even” mode.)
Some ECMAScript operators deal only with integers in specific ranges such as -231 through 231-1, inclusive, or in the range 0 through 216-1, inclusive. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.
An Object is logically a collection of properties. Each property is either a data property, or an accessor property:
A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.
An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.
Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty string, are valid as property keys. A property name is a property key that is a String value.
An integer index is a String-valued property key that is a canonical numeric String (see 7.1.16) and whose numeric value is either +0 or a positive integer ≤ 253-1. An array index is an integer index whose numeric value i is in the range +0 ≤ i < 232-1.
Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Ordinary objects are the most common form of objects and have the default object semantics. An exotic object is any form of object whose property semantics differ in any way from the default semantics.
Attributes are used in this specification to define and explain the state of Object properties. A data property associates a key value with the attributes listed in Table 2.
The value retrieved by a get access of the property.
[[Writable]]
Boolean
If false, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]
Boolean
If true, the property will be enumerated by a for-in enumeration (see 13.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]
Boolean
If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.
An accessor property associates a key value with the attributes listed in Table 3.
Table 3: Attributes of an Accessor Property
Attribute Name
Value Domain
Description
[[Get]]
Object | Undefined
If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 6) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]
Object | Undefined
If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 6) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]
Boolean
If true, the property is to be enumerated by a for-in enumeration (see 13.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]
Boolean
If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.
If the initial values of a property's attributes are not explicitly specified by this specification, the default value defined in Table 4 is used.
Table 4: Default Attribute Values
Attribute Name
Default Value
[[Value]]
undefined
[[Get]]
undefined
[[Set]]
undefined
[[Writable]]
false
[[Enumerable]]
false
[[Configurable]]
false
6.1.7.2Object Internal Methods and Internal Slots#
The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.
Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.
Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].
Table 5 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.
The “Signature” column of Table 5 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type. An internal method implicitly returns a Completion Record. In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.
Table 5: Essential Internal Methods
Internal Method
Signature
Description
[[GetPrototypeOf]]
() → Object | Null
Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this object. Returns true if the operation was successful or false if the operation was unsuccessful.
Return a Property Descriptor for the own property of this object whose key is propertyKey, or undefined if no such property exists.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as the this value when evaluating the code.
[[Set]]
(propertyKey, value, Receiver) → Boolean
Set the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as the this value when evaluating the code. Returns true if the property value was set or false if it could not be set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is propertyKey from this object. Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean
Create or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Return true if that property was successfully created/updated or false if the property could not be created or updated.
Return a List whose elements are all of the own property keys for the object.
Table 6 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal methods. A constructor (also referred to as a constructor function) is a function object that supports the [[Construct]] internal method.
Table 6: Additional Essential Internal Methods of Function Objects
Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
Creates an object. Invoked via the new or super operators. The first argument to the internal method is a list containing the arguments of the operator. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. A function object is not necessarily a constructor and such non-constructor function objects do not have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 9. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.
6.1.7.3Invariants of the Essential Internal Methods#
The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.
Definitions:
The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a non-extensible target.
All references to SameValue are according to the definition of the SameValue algorithm.
[[GetPrototypeOf]] ( )
The Type of the return value must be either Object or Null.
If target is non-extensible, and [[GetPrototypeOf]] returns a value v, then any future calls to [[GetPrototypeOf]] should return the SameValue as v.
Note 1
An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.
[[SetPrototypeOf]] (V)
The Type of the return value must be Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The Type of the return value must be Boolean.
If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.
[[PreventExtensions]] ( )
The Type of the return value must be Boolean.
If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.
[[GetOwnProperty]] (P)
The Type of the return value must be either Property Descriptor or Undefined.
If the Type of the return value is Property Descriptor, the return value must be a complete property descriptor (see 6.2.4.6).
If a property P is described as a data property with Desc.[[Value]] equal to v and Desc.[[Writable]] and Desc.[[Configurable]] are both false, then the SameValue must be returned for the Desc.[[Value]] attribute of the property on all future calls to [[GetOwnProperty]] ( P ).
If P's attributes other than [[Writable]] may change over time or if the property might disappear, then P's [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).
Note 2
As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the Desc.[[Writable]] and Desc.[[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other internal methods.
[[DefineOwnProperty]] (P, Desc)
The Type of the return value must be Boolean.
[[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:
P is a non-configurable writable own data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.
All attributes in Desc are the SameValue as P's attributes.
[[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] ( P )
The Type of the return value must be Boolean.
If P was previously observed as a non-configurable data or accessor own property of the target, [[HasProperty]] must return true.
[[Get]] (P, Receiver)
If P was previously observed as a non-configurable, non-writable own data property of the target with value v, then [[Get]] must return the SameValue.
If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.
[[Set]] ( P, V, Receiver)
The Type of the return value must be Boolean.
If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.
[[Delete]] ( P )
The Type of the return value must be Boolean.
If P was previously observed to be a non-configurable own data or accessor property of the target, [[Delete]] must return false.
The Type of each element of the returned List is either String or Symbol.
The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.
If the object is non-extensible, the returned List must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have realm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per realm.
Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. Determination of the current realm and its intrinsics is described in 8.3. The well-known intrinsics are listed in Table 7.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types are Reference, List, Completion, Property Descriptor, Lexical Environment, Environment Record, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.
The List type is used to explain the evaluation of argument lists (see 12.3.6) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.
For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».
The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, {[[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty} defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor{[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true}.
The Completion type is a Record used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.
Values of the Completion type are Record values whose fields are defined as by Table 8. Such values are referred to as Completion Records.
The algorithms of this specification often implicitly return Completion Records whose [[Type]] is normal. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not a Completion Record, such as:
However, if the value expression of a “return” statement is a Completion Record construction literal, the resulting Completion Record is returned. If the value expression is a call to an abstract operation, the “return” statement simply returns the Completion Record produced by the abstract operation.
The abstract operation Completion(completionRecord) is used to emphasize that a previously computed Completion Record is being returned. The Completion abstract operation takes a single argument, completionRecord, and performs the following steps:
The Reference type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the super keyword and other language features. For example, the left-hand operand of an assignment is expected to produce a reference.
A Reference is a resolved name or property binding. A Reference consists of three components, the base value, the referenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a Boolean, a String, a Symbol, a Number, or an Environment Record. A base value of undefined indicates that the Reference could not be resolved to a binding. The referenced name is a String or Symbol value.
A Super Reference is a Reference that is used to represents a name binding that was expressed using the super keyword. A Super Reference has an additional thisValue component and its base value will never be an Environment Record.
The following abstract operations are used in this specification to access the components of references:
GetBase(V). Returns the base value component of the reference V.
GetReferencedName(V). Returns the referenced name component of the reference V.
IsStrictReference(V). Returns the strict reference flag component of the reference V.
HasPrimitiveBase(V). Returns true if Type(base) is Boolean, String, Symbol, or Number.
IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is true; otherwise returns false.
IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.
IsSuperReference(V). Returns true if this reference has a thisValue component.
The following abstract operations are used in this specification to operate on references:
Return ? base.GetBindingValue(GetReferencedName(V), IsStrictReference(V)) (see 8.1.1).
Note
The object that may be created in step 5.a.ii is not accessible outside of the above abstract operation and the ordinary object [[Get]] internal method. An implementation might choose to avoid the actual creation of the object.
Return ? base.SetMutableBinding(GetReferencedName(V), W, IsStrictReference(V)) (see 8.1.1).
Note
The object that may be created in step 6.a.ii is not accessible outside of the above algorithm and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.
The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field's name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. In addition, any field may be present or absent. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither. A generic Property Descriptor is a Property Descriptor value that is neither a data Property Descriptor nor an accessor Property Descriptor. A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the fields that correspond to the property attributes defined in either Table 2 or Table 3.
The following abstract operations are used in this specification to operate upon Property Descriptor values:
If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
Else,
If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
Return Desc.
6.2.5The Lexical Environment and Environment Record Specification Types#
The Lexical Environment and Environment Record types are used to explain the behaviour of name resolution in nested functions and blocks. These types and the operations upon them are defined in 8.1.
The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A Data Block value is created with a fixed number of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined integer indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3rd byte.
The following abstract operations are used in this specification to operate upon Data Block values:
These operations are not a part of the ECMAScript language; they are defined here to solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined throughout this specification.
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.
The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. Conversion occurs according to Table 9:
Table 9: ToPrimitive Conversions
Input Type
Result
Undefined
Return input.
Null
Return input.
Boolean
Return input.
Number
Return input.
String
Return input.
Symbol
Return input.
Object
Perform the steps following this table.
When Type(input) is Object, the following steps are taken:
If PreferredType was not passed, let hint be "default".
Else if PreferredType is hint String, let hint be "string".
Else PreferredType is hint Number, let hint be "number".
Let exoticToPrim be ? GetMethod(input, @@toPrimitive).
If exoticToPrim is not undefined, then
Let result be ? Call(exoticToPrim, input, « hint »).
When ToPrimitive is called with no hint, then it generally behaves as if the hint were Number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of the objects defined in this specification only Date objects (see 20.3.4.45) and Symbol objects (see 19.4.3.4) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint were String.
ToNumber applied to Strings applies the following grammar to the input String interpreted as a sequence of UTF-16 encoded code points (6.1.4). If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.
Note 1
The terminal symbols of this grammar are all composed of Unicode BMP code points so the result will be NaN if the string contains the UTF-16 encoding of any supplementary code points or any unpaired surrogate code points.
The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 11.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below. The MV on any grammar symbol, not provided below, is the MV for that symbol defined in 11.8.3.1.
Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space code point in the String numeric literal is "-", in which case the rounded value is -0. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in 6.1.6), unless the literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of an ExponentPart and
it is not 0; or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.
The abstract operation ToInt32 converts argument to one of 232 integer values in the range -231 through 231-1, inclusive. This abstract operation functions as follows:
The abstract operation ToUint32 converts argument to one of 232 integer values in the range 0 through 232-1, inclusive. This abstract operation functions as follows:
The abstract operation ToInt16 converts argument to one of 216 integer values in the range -32768 through 32767, inclusive. This abstract operation functions as follows:
The abstract operation ToUint16 converts argument to one of 216 integer values in the range 0 through 216-1, inclusive. This abstract operation functions as follows:
The abstract operation ToInt8 converts argument to one of 28 integer values in the range -128 through 127, inclusive. This abstract operation functions as follows:
The abstract operation ToUint8 converts argument to one of 28 integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:
The abstract operation ToUint8Clamp converts argument to one of 28 integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:
Unlike the other ECMAScript integer conversion abstract operation, ToUint8Clamp rounds rather than truncates non-integer values and does not convert +∞ to 0. ToUint8Clamp does “round half to even” tie-breaking. This differs from Math.round which does “round half up” tie-breaking.
The abstract operation ToString converts a Number m to String format as follows:
If m is NaN, return the String "NaN".
If m is +0 or -0, return the String "0".
If m is less than zero, return the String concatenation of the String "-" and ToString(-m).
If m is +∞, return the String "Infinity".
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k-1 ≤ s < 10k, the Number value for s × 10n-k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
If k ≤ n ≤ 21, return the String consisting of the code units of the k digits of the decimal representation of s (in order, with no leading zeroes), followed by n-k occurrences of the code unit 0x0030 (DIGIT ZERO).
If 0 < n ≤ 21, return the String consisting of the code units of the most significant n digits of the decimal representation of s, followed by the code unit 0x002E (FULL STOP), followed by the code units of the remaining k-n digits of the decimal representation of s.
If -6 < n ≤ 0, return the String consisting of the code unit 0x0030 (DIGIT ZERO), followed by the code unit 0x002E (FULL STOP), followed by -n occurrences of the code unit 0x0030 (DIGIT ZERO), followed by the code units of the k digits of the decimal representation of s.
Otherwise, if k = 1, return the String consisting of the code unit of the single digit of s, followed by code unit 0x0065 (LATIN SMALL LETTER E), followed by the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n-1 is positive or negative, followed by the code units of the decimal representation of the integer abs(n-1) (with no leading zeroes).
Return the String consisting of the code units of the most significant digit of the decimal representation of s, followed by code unit 0x002E (FULL STOP), followed by the code units of the remaining k-1 digits of the decimal representation of s, followed by code unit 0x0065 (LATIN SMALL LETTER E), followed by code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n-1 is positive or negative, followed by the code units of the decimal representation of the integer abs(n-1) (with no leading zeroes).
Note 1
The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:
If x is any Number value other than -0, then ToNumber(ToString(x)) is exactly the same Number value as x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.
Note 2
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k-1 ≤ s < 10k, the Number value for s × 10n-k is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10n-k is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:
The abstract operation ToLength converts argument to an integer suitable for use as the length of an array-like object. It performs the following steps:
The abstract operation CanonicalNumericIndexString returns argument converted to a numeric value if it is a String representation of a Number that would be produced by ToString, or the string "-0". Otherwise, it returns undefined. This abstract operation functions as follows:
The abstract operation RequireObjectCoercible throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 14:
The abstract operation IsCallable determines if argument, which must be an ECMAScript language value, is a callable function with a [[Call]] internal method.
The abstract operation IsConstructor determines if argument, which must be an ECMAScript language value, is a function object with a [[Construct]] internal method.
The abstract operation IsExtensible is used to determine whether additional properties can be added to the object that is O. A Boolean value is returned. This abstract operation performs the following steps:
The abstract operation IsPropertyKey determines if argument, which must be an ECMAScript language value, is a value that may be used as a property key.
The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:
If Type(x) is different from Type(y), return false.
The internal comparison abstract operation SameValueZero(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:
If Type(x) is different from Type(y), return false.
The internal comparison abstract operation SameValueNonNumber(x, y), where neither x nor y are Number values, produces true or false. Such a comparison is performed as follows:
If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true; otherwise, return false.
The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows:
If py is a prefix of px, return false. (A String value p is a prefix of String value q if q can be the result of concatenating p and some other String r. Note that any String is a prefix of itself, because r may be the empty String.)
If px is a prefix of py, return true.
Let k be the smallest nonnegative integer such that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
Let m be the integer that is the code unit value at index k within px.
Let n be the integer that is the code unit value at index k within py.
If m < n, return true. Otherwise, return false.
Else,
Let nx be ? ToNumber(px). Because px and py are primitive values evaluation order is not important.
If nx and ny are the same Number value, return false.
If nx is +0 and ny is -0, return false.
If nx is -0 and ny is +0, return false.
If nx is +∞, return false.
If ny is +∞, return true.
If ny is -∞, return false.
If nx is -∞, return true.
If the mathematical value of nx is less than the mathematical value of ny —note that these mathematical values are both finite and not both zero—return true. Otherwise, return false.
Note 1
Step 3 differs from step 7 in the algorithm for the addition operator + (12.8.3) in using “and” instead of “or”.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.
The abstract operation Get is used to retrieve the value of a specific property of an object. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:
The abstract operation GetV is used to retrieve the value of a specific property of an ECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. The operation is called with arguments V and P where V is the value and P is the property key. This abstract operation performs the following steps:
The abstract operation Set is used to set the value of a specific property of an object. The operation is called with arguments O, P, V, and Throw where O is the object, P is the property key, V is the new value for the property and Throw is a Boolean flag. This abstract operation performs the following steps:
The abstract operation CreateDataProperty is used to create a new own property of an object. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the value for the property. This abstract operation performs the following steps:
Let newDesc be the PropertyDescriptor{[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.
The abstract operation CreateMethodProperty is used to create a new own property of an object. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the value for the property. This abstract operation performs the following steps:
Let newDesc be the PropertyDescriptor{[[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}.
Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.
The abstract operation CreateDataPropertyOrThrow is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the value for the property. This abstract operation performs the following steps:
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception.
The abstract operation DefinePropertyOrThrow is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. The operation is called with arguments O, P, and desc where O is the object, P is the property key, and desc is the Property Descriptor for the property. This abstract operation performs the following steps:
The abstract operation DeletePropertyOrThrow is used to remove a specific own property of an object. It throws an exception if the property is not configurable. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:
The abstract operation GetMethod is used to get the value of a specific property of an ECMAScript language value when the value of the property is expected to be a function. The operation is called with arguments V and P where V is the ECMAScript language value, P is the property key. This abstract operation performs the following steps:
The abstract operation HasProperty is used to determine whether an object has a property with the specified property key. The property may be either an own or inherited. A Boolean value is returned. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:
The abstract operation HasOwnProperty is used to determine whether an object has an own property with the specified property key. A Boolean value is returned. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:
The abstract operation Call is used to call the [[Call]] internal method of a function object. The operation is called with arguments F, V, and optionally argumentsList where F is the function object, V is an ECMAScript language value that is the this value of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new empty List is used as its value. This abstract operation performs the following steps:
If argumentsList was not passed, let argumentsList be a new empty List.
If IsCallable(F) is false, throw a TypeError exception.
The abstract operation Construct is used to call the [[Construct]] internal method of a function object. The operation is called with arguments F, and optionally argumentsList, and newTarget where F is the function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new empty List is used as its value. If newTarget is not present, F is used as its value. This abstract operation performs the following steps:
If newTarget was not passed, let newTarget be F.
If argumentsList was not passed, let argumentsList be a new empty List.
The abstract operation TestIntegrityLevel is used to determine if the set of own properties of an object are fixed. This abstract operation performs the following steps:
The abstract operation CreateArrayFromList is used to create an Array object whose elements are provided by a List. This abstract operation performs the following steps:
Assert: elements is a List whose elements are all ECMAScript language values.
The abstract operation CreateListFromArrayLike is used to create a List value whose elements are provided by the indexed properties of an array-like object, obj. The optional argument elementTypes is a List containing the names of ECMAScript Language Types that are allowed for element values of the List that is created. This abstract operation performs the following steps:
If elementTypes was not passed, let elementTypes be « Undefined, Null, Boolean, String, Symbol, Number, Object ».
If Type(obj) is not Object, throw a TypeError exception.
The abstract operation Invoke is used to call a method property of an ECMAScript language value. The operation is called with arguments V, P, and optionally argumentsList where V serves as both the lookup point for the property and the this value of the call, P is the property key, and argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new empty List is used as its value. This abstract operation performs the following steps:
The abstract operation OrdinaryHasInstance implements the default algorithm for determining if an object O inherits from the instance object inheritance path provided by constructor C. This abstract operation performs the following steps:
7.3.20SpeciesConstructor ( O, defaultConstructor )#
The abstract operation SpeciesConstructor is used to retrieve the constructor that should be used to create new objects that are derived from the argument object O. The defaultConstructor argument is the constructor to use if a constructor @@species property cannot be found starting from O. This abstract operation performs the following steps:
If desc.[[Enumerable]] is true, append key to names.
Order the elements of names so they are in the same relative order as would be produced by the Iterator that would be returned if the EnumerateObjectProperties internal method was invoked with O.
The abstract operation IteratorStep with argument iterator requests the next value from iterator and returns either false indicating that the iterator has reached its end or the IteratorResult object if a next value is available. IteratorStep performs the following steps:
The abstract operation IteratorClose with arguments iterator and completion is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state:
The abstract operation CreateIterResultObject with arguments value and done creates an object that supports the IteratorResult interface by performing the following steps:
The abstract operation CreateListIterator with argument list creates an Iterator (25.1.1.2) object whose next method returns the successive elements of list. It performs the following steps:
A Lexical Environment is a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.
An Environment Record records the identifier bindings that are created within the scope of its associated Lexical Environment. It is referred to as the Lexical Environment's EnvironmentRecord
The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical Environments. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Lexical Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the current evaluation of the surrounding function.
A global environment is a Lexical Environment which does not have an outer environment. The global environment's outer environment reference is null. A global environment's EnvironmentRecord may be prepopulated with identifier bindings and includes an associated global object whose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.
A module environment is a Lexical Environment that contains the bindings for the top level declarations of a Module. It also contains the bindings that are explicitly imported by the Module. The outer environment of a module environment is a global environment.
A function environment is a Lexical Environment that corresponds to the invocation of an ECMAScript function object. A function environment may establish a new this binding. A function environment also captures the state necessary to support super method invocations.
Lexical Environments and Environment Record values are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.
There are two primary kinds of Environment Record values used in this specification: declarative Environment Records and object Environment Records. Declarative Environment Records are used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values. Object Environment Records are used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object. Global Environment Records and function Environment Records are specializations that are used for specifically for Script global declarations and for top-level declarations within functions.
For specification purposes Environment Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses, declarative Environment Record, object Environment Record, and global Environment Record. Function Environment Records and module Environment Records are subclasses of declarative Environment Record. The abstract class includes the abstract specification methods defined in Table 15. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.
Table 15: Abstract Methods of Environment Records
Method
Purpose
HasBinding(N)
Determine if an Environment Record has a binding for the String value N. Return true if it does and false if it does not
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an Environment Record. The String value N is the text of the bound name. If the Boolean argument D is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an Environment Record. The String value N is the text of the bound name. If S is true then attempts to access the value of the binding before it is initialized or set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an Environment Record. The String value N is the text of the bound name. S is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If S is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
DeleteBinding(N)
Delete a binding from an Environment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding()
Determine if an Environment Record establishes a this binding. Return true if it does and false if it does not.
HasSuperBinding()
Determine if an Environment Record establishes a super method binding. Return true if it does and false if it does not.
WithBaseObject()
If this Environment Record is associated with a with statement, return the with object. Otherwise, return undefined.
Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for declarative Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:
Let envRec be the declarative Environment Record for which the method was invoked.
If envRec has a binding for the name that is the value of N, return true.
The concrete Environment Record method CreateMutableBinding for declarative Environment Records creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument D has the value true the new binding is marked as being subject to deletion.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
The concrete Environment Record method CreateImmutableBinding for declarative Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If the Boolean argument S has the value true the new binding is marked as a strict binding.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
The concrete Environment Record method InitializeBinding for declarative Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialized.
The concrete Environment Record method SetMutableBinding for declarative Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N normally already exist, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true.
Let envRec be the declarative Environment Record for which the method was invoked.
The concrete Environment Record method GetBindingValue for declarative Environment Records simply returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
The concrete Environment Record method DeleteBinding for declarative Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec has a binding for the name that is the value of N.
If the binding for N in envRec cannot be deleted, return false.
Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for object Environment Records.
Object Environment Records created for with statements (13.11) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a withEnvironment Boolean value that is associated with each object Environment Record. By default, the value of withEnvironment is false for any object Environment Record.
The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for object Environment Records determines if its associated binding object has a property whose name is the value of the argument N:
Let envRec be the object Environment Record for which the method was invoked.
The concrete Environment Record method CreateMutableBinding for object Environment Records creates in an Environment Record's associated binding object a property whose name is the String value and initializes it to the value undefined. If Boolean argument D has the value true the new property's [[Configurable]] attribute is set to true; otherwise it is set to false.
Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
If D is true, let configValue be true; otherwise let configValue be false.
Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.
The concrete Environment Record method InitializeBinding for object Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.
Let envRec be the object Environment Record for which the method was invoked.
Assert: envRec must have an uninitialized binding for N.
Record that the binding for N in envRec has been initialized.
Return ? envRec.SetMutableBinding(N, V, false).
Note
In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, implementations do not need to explicitly track the initialization state of individual object Environment Record bindings.
The concrete Environment Record method SetMutableBinding for object Environment Records attempts to set the value of the Environment Record's associated binding object's property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
Let envRec be the object Environment Record for which the method was invoked.
The concrete Environment Record method GetBindingValue for object Environment Records returns the value of its associated binding object's property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:
Let envRec be the object Environment Record for which the method was invoked.
The concrete Environment Record method DeleteBinding for object Environment Records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true.
Let envRec be the object Environment Record for which the method was invoked.
A function Environment Record is a declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in Table 16.
Table 16: Additional Fields of Function Environment Records
Field Name
Value
Meaning
[[ThisValue]]
Any
This is the this value used for this invocation of the function.
[[ThisBindingStatus]]
"lexical" | "initialized" | "uninitialized"
If the value is "lexical", this is an ArrowFunction and does not have a local this value.
[[FunctionObject]]
Object
The function object whose invocation caused this Environment Record to be created.
[[HomeObject]]
Object | undefined
If the associated function has super property accesses and is not an ArrowFunction, [[HomeObject]] is the object that the function is bound to as a method. The default value for [[HomeObject]] is undefined.
[[NewTarget]]
Object | undefined
If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] newTarget parameter. Otherwise, its value is undefined.
Function Environment Records support all of the declarative Environment Record methods listed in Table 15 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed in Table 17:
Table 17: Additional Methods of Function Environment Records
Method
Purpose
BindThisValue(V)
Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()
Return the value of this Environment Record's this binding. Throws a ReferenceError if the this binding has not been initialized.
GetSuperBase()
Return the object that is the base for super property accesses bound in this Environment Record. The object is derived from this Environment Record's [[HomeObject]] field. The value undefined indicates that super property accesses will produce runtime errors.
The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:
A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A global Environment Record provides the bindings for built-in globals (clause 18), properties of the global object, and for all top-level declarations (13.2.8, 13.2.10) that occur within a Script.
Properties may be created directly on a global object. Hence, the object Environment Record component of a global Environment Record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, or VariableDeclaration declarations and bindings created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a global Environment Record maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.
Global Environment Records have the additional fields listed in Table 18 and the additional methods listed in Table 19.
Table 18: Additional Fields of Global Environment Records
Determines if the argument is the name of a global object property that may not be shadowed by a global lexically binding.
CanDeclareGlobalVar (N)
Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N)
Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
CreateGlobalVarBinding(N, D)
Used to create and initialize to undefined a global var binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a var. The String value N is the bound name. If D is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)
Create and initialize a global function binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a function. The String value N is the bound name. V is the initialization value. If the Boolean argument D is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.
The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for global Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:
Let envRec be the global Environment Record for which the method was invoked.
The concrete Environment Record method CreateMutableBinding for global Environment Records creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If Boolean argument D has the value true the new binding is marked as being subject to deletion.
Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, throw a TypeError exception.
The concrete Environment Record method CreateImmutableBinding for global Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If the Boolean argument S has the value true the new binding is marked as a strict binding.
Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, throw a TypeError exception.
The concrete Environment Record method InitializeBinding for global Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.
Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
Return DclRec.InitializeBinding(N, V).
Assert: If the binding exists, it must be in the object Environment Record.
The concrete Environment Record method SetMutableBinding for global Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, a TypeError is thrown if S is true. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
Let envRec be the global Environment Record for which the method was invoked.
The concrete Environment Record method GetBindingValue for global Environment Records returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
Let envRec be the global Environment Record for which the method was invoked.
The concrete Environment Record method DeleteBinding for global Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.
Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
Return DclRec.DeleteBinding(N).
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
The concrete Environment Record method HasVarDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration:
Let envRec be the global Environment Record for which the method was invoked.
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames contains the value of N, return true.
The concrete Environment Record method HasLexicalDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration:
Let envRec be the global Environment Record for which the method was invoked.
The concrete Environment Record method HasRestrictedGlobalProperty for global Environment Records determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexically binding:
Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return false.
If existingProp.[[Configurable]] is true, return false.
Return true.
Note
Properties may exist upon a global object that were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of the global object. The global property undefined is an example of such a property.
The concrete Environment Record method CanDeclareGlobalVar for global Environment Records determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed.
Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
The concrete Environment Record method CanDeclareGlobalFunction for global Environment Records determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is true, return true.
If IsDataDescriptor(existingProp) is true and existingProp has attribute values {[[Writable]]: true, [[Enumerable]]: true}, return true.
The concrete Environment Record method CreateGlobalVarBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is reused and assumed to be initialized.
Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
The concrete Environment Record method CreateGlobalFunctionBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is replaced.
Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined or existingProp.[[Configurable]] is true, then
Let desc be the PropertyDescriptor{[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D}.
Else,
Let desc be the PropertyDescriptor{[[Value]]: V }.
Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Steps 10-12 are equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.
A module Environment Record is a declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.
Module Environment Records support all of the declarative Environment Record methods listed in Table 15 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed in Table 20:
Table 20: Additional Methods of Module Environment Records
Method
Purpose
CreateImportBinding(N, M, N2)
Create an immutable indirect binding in a module Environment Record. The String value N is the text of the bound name. M is a Module Record, and N2 is a binding that exists in M's module Environment Record.
The concrete Environment Record method GetBindingValue for module Environment Records returns the value of its bound identifier whose name is the value of the argument N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
Let envRec be the module Environment Record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N is an indirect binding, then
Let M and N2 be the indirection values provided when this binding for N was created.
Let targetEnv be M.[[Environment]].
If targetEnv is undefined, throw a ReferenceError exception.
The concrete Environment Record method CreateImportBinding for module Environment Records creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. M is a Module Record, and N2 is the name of a binding that exists in M's module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding.
Let envRec be the module Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
The abstract operation GetIdentifierReference is called with a Lexical Environmentlex, a String name, and a Boolean flag strict. The value of lex may be null. When called, the following steps are performed:
If lex is the value null, then
Return a value of type Reference whose base value is undefined, whose referenced name is name, and whose strict reference flag is strict.
When the abstract operation NewObjectEnvironment is called with an Object O and a Lexical EnvironmentE as arguments, the following steps are performed:
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.
A realm is represented in this specification as a Realm Record with the fields specified in Table 21:
Table 21: Realm Record Fields
Field Name
Value
Meaning
[[Intrinsics]]
Record whose field names are intrinsic keys and whose values are objects
The intrinsic values used by code associated with this realm
A List of Record { [[Strings]]: List, [[Array]]: Object}.
Template objects are canonicalized separately for each realm using its Realm Record's [[TemplateMap]]. Each [[Strings]] value is a List containing, in source text order, the raw String values of a TemplateLiteral that has been evaluated. The associated [[Array]] value is the corresponding template object that is passed to a tag function.
An implementation may define other, implementation specific fields.
Set fields of intrinsics with the values listed in Table 7 that have not already been handled above. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 18-26. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(realmRec, <steps>, <prototype>, <slots>) where <steps> is the definition of that function provided by this specification, <prototype> is the specified value of the function's [[Prototype]] internal slot and <slots> is a list of the names, if any, of the function's specified internal slots. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
The abstract operation SetDefaultGlobalBindings with argument realmRec performs the following steps:
Let global be realmRec.[[GlobalObject]].
For each property of the Global Object specified in clause 18, do
Let name be the String value of the property name.
Let desc be the fully populated data property descriptor for the property containing the specified attributes for the property. For properties listed in 18.2, 18.3, or 18.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context that is actually executing code. This is known as the running execution context.
The execution context stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.
An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 22.
Table 22: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function
If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null.
The Realm Record from which associated code accesses ECMAScript resources.
ScriptOrModule
The Module Record or Script Record from which associated code originates. If there is no originating script or module, as is the case for the original execution context created in InitializeHostDefinedRealm, the value is null.
Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.
The value of the Realm component of the running execution context is also called the current Realm Record. The value of the Function component of the running execution context is also called the active function object.
Execution contexts for ECMAScript code have the additional state components listed in Table 23.
Table 23: Additional State Components for ECMAScript Code Execution Contexts
Component
Purpose
LexicalEnvironment
Identifies the Lexical Environment used to resolve identifier references made by code within this execution context.
The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value.
Execution contexts representing the evaluation of generator objects have the additional state components listed in Table 24.
Table 24: Additional State Components for Generator Execution Contexts
Component
Purpose
Generator
The GeneratorObject that this execution context is evaluating.
In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.
The GetActiveScriptOrModule abstract operation is used to determine the running script or module, based on the active function object. GetActiveScriptOrModule performs the following steps:
The ResolveBinding abstract operation is used to determine the binding of name passed as a String value. The optional argument env can be used to explicitly provide the Lexical Environment that is to be searched for the binding. During execution of ECMAScript code, ResolveBinding is performed using the following algorithm:
If env was not passed or if env is undefined, then
The abstract operation GetThisEnvironment finds the Environment Record that currently supplies the binding of the keyword this. GetThisEnvironment performs the following steps:
The abstract operation ResolveThisBinding determines the binding of the keyword this using the LexicalEnvironment of the running execution context. ResolveThisBinding performs the following steps:
The abstract operation GetNewTarget determines the NewTarget value using the LexicalEnvironment of the running execution context. GetNewTarget performs the following steps:
The abstract operation GetGlobalObject returns the global object used by the currently running execution context. GetGlobalObject performs the following steps:
A Job is an abstract operation that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress. A Job abstract operation may be defined to accept an arbitrary set of job parameters.
Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 25. Once execution of a Job is initiated, the Job always executes to completion. No other Job may be initiated until the currently running Job completes. However, the currently running Job or external events may cause the enqueuing of additional PendingJobs that may be initiated sometime after completion of the currently running Job.
This is the abstract operation that is performed when execution of this PendingJob is initiated. Jobs are abstract operations that use NextJob rather than Return to indicate that they have completed.
The script or module for the initial execution context when this PendingJob is initiated.
[[HostDefined]]
Any, default value is undefined.
Field reserved for use by host environments that need to associate additional information with a pending Job.
A Job Queue is a FIFO queue of PendingJob records. Each Job Queue has a name and the full set of available Job Queues are defined by an ECMAScript implementation. Every ECMAScript implementation has at least the Job Queues defined in Table 26.
Table 26: Required Job Queues
Name
Purpose
ScriptJobs
Jobs that validate and evaluate ECMAScript Script and Module source text. See clauses 10 and 15.
PromiseJobs
Jobs that are responses to the settlement of a Promise (see 25.4).
A request for the future execution of a Job is made by enqueueing, on a Job Queue, a PendingJob record that includes a Job abstract operation name and any necessary argument values. When there is no running execution context and the execution context stack is empty, the ECMAScript implementation removes the first PendingJob from a Job Queue and uses the information contained in it to create an execution context and starts execution of the associated Job abstract operation.
The PendingJob records from a single Job Queue are always initiated in FIFO order. This specification does not define the order in which multiple Job Queues are serviced. An ECMAScript implementation may interweave the FIFO evaluation of the PendingJob records of a Job Queue with the evaluation of the PendingJob records of one or more other Job Queues. An implementation must define what occurs when there are no running execution context and all Job Queues are empty.
Note
Typically an ECMAScript implementation will have its Job Queues pre-initialized with at least one PendingJob and one of those Jobs will be the first to be executed. An implementation might choose to free all resources and terminate if the current Job completes and all Job Queues are empty. Alternatively, it might choose to wait for a some implementation specific agent or mechanism to enqueue new PendingJob requests.
The following abstract operations are used to create and manage Jobs and Job Queues:
Let callerScriptOrModule be callerContext's ScriptOrModule.
Let pending be PendingJob{ [[Job]]: job, [[Arguments]]: arguments, [[Realm]]: callerRealm, [[ScriptOrModule]]: callerScriptOrModule, [[HostDefined]]: undefined }.
Perform any implementation or host environment defined processing of pending. This may include modifying the [[HostDefined]] field or any other field of pending.
Add pending at the back of the Job Queue named by queueName.
If the host requires use of an exotic object to serve as realm's global object, let global be such an object created in an implementation defined manner. Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
If the host requires that the this binding in realm's global scope return an object other than the global object, let thisValue be such an object created in an implementation defined manner. Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
Create any implementation defined global object properties on globalObj.
In an implementation dependent manner, obtain the ECMAScript source texts (see clause 10) and any associated host-defined values for zero or more ECMAScript scripts and/or ECMAScript modules. For each such sourceText and hostDefined,
If sourceText is the source code of a script, then
9.1Ordinary Object Internal Methods and Internal Slots#
All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.
Every ordinary object has a Boolean-valued [[Extensible]] internal slot that controls whether or not properties may be added to the object. If the value of the [[Extensible]] internal slot is false then additional properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Prototype]] internal slot of the object may not be modified. Once the value of an object's [[Extensible]] internal slot has been set to false it may not be subsequently changed to true.
In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.
Each ordinary object internal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked when ordinary object internal methods are applied to them.
If the [[GetPrototypeOf]] internal method of p is not the ordinary object internal method defined in 9.1.1, let done be true.
Else, let p be the value of p's [[Prototype]] internal slot.
Set the value of the [[Prototype]] internal slot of O to V.
Return true.
Note
The loop in step 8 guarantees that there will be no circularities in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
When the abstract operation OrdinaryDefineOwnProperty is called with Object O, property key P, and Property DescriptorDesc, the following steps are taken:
Let current be ? O.[[GetOwnProperty]](P).
Let extensible be the value of the [[Extensible]] internal slot of O.
When the abstract operation IsCompatiblePropertyDescriptor is called with Boolean value Extensible, and Property Descriptors Desc, and Current, the following steps are taken:
When the abstract operation ValidateAndApplyPropertyDescriptor is called with Object O, property key P, Boolean value extensible, and Property Descriptors Desc, and current, the following steps are taken:
This algorithm contains steps that test various fields of the Property DescriptorDesc for specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent then its value is considered to be false.
Note 1
If undefined is passed as the O argument only validation is performed and no object updates are performed.
Assert: If O is not undefined, then IsPropertyKey(P) is true.
If O is not undefined, create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
If O is not undefined, create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
Return true.
Return true, if every field in Desc is absent.
Return true, if every field in Desc also occurs in current and the value of every field in Desc is the same value as the corresponding field in current when compared using the SameValue algorithm.
If the [[Configurable]] field of current is false, then
Return false, if the [[Configurable]] field of Desc is true.
Return false, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
If O is not undefined, convert the property named P of object O from a data property to an accessor property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
Else,
If O is not undefined, convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
If the [[Configurable]] field of current is false, then
Return false, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.
Return false, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.
If O is not undefined, then
For each field of Desc that is present, set the corresponding attribute of the property named P of object O to the value of the field.
Return true.
Note 2
Step 8.b allows any field of Desc to be different from the corresponding field of current if current's [[Configurable]] field is true. This even permits changing the [[Value]] of a property whose [[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] is first set to true, a new [[Value]] is set, and then [[Writable]] is set to false.
When the abstract operation OrdinarySet is called with Object O, property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
The abstract operation ObjectCreate with argument proto (an object or null) is used to specify the runtime creation of new ordinary objects. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty List is used. This abstract operation performs the following steps:
If internalSlotsList was not provided, let internalSlotsList be a new empty List.
Let obj be a newly created object with an internal slot for each name in internalSlotsList.
Set obj's essential internal methods to the default ordinary object definitions specified in 9.1.
Set the [[Prototype]] internal slot of obj to proto.
Set the [[Extensible]] internal slot of obj to true.
The abstract operation OrdinaryCreateFromConstructor creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's prototype property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. The optional internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty List is used. This abstract operation performs the following steps:
Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
The abstract operation GetPrototypeFromConstructor determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor's prototype property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. This abstract operation performs the following steps:
Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Let proto be realm's intrinsic object named intrinsicDefaultProto.
Return proto.
Note
If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the realm of the constructor function rather than from the running execution context.
ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (10.2.1) or non-strict mode code. An ECMAScript function object whose code is strict mode code is called a strict function. One whose code is not strict mode code is called a non-strict function.
ECMAScript function objects have the additional internal slots listed in Table 27.
Table 27: Internal Slots of ECMAScript Function Objects
The script or module in which the function was created.
[[ThisMode]]
(lexical, strict, global)
Defines how this references are interpreted within the formal parameters and code body of the function. lexical means that this refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object.
[[Strict]]
Boolean
true if this is a strict mode function, false if this is not a strict mode function.
[[HomeObject]]
Object
If the function uses super, this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin.
All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.
The [[Call]] internal method for an ECMAScript function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
Assert: F is an ECMAScript function object.
If F's [[FunctionKind]] internal slot is "classConstructor", throw a TypeError exception.
When calleeContext is removed from the execution context stack in step 8 it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.
When the abstract operation PrepareForOrdinaryCall is called with function object F and ECMAScript language valuenewTarget, the following steps are taken:
When the abstract operation OrdinaryCallBindThis is called with function object F, execution contextcalleeContext, and ECMAScript value thisArgument, the following steps are taken:
Let thisMode be the value of F's [[ThisMode]] internal slot.
The [[Construct]] internal method for an ECMAScript Function object F is called with parameters argumentsList and newTarget. argumentsList is a possibly empty List of ECMAScript language values. The following steps are taken:
The abstract operation FunctionAllocate requires the three arguments functionPrototype, strict and functionKind. FunctionAllocate performs the following steps:
Assert: functionKind is either "normal", "non-constructor" or "generator".
If functionKind is "normal", let needsConstruct be true.
Else, let needsConstruct be false.
If functionKind is "non-constructor", let functionKind be "normal".
Let F be a newly created ECMAScript function object with the internal slots listed in Table 27. All of those internal slots are initialized to undefined.
Set F's essential internal methods to the default ordinary object definitions specified in 9.1.
Set F's [[Call]] internal method to the definition specified in 9.2.1.
If needsConstruct is true, then
Set F's [[Construct]] internal method to the definition specified in 9.2.2.
Set the [[ConstructorKind]] internal slot of F to "base".
Set the [[Strict]] internal slot of F to strict.
Set the [[FunctionKind]] internal slot of F to functionKind.
Set the [[Prototype]] internal slot of F to functionPrototype.
Set the [[Extensible]] internal slot of F to true.
The abstract operation FunctionInitialize requires the arguments: a function object F, kind which is one of (Normal, Method, Arrow), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope. FunctionInitialize performs the following steps:
Assert: F is an extensible object that does not have a length own property.
Let len be the ExpectedArgumentCount of ParameterList.
The abstract operation FunctionCreate requires the arguments: kind which is one of (Normal, Method, Arrow), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope, a Boolean flag Strict, and optionally, an object prototype. FunctionCreate performs the following steps:
The abstract operation GeneratorFunctionCreate requires the arguments: kind which is one of (Normal, Method), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope, and a Boolean flag Strict. GeneratorFunctionCreate performs the following steps:
Let functionPrototype be the intrinsic object %Generator%.
Let F be FunctionAllocate(functionPrototype, Strict, "generator").
The abstract operation AddRestrictedFunctionProperties is called with a function object F and Realm Recordrealm as its argument. It performs the following steps:
The %ThrowTypeError% intrinsic is an anonymous built-in function object that is defined once for each realm. When %ThrowTypeError% is called it performs the following steps:
Throw a TypeError exception.
The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is false.
The length property of a %ThrowTypeError% function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The abstract operation MakeConstructor requires a Function argument F and optionally, a Boolean writablePrototype and an object prototype. If prototype is provided it is assumed to already contain, if needed, a "constructor" property whose value is F. This operation converts F into a constructor by performing the following steps:
Assert: F is an ECMAScript function object.
Assert: F has a [[Construct]] internal method.
Assert: F is an extensible object that does not have a prototype own property.
If the writablePrototype argument was not provided, let writablePrototype be true.
The abstract operation SetFunctionName requires a Function argument F, a String or Symbol argument name and optionally a String argument prefix. This operation adds a name property to F by performing the following steps:
Assert: F is an extensible object that does not have a name own property.
When an execution context is established for evaluating an ECMAScript function a new function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.
FunctionDeclarationInstantiation is performed as follows using arguments func and argumentsList. func is the function object for which the execution context is being established.
Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
If the value of the [[ThisMode]] internal slot of func is lexical, then
NOTE Arrow functions never have an arguments objects.
Let argumentsObjectNeeded be false.
Else if "arguments" is an element of parameterNames, then
Let argumentsObjectNeeded be false.
Else if hasParameterExpressions is false, then
If "arguments" is an element of functionNames or if "arguments" is an element of lexicalNames, then
Let argumentsObjectNeeded be false.
For each String paramName in parameterNames, do
Let alreadyDeclared be envRec.HasBinding(paramName).
NOTE Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
NOTE mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
Let iteratorRecord be Record {[[Iterator]]: CreateListIterator(argumentsList), [[Done]]: false}.
If hasDuplicates is true, then
Perform ? IteratorBindingInitialization for formals with iteratorRecord and undefined as arguments.
Else,
Perform ? IteratorBindingInitialization for formals with iteratorRecord and env as arguments.
If hasParameterExpressions is false, then
NOTE Only a single lexical environment is needed for the parameters and top-level vars.
Let instantiatedVarNames be a copy of the ListparameterNames.
For each n in varNames, do
If n is not an element of instantiatedVarNames, then
Append n to instantiatedVarNames.
Perform ! envRec.CreateMutableBinding(n, false).
Call envRec.InitializeBinding(n, undefined).
Let varEnv be env.
Let varEnvRec be envRec.
Else,
NOTE A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
NOTE: Non-strict functions use a separate lexical Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
Set the LexicalEnvironment of calleeContext to lexEnv.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
For each element d in lexDeclarations do
NOTE A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
B.3.3 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.
Note 3
Parameter Initializers may contain direct eval expressions. Any top level declarations of such evals are only visible to the eval code (10.2). The creation of the environment for such declarations is described in 14.1.19.
The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (9.2) whose behaviour is provided using ECMAScript code or as implementation provided exotic function objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.
If a built-in function object is implemented as an exotic object it must have the ordinary object behaviour specified in 9.1. All such exotic function objects also have [[Prototype]], [[Extensible]], [[Realm]], and [[ScriptOrModule]] internal slots.
Unless otherwise specified every built-in function object has the %FunctionPrototype% object as the initial value of its [[Prototype]] internal slot.
The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides the this value, the [[Call]] argumentsList provides the named parameters, and the NewTarget value is undefined. When invoked with [[Construct]], the this value is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScript function object then this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict mode functions. If a built-in constructor has any [[Call]] behaviour other than throwing a TypeError exception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[FunctionKind]] internal slot to have the value "classConstructor".
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in constructor's named parameters.
Built-in functions that are not constructors do not have a prototype property unless otherwise specified in the description of a particular function.
If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:
The [[Call]] internal method for a built-in function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.
The [[Construct]] internal method for built-in function object F is called with parameters argumentsList and newTarget. The steps performed are the same as [[Call]] (see 9.3.1) except that step 10 is replaced by:
Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.
The abstract operation CreateBuiltinFunction takes arguments realm, prototype, and steps. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty List is used. CreateBuiltinFunction returns a built-in function object created by the following steps:
Assert: steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList. The initial value of each of those internal slots is undefined.
Set the [[Prototype]] internal slot of func to prototype.
Set the [[Extensible]] internal slot of func to true.
Set the [[ScriptOrModule]] internal slot of func to null.
Return func.
Each built-in function defined in this specification is created as if by calling the CreateBuiltinFunction abstract operation, unless otherwise specified.
9.4Built-in Exotic Object Internal Methods and Slots#
This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:
A bound function is an exotic object that wraps another function object. A bound function is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function generally results in a call of its wrapped function.
Bound function objects do not have the internal slots of ECMAScript function objects defined in Table 27. Instead they have the internal slots defined in Table 28.
Table 28: Internal Slots of Exotic Bound Function Objects
Internal Slot
Type
Description
[[BoundTargetFunction]]
Callable Object
The wrapped function object.
[[BoundThis]]
Any
The value that is always passed as the this value when calling the wrapped function.
A list of values whose elements are used as the first arguments to any call to the wrapped function.
Bound function objects provide all of the essential internal methods as specified in 9.1. However, they use the following definitions for the essential internal methods of function objects.
When the [[Call]] internal method of an exotic bound function object, F, which was created using the bind function is called with parameters thisArgument and argumentsList, a List of ECMAScript language values, the following steps are taken:
Let target be the value of F's [[BoundTargetFunction]] internal slot.
Let boundThis be the value of F's [[BoundThis]] internal slot.
Let boundArgs be the value of F's [[BoundArguments]] internal slot.
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
When the [[Construct]] internal method of an exotic bound function object, F that was created using the bind function is called with a list of arguments argumentsList and newTarget, the following steps are taken:
Let target be the value of F's [[BoundTargetFunction]] internal slot.
Assert: target has a [[Construct]] internal method.
Let boundArgs be the value of F's [[BoundArguments]] internal slot.
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
If SameValue(F, newTarget) is true, let newTarget be target.
The abstract operation BoundFunctionCreate with arguments targetFunction, boundThis and boundArgs is used to specify the creation of new Bound Function exotic objects. It performs the following steps:
An Array object is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 232. The value of the length property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is an array index, the value of the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the value of the length property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.
Note
A String property name P is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232-1.
Array exotic objects always have a non-configurable property named "length".
Array exotic objects provide an alternative definition for the [[DefineOwnProperty]] internal method. Except for that internal method, Array exotic objects provide all of the other essential internal methods as specified in 9.1.
When the [[DefineOwnProperty]] internal method of an Array exotic object A is called with property key P, and Property DescriptorDesc, the following steps are taken:
Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
The abstract operation ArrayCreate with argument length (either 0 or a positive integer) and optional argument proto is used to specify the creation of new Array exotic objects. It performs the following steps:
Assert: length is an integer Number ≥ 0.
If length is -0, let length be +0.
If length>232-1, throw a RangeError exception.
If the proto argument was not passed, let proto be the intrinsic object %ArrayPrototype%.
Let A be a newly created Array exotic object.
Set A's essential internal methods except for [[DefineOwnProperty]] to the default ordinary object definitions specified in 9.1.
Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.2.1.
Set the [[Prototype]] internal slot of A to proto.
Set the [[Extensible]] internal slot of A to true.
The abstract operation ArraySpeciesCreate with arguments originalArray and length is used to specify the creation of a new Array object using a constructor function that is derived from originalArray. It performs the following steps:
If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running execution context, then a new Array is created using the realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.
Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
Return OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor{[[Writable]]: false}). This call will always return true.
Return true.
Note
In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2nd Edition of this specification.
A String object is an exotic object that encapsulates a String value and exposes virtual integer indexed data properties corresponding to the individual code unit elements of the String value. Exotic String objects always have a data property named "length" whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.
Exotic String objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.
Exotic String objects provide alternative definitions for the following internal methods. All of the other exotic String object essential internal methods that are not defined below are as specified in 9.1.
The abstract operation StringCreate with arguments value and prototype is used to specify the creation of new exotic String objects. It performs the following steps:
Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.
Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (19.1.3.6) to identify them as such.
Arguments exotic objects provide alternative definitions for the following internal methods. All of the other exotic arguments object essential internal methods that are not defined below are as specified in 9.1
Note 1
For non-strict functions the integer indexed data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function's execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object's properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.
Note 2
The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.
Note 3
Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access. The "callee" property has a more specific meaning for non-strict functions and a "caller" property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by conforming ECMAScript implementations.
The [[DefineOwnProperty]] internal method of an arguments exotic object when called with a property key P and Property DescriptorDesc performs the following steps:
Let args be the arguments object.
Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
The [[Get]] internal method of an arguments exotic object when called with a property key P and ECMAScript language valueReceiver performs the following steps:
Let args be the arguments object.
Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
The [[Set]] internal method of an arguments exotic object when called with property key P, value V, and ECMAScript language valueReceiver performs the following steps:
The abstract operation CreateMappedArgumentsObject is called with object func, parsed grammar phrase formals, ListargumentsList, and Environment Recordenv. The following steps are performed:
Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let len be the number of elements in argumentsList.
Let obj be a newly created arguments exotic object with a [[ParameterMap]] internal slot.
Set the [[GetOwnProperty]] internal method of obj as specified in 9.4.4.1.
Set the [[DefineOwnProperty]] internal method of obj as specified in 9.4.4.2.
Set the [[Get]] internal method of obj as specified in 9.4.4.3.
Set the [[Set]] internal method of obj as specified in 9.4.4.4.
Set the [[HasProperty]] internal method of obj as specified in 9.4.4.5.
Set the [[Delete]] internal method of obj as specified in 9.4.4.6.
Set the remainder of obj's essential internal methods to the default ordinary object definitions specified in 9.1.
The abstract operation MakeArgGetter called with String name and Environment Recordenv creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps:
An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter function f that expects no arguments is called it performs the following steps:
Let name be the value of f's [[Name]] internal slot.
Let env be the value of f's [[Env]] internal slot.
Return env.GetBindingValue(name, false).
Note
ArgGetter functions are never directly accessible to ECMAScript code.
The abstract operation MakeArgSetter called with String name and Environment Recordenv creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps:
An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter function f is called with argument value it performs the following steps:
Let name be the value of f's [[Name]] internal slot.
Let env be the value of f's [[Env]] internal slot.
Return env.SetMutableBinding(name, value, false).
Note
ArgSetter functions are never directly accessible to ECMAScript code.
An Integer Indexed object is an exotic object that performs special handling of integer index property keys.
Integer Indexed exotic objects have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Integer Indexed exotic objects provide alternative definitions for the following internal methods. All of the other Integer Indexed exotic object essential internal methods that are not defined below are as specified in 9.1.
When the [[DefineOwnProperty]] internal method of an Integer Indexed exotic object O is called with property key P, and Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of an Integer Indexed exotic object O is called with property key P and ECMAScript language valueReceiver, the following steps are taken:
When the [[Set]] internal method of an Integer Indexed exotic object O is called with property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
The abstract operation IntegerIndexedObjectCreate with arguments prototype and internalSlotsList is used to specify the creation of new Integer Indexed exotic objects. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs the following steps:
Assert: internalSlotsList contains the names [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]].
Let A be a newly created object with an internal slot for each name in internalSlotsList.
Set A's essential internal methods to the default ordinary object definitions specified in 9.1.
Set the [[GetOwnProperty]] internal method of A as specified in 9.4.5.1.
Set the [[HasProperty]] internal method of A as specified in 9.4.5.2.
Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.5.3.
Set the [[Get]] internal method of A as specified in 9.4.5.4.
Set the [[Set]] internal method of A as specified in 9.4.5.5.
Set the [[OwnPropertyKeys]] internal method of A as specified in 9.4.5.6.
Set the [[Prototype]] internal slot of A to prototype.
Set the [[Extensible]] internal slot of A to true.
A module namespace object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 15.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace objects are not extensible.
Module namespace objects have the internal slots defined in Table 29.
Table 29: Internal Slots of Module Namespace Exotic Objects
A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using Array.prototype.sort using SortCompare as comparefn.
Module namespace exotic objects provide alternative definitions for all of the internal methods.
When the [[DefineOwnProperty]] internal method of a module namespace exotic object O is called with property key P and Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of a module namespace exotic object O is called with property key P and ECMAScript language valueReceiver, the following steps are taken:
ResolveExport is idempotent and side-effect free. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.
When the [[Set]] internal method of a module namespace exotic object O is called with property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
The abstract operation ModuleNamespaceCreate with arguments module, and exports is used to specify the creation of new module namespace exotic objects. It performs the following steps:
9.5Proxy Object Internal Methods and Internal Slots#
A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy objects has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or null. Methods (see Table 30) of a handler object may be used to augment the implementation for one or more of the proxy object's internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy's target object.
Table 30: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[DefineOwnProperty]]
defineProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that proxy object to throw a TypeError exception.
Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.
In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is any ECMAScript language value and Desc is a Property Descriptor record.
If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
Return handlerProto.
Note
[[GetPrototypeOf]] for proxy objects enforces the following invariant:
The result of [[GetPrototypeOf]] must be either an Object or null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
Let targetResult be ? target.[[IsExtensible]]().
If SameValue(booleanTrapResult, targetResult) is false, throw a TypeError exception.
Return booleanTrapResult.
Note
[[IsExtensible]] for proxy objects enforces the following invariant:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.
If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
Throw a TypeError exception.
Return resultDesc.
Note
[[GetOwnProperty]] for proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.
When the [[DefineOwnProperty]] internal method of a Proxy exotic object O is called with property key P and Property DescriptorDesc, the following steps are taken:
If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
Return true.
Note
[[DefineOwnProperty]] for proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
When the [[Get]] internal method of a Proxy exotic object O is called with property key P and ECMAScript language valueReceiver, the following steps are taken:
Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined, then
If IsDataDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false and targetDesc.[[Get]] is undefined, then
If trapResult is not undefined, throw a TypeError exception.
Return trapResult.
Note
[[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.
When the [[Set]] internal method of a Proxy exotic object O is called with property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, V, Receiver »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined, then
If IsDataDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false, then
If targetDesc.[[Set]] is undefined, throw a TypeError exception.
Return true.
Note
[[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.
The [[Call]] internal method of a Proxy exotic object O is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
Let handler be the value of the [[ProxyHandler]] internal slot of O.
A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
The [[Construct]] internal method of a Proxy exotic object O is called with parameters argumentsList which is a possibly empty List of ECMAScript language values and newTarget. The following steps are taken:
Let handler be the value of the [[ProxyHandler]] internal slot of O.
Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
If Type(newObj) is not Object, throw a TypeError exception.
Return newObj.
Note 1
A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for proxy objects enforces the following invariants:
The abstract operation ProxyCreate with arguments target and handler is used to specify the creation of new Proxy exotic objects. It performs the following steps:
If Type(target) is not Object, throw a TypeError exception.
If target is a Proxy exotic object and the value of the [[ProxyHandler]] internal slot of target is null, throw a TypeError exception.
If Type(handler) is not Object, throw a TypeError exception.
If handler is a Proxy exotic object and the value of the [[ProxyHandler]] internal slot of handler is null, throw a TypeError exception.
Let P be a newly created object.
Set P's essential internal methods (except for [[Call]] and [[Construct]]) to the definitions specified in 9.5.
ECMAScript code is expressed using Unicode, version 8.0.0 or later. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.
Note
In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.
Function code is generally provided as the bodies of Function Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3) and Generator Definitions (14.4). Function code is also derived from the arguments to the Function constructor (19.2.1.1) and the GeneratorFunction constructor (25.2.1.1).
An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:
Function code that is supplied as the arguments to the built-in Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.
ECMAScript code that is not strict mode code is called non-strict code.
An ECMAScript implementation may support the evaluation of exotic function objects whose evaluative behaviour is expressed in some implementation defined form of executable code other than via ECMAScript code. Whether a function object is an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.
The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName after the first character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 11.2).
The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 31.
White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.
The ECMAScript white space code points are listed in Table 32.
Table 32: White Space Code Points
Code Point
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+0020
SPACE
<SP>
U+00A0
NO-BREAK SPACE
<NBSP>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
Other category “Zs”
Any other Unicode “Separator, space” code point
<USP>
ECMAScript implementations must recognize as WhiteSpace code points listed in the “Separator, space” (Zs) category.
Note
Other than for the code points listed in Table 32, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Zs”.
Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.
Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.
The ECMAScript line terminator code points are listed in Table 33.
Table 33: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points in Table 33 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 32. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 11.9).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.
IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in version 8.0.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of an IdentifierName.
Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point to the IdentifierName. The code point is expressed by the HexDigits of the UnicodeEscapeSequence (see 11.8.4). The \ preceding the UnicodeEscapeSequence and the u and { } code units, if they appear, do not contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a code point into an IdentifierName that would otherwise be illegal. In other words, if a \UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.
Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
Return the String value consisting of the sequence of code units corresponding to IdentifierName. In determining the sequence any occurrences of \UnicodeEscapeSequence are first replaced with the code point represented by the UnicodeEscapeSequence and then the code points of the entire IdentifierName are converted to code units by UTF16Encoding each code point.
In some contexts yield is given the semantics of an Identifier. See 12.1.1. In strict mode code, let and static are treated as reserved keywords through static semantic restrictions (see 12.1.1, 13.3.1.1, 13.7.5.1, and 14.5.1) rather than the lexical grammar.
await is only treated as a FutureReservedWord when Module is the goal symbol of the syntactic grammar.
Note
Use of the following tokens within strict mode code is also reserved. That usage is restricted using static semantic restrictions (see 12.1.1) rather than the lexical grammar:
A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.
Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and
it is not 0; or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.
A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), U+2028 (LINE SEPARATOR), U+2029 (PARAGRAPH SEPARATOR), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 10.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.
A line terminator code point cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to cause a line terminator code point to be part of the String value of a string literal is to use an escape sequence such as \n or \u000A.
A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of code unit values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 11.8.3.
The SV of StringLiteral::"" is the empty code unit sequence.
The SV of StringLiteral::'' is the empty code unit sequence.
A regular expression literal is an input element that is converted to a RegExp object (see 21.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 21.2.3).
The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (21.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.
Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.
A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (SV, 11.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 11.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.
The TRV of UnicodeEscapeSequence::u{HexDigits} is the sequence consisting of code unit value 0x0075 followed by code unit value 0x007B followed by TRV of HexDigits followed by code unit value 0x007D.
The TRV of LineTerminatorSequence::<CR><LF> is the sequence consisting of the code unit value 0x000A.
Note
TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.
In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 11.
There are three basic rules of semicolon insertion:
When, as a Script or Module is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }.
The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (13.7.2).
When, as the Script or Module is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScript Script or Module, then a semicolon is automatically inserted at the end of the input stream.
When, as the Script or Module is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 13.7.4).
Note
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should appear on the same line as its operand.
An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.
It is a Syntax Error if the code matched by this production is contained in strict mode code and the StringValue of Identifier is "arguments" or "eval".
It is a Syntax Error if this phrase is contained in strict mode code and the StringValue of IdentifierName is: "implements", "interface", "let", "package", "private", "protected", "public", "static", or "yield".
It is a Syntax Error if StringValue of IdentifierName is the same String value as the StringValue of any ReservedWord except for yield.
Note
StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.
undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 9.2.12). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
In non-strict code, the keyword yield may be used as an identifier. Evaluating the IdentifierReference production resolves the binding of yield as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code. See 13.3.1 for the handling of yield in binding creation contexts.
An ArrayLiteral is an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.
CreateDataProperty is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].
An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.
In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.
Always throw a Syntax Error if code matches this production.
Note
This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It cannot occur in an actual object initializer.
It is a Syntax Error if BodyText of RegularExpressionLiteral cannot be recognized using the goal symbol Pattern of the ECMAScript RegExp grammar specified in 21.2.1.
It is a Syntax Error if FlagText of RegularExpressionLiteral contains any code points other than "g", "i", "m", "u", or "y", or if it contains the same code point more than once.
Return a List whose first element is siteObj, whose second elements is firstSub, and whose subsequent elements are the elements of restSub, in order. restSub may contain no elements.
Append the Record{[[Strings]]: rawStrings, [[Array]]: template} to templateRegistry.
Return template.
Note 1
The creation of a template object cannot result in an abrupt completion.
Note 2
Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (12.2.9.5). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template objects.
This algorithm does not apply GetValue to the result of evaluating Expression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.
The abstract operation EvaluateCall takes as arguments a value ref, a syntactic grammar production arguments, and a Boolean argument tailPosition. It performs the following steps:
The abstract operation EvaluateDirectCall takes as arguments a value func, a value thisValue, a syntactic grammar production arguments, and a Boolean argument tailPosition. It performs the following steps:
Let argList be ? ArgumentListEvaluation(arguments).
If Type(func) is not Object, throw a TypeError exception.
If IsCallable(func) is false, throw a TypeError exception.
Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
Return a value of type Reference that is a Super Reference whose base value is bv, whose referenced name is propertyKey, whose thisValue is actualThis, and whose strict reference flag is strict.
A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (12.2.9). The actual arguments include a template object (12.2.9.3) and the values produced by evaluating the expressions embedded within the TemplateLiteral.
When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError exception is thrown.
Object (standard exotic and does not implement [[Call]])
"object"
Object (implements [[Call]])
"function"
Object (non-standard exotic and does not implement [[Call]])
Implementation-defined. Must not be "undefined", "boolean", "function", "number", "symbol", or "string".
Note
Implementations are discouraged from defining new typeof result values for non-standard exotic objects. If possible "object" should be used for such objects.
The *MultiplicativeOperator performs multiplication, producing the product of its operands. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.
The result of a floating-point multiplication is governed by the rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is NaN, the result is NaN.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Multiplication of an infinity by a zero results in NaN.
Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.
Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity nor NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.
The /MultiplicativeOperator performs division, producing the quotient of its operands. The left operand is the dividend and the right operand is the divisor. ECMAScript does not perform integer division. The operands and result of all division operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE 754-2008 arithmetic:
If either operand is NaN, the result is NaN.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in NaN.
Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.
Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.
Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.
Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.
The %MultiplicativeOperator yields the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor.
Note
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.
The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2008. The IEEE 754-2008 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.
The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:
If either operand is NaN, the result is NaN.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n - (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode.
Returns an implementation-dependent approximation of the result of raising base to the power exponent.
If exponent is NaN, the result is NaN.
If exponent is +0, the result is 1, even if base is NaN.
If exponent is -0, the result is 1, even if base is NaN.
If base is NaN and exponent is nonzero, the result is NaN.
If abs(base) > 1 and exponent is +∞, the result is +∞.
If abs(base) > 1 and exponent is -∞, the result is +0.
If abs(base) is 1 and exponent is +∞, the result is NaN.
If abs(base) is 1 and exponent is -∞, the result is NaN.
If abs(base) < 1 and exponent is +∞, the result is +0.
If abs(base) < 1 and exponent is -∞, the result is +∞.
If base is +∞ and exponent > 0, the result is +∞.
If base is +∞ and exponent < 0, the result is +0.
If base is -∞ and exponent > 0 and exponent is an odd integer, the result is -∞.
If base is -∞ and exponent > 0 and exponent is not an odd integer, the result is +∞.
If base is -∞ and exponent < 0 and exponent is an odd integer, the result is -0.
If base is -∞ and exponent < 0 and exponent is not an odd integer, the result is +0.
If base is +0 and exponent > 0, the result is +0.
If base is +0 and exponent < 0, the result is +∞.
If base is -0 and exponent > 0 and exponent is an odd integer, the result is -0.
If base is -0 and exponent > 0 and exponent is not an odd integer, the result is +0.
If base is -0 and exponent < 0 and exponent is an odd integer, the result is -∞.
If base is -0 and exponent < 0 and exponent is not an odd integer, the result is +∞.
If base < 0 and base is finite and exponent is finite and exponent is not an integer, the result is NaN.
Note
The result of base**exponent when base is 1 or -1 and exponent is +Infinity or -Infinity differs from IEEE 754-2008. The first edition of ECMAScript specified a result of NaN for this operation, whereas later versions of IEEE 754-2008 specified 1. The historical ECMAScript behaviour is preserved for compatibility reasons.
Return the result of applying the addition operation to lnum and rnum. See the Note below 12.8.5.
Note 1
No hint is provided in the calls to ToPrimitive in steps 5 and 6. All standard objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Exotic objects may handle the absence of a hint in some other manner.
Note 2
Step 7 differs from step 5 of the Abstract Relational Comparison algorithm, by using the logical-or operation instead of the logical-and operation.
The + operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The - operator performs subtraction, producing the difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is NaN, the result is NaN.
The sum of two infinities of opposite sign is NaN.
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeroes is -0. The sum of two positive zeroes, or of two zeroes of opposite sign, is +0.
The sum of a zero and a nonzero finite value is equal to the nonzero operand.
The sum of two nonzero finite values of the same magnitude and opposite sign is +0.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.
Note
The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands a and b, it is always the case that a-b produces the same result as a+(-b).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator(O, C) implements the generic algorithm for determining if an object O inherits from the inheritance path defined by constructor C. This abstract operation performs the following steps:
If Type(C) is not Object, throw a TypeError exception.
Let instOfHandler be ? GetMethod(C, @@hasInstance).
Steps 5 and 6 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If a function object does not define or inherit @@hasInstance it uses the default instanceof semantics.
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
If r is true, return false. Otherwise, return true.
Note 1
Given the above definition of equality:
String comparison can be forced by: "" + a == "" + b.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:
new String("a") == "a" and "a" == new String("a") are both true.
new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.
The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.
The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.
When an assignment occurs within strict mode code, it is an runtime error if lref in step 1.f of the first algorithm or step 7 of the second algorithm it is an unresolvable reference. If it is, a ReferenceError exception is thrown. The LeftHandSideExpression also may not be a reference to a data property with the attribute value {[[Writable]]: false}, to an accessor property with the attribute value {[[Set]]: undefined}, nor to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.
Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.
The value of a StatementList is the value of the last value producing item in the StatementList. For example, the following calls to the eval function all return the value 1:
When a Block or CaseBlock production is evaluated a new declarative Environment Record is created and bindings for each block scoped variable, constant, function, generator function, or class declared in the block are instantiated in the Environment Record.
BlockDeclarationInstantiation is performed as follows using arguments code and env. code is the grammar production corresponding to the body of the block. env is the Lexical Environment in which bindings are to be created.
If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's object Environment Record, then step 6 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
With parameters value, environment, and propertyName.
Note
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.
undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 9.2.12). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
The abstract operation ForIn/OfHeadEvaluation is called with arguments TDZnames, expr, and iterationKind. The value of iterationKind is either enumerate or iterate.
The abstract operation ForIn/OfBodyEvaluation is called with arguments lhs, stmt, iterator, lhsKind, and labelSet. The value of lhsKind is either assignment, varBinding or lexicalBinding.
Return an Iterator object (25.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
Note
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
let visited = newSet;
for (let key ofReflect.ownKeys(obj)) {
if (typeof key === "string") {
let desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc && !visited.has(key)) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
}
let proto = Reflect.getPrototypeOf(obj)
if (proto === null) return;
for (let protoName of EnumerateObjectProperties(proto)) {
if (!visited.has(protoName)) yield protoName;
}
}
It is a Syntax Error if this production is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement or a SwitchStatement.
A return statement causes a function to cease execution and return a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression.
The with statement adds an object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.
No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.
CaseSelectorEvaluation does not execute the associated StatementList. It simply evaluates the Expression and returns the value, which the CaseBlock algorithm uses to determine which StatementList to start executing.
A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.
The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.
Evaluating the DebuggerStatement production may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.
Various ECMAScript language elements cause the creation of ECMAScript function objects (9.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (9.2.1).
14.1.1Directive Prologues and the Use Strict Directive#
A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the initial StatementListItem or ModuleItem productions of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.
A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.
Note
The ExpressionStatement productions of a Directive Prologue are evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings for ExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive and which does not have a meaning defined by the implementation.
Multiple occurrences of the same BindingIdentifier in a FormalParameterList is only allowed for functions and generator functions which have simple parameter lists and which are not defined in strict mode code.
The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.
14.1.10Static Semantics: IsAnonymousFunctionDefinition ( production )#
The abstract operation IsAnonymousFunctionDefinition determines if its argument is a function definition that does not bind a name. The argument production is the result of parsing an AssignmentExpression or Initializer. The following steps are taken:
If IsFunctionDefinition of production is false, return false.
Let hasName be the result of HasName of production.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
If ContainsExpression of BindingElement is false, return the result of performing IteratorBindingInitialization for BindingElement using iteratorRecord and environment as the arguments.
If ContainsExpression of BindingRestElement is false, return the result of performing IteratorBindingInitialization for BindingRestElement using iteratorRecord and environment as the arguments.
A prototype property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.
Normally, Contains does not look inside most function forms. However, Contains is used to detect new.target, this, and super usage within an ArrowFunction.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 4 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the scope that is captured by the function object of the ArrowFunction.
YieldExpression cannot be used within the FormalParameters of a generator function because any expressions that are part of FormalParameters are evaluated before the resulting generator object is in a resumable state.
Note 3
Abstract operations relating to generator objects are defined in 25.3.3.
Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
If Type(innerResult) is not Object, throw a TypeError exception.
NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
Let constructorInfo be the result of performing DefineMethod for constructor with arguments proto and constructorParent as the optional functionPrototype argument.
Return the result of ClassDefinitionEvaluation of ClassTail with argument undefined.
Note
ClassDeclaration:classClassTail only occurs as part of an ExportDeclaration and the setting of a name property and establishing its binding are handled as part of the evaluation action for that production. See 15.2.3.11.
Return the result of HasProductionInTailPosition of body with argument nonterminal.
Note
Tail Position calls are only defined in strict mode code because of a common non-standard language extension (see 9.2.7) that enables observation of the chain of caller contexts.
nonterminal is a parsed grammar production that represents a specific range of source text. When the following algorithms compare nonterminal to other grammar symbols they are testing whether the same source text was matched by both symbols.
A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. Function calls cannot return reference values, so such a GetValue operation will always returns the same value as the actual function call result.
Assert: leafContext has no further use. It will never be activated as the running execution context.
A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.
It is a Syntax Error if StatementList Contains super unless the source code containing super is eval code that is being processed by a direct eval that is contained in function code that is not the function code of an ArrowFunction.
It is a Syntax Error if StatementList Contains NewTarget unless the source code containing NewTarget is eval code that is being processed by a direct eval that is contained in function code that is not the function code of an ArrowFunction.
It is a Syntax Error if ContainsDuplicateLabels of StatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of StatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of StatementList with arguments « » and « » is true.
The abstract operation ParseScript with arguments sourceText, realm, and hostDefined creates a Script Record based upon the result of parsing sourceText as a Script. ParseScript performs the following steps:
Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Script as the goal symbol and analyze the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError or ReferenceError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation dependent, but at least one must be present.
An implementation may parse script source text and analyze it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.
When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.
GlobalDeclarationInstantiation is performed as follows using arguments script and env. script is the ScriptBody for which the execution context is being established. env is the global lexical environment in which bindings are to be created.
If vn is not an element of declaredFunctionNames, then
Let vnDefinable be ? envRec.CanDeclareGlobalVar(vn).
If vnDefinable is false, throw a TypeError exception.
If vn is not an element of declaredVarNames, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex B.3.3.2 adds additional steps at this point.
Let lexDeclarations be the LexicallyScopedDeclarations of script.
For each element d in lexDeclarations do
NOTE Lexically declared names are only instantiated here but not initialized.
Early errors specified in 15.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.
It is a Syntax Error if the LexicallyDeclaredNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of ModuleItemList also occurs in the VarDeclaredNames of ModuleItemList.
It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of ModuleItemList does not also occur in either the VarDeclaredNames of ModuleItemList, or the LexicallyDeclaredNames of ModuleItemList.
It is a Syntax Error if ContainsDuplicateLabels of ModuleItemList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of ModuleItemList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of ModuleItemList with arguments « » and « » is true.
Note
The duplicate ExportedNames rule implies that multiple export defaultExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.
The abstract operation ImportedLocalNames with argument importEntries creates a List of all of the local name bindings defined by a List of ImportEntry Records (see Table 40). ImportedLocalNames performs the following steps:
A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.
For specification purposes Module Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with concrete subclasses. This specification only defines a single Module Record concrete subclass named Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.
Module Record defines the fields listed in Table 37. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in Table 38. All Module definition subclasses must provide concrete implementations of these abstract methods.
Return the binding of a name exported by this module. Bindings are represented by a Record of the form {[[Module]]: Module Record, [[BindingName]]: String}
ModuleDeclarationInstantiation()
Transitively resolve all module dependencies and create a module Environment Record for the module.
ModuleEvaluation()
Do nothing if this module has already been evaluated. Otherwise, transitively evaluate all module dependences of this module and then evaluate this module.
ModuleDeclarationInstantiation must be completed prior to invoking this method.
A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (10) that was parsed using the goal symbol Module. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, instantiate, and evaluate the module.
In addition to the fields, defined in Table 37, Source Text Module Records have the additional fields listed in Table 39. Each of these fields initially has the value undefined.
Table 39: Additional Fields of Source Text Module Records
Field Name
Value Type
Meaning
[[ECMAScriptCode]]
a parse result
The result of parsing the source text of this module using Module as the goal symbol.
A List of all the ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The List is source code occurrence ordered.
A List of ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module.
An ImportEntry Record is a Record that digests information about a single declarative import. Each ImportEntry Record has the fields defined in Table 40:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value "*" indicates that the import request is for the target module's namespace object.
[[LocalName]]
String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table 41 gives examples of ImportEntry records fields used to represent the syntactic import forms:
Table 41 (Informative): Import Forms Mappings to ImportEntry Records
An ExportEntry Record is a Record that digests information about a single declarative export. Each ExportEntry Record has the fields defined in Table 42:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have a ModuleSpecifier. "*" indicates that the export request is for all exported bindings.
[[LocalName]]
String | null
The name that is used to locally access the exported value from within the importing module. null if the exported value is not locally accessible from within the module.
Note 2
Table 43 gives examples of the ExportEntry record fields used to represent the syntactic export forms:
Table 43 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form
[[ExportName]]
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
export var v;
"v"
null
null
"v"
export default function f(){}
"default"
null
null
"f"
export default function(){}
"default"
null
null
"*default*"
export default 42;
"default"
null
null
"*default*"
export {x};
"x"
null
null
"x"
export {v as x};
"x"
null
null
"v"
export {x} from "mod";
"x"
"mod"
"x"
null
export {v as x} from "mod";
"x"
"mod"
"v"
null
export * from "mod";
null
"mod"
"*"
null
The following definitions specify the required concrete methods and other abstract operations for Source Text Module Records
The abstract operation ParseModule with arguments sourceText, realm, and hostDefined creates a Source Text Module Record based upon the result of parsing sourceText as a Module. ParseModule performs the following steps:
Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Module as the goal symbol and analyze the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError or ReferenceError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation dependent, but at least one must be present.
If ee.[[LocalName]] is not an element of importedBoundNames, then
Append ee to localExportEntries.
Else,
Let ie be the element of importEntries whose [[LocalName]] is the same as ee.[[LocalName]].
If ie.[[ImportName]] is "*", then
Assert: this is a re-export of an imported module namespace object.
Append ee to localExportEntries.
Else, this is a re-export of a single name
Append to indirectExportEntries the Record {[[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]: null, [[ExportName]]: ee.[[ExportName]] }.
An implementation may parse module source text and analyze it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.
The ResolveExport concrete method of a Source Text Module Record with arguments exportName, resolveSet, and exportStarSet performs the following steps:
Let resolution be ? importedModule.ResolveExport(exportName, resolveSet, exportStarSet).
If resolution is "ambiguous", return "ambiguous".
If resolution is not null, then
If starResolution is null, let starResolution be resolution.
Else,
Assert: there is more than one * import that includes the requested name.
If resolution.[[Module]] and starResolution.[[Module]] are not the same Module Record or SameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) is false, return "ambiguous".
Return starResolution.
Note
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the Module Record this method was invoked on or some other module that is imported by that module. The parameter resolveSet is use to detect unresolved circular import/export paths. If a pair consisting of specific Module Record and exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.
If a defining module is found a Record {[[Module]], [[BindingName]]} is returned. This record identifies the resolved binding of the originally requested export. If no definition was found or the request is found to be circular, null is returned. If the request is found to be ambiguous, the string "ambiguous" is returned.
For each String required that is an element of module.[[RequestedModules]] do,
NOTE: Before instantiating a module, all of the modules it requested must be available. An implementation may perform this test at any time prior to this point.
HostResolveImportedModule is an implementation defined abstract operation that provides the concrete Module Record subclass instance that corresponds to the ModuleSpecifier String, specifier, occurring within the context of the module represented by the Module RecordreferencingModule.
The implementation of HostResolveImportedModule must conform to the following requirements:
The normal return value must be an instance of a concrete subclass of Module Record.
If a Module Record corresponding to the pair referencingModule, specifier does not exist or cannot be created, an exception must be thrown.
This operation must be idempotent if it completes normally. Each time it is called with a specific referencingModule, specifier pair as arguments it must return the same Module Record instance.
Multiple different referencingModule, specifier pairs may map to the same Module Record instance. The actual mapping semantic is implementation defined but typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic case folding and expansion of relative and abbreviated path specifiers.
An implementation may parse a sourceText as a Module, analyze it for Early Error conditions, and instantiate it prior to the execution of the TopLevelModuleEvaluationJob for that sourceText. An implementation may also resolve, pre-parse and pre-analyze, and pre-instantiate module dependencies of sourceText. However, the reporting of any errors detected by these actions must be deferred until the TopLevelModuleEvaluationJob is actually executed.
For each IdentifierNamen in ReferencedBindings of ExportClause: It is a Syntax Error if StringValue of n is a ReservedWord or if the StringValue of n is one of: "implements", "interface", "let", "package", "private", "protected", "public", or "static".
Return a new List containing the Record {[[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
Let sourceName be the StringValue of the first IdentifierName.
Let exportName be the StringValue of the second IdentifierName.
If module is null, then
Let localName be sourceName.
Let importName be null.
Else,
Let localName be null.
Let importName be sourceName.
Return a new List containing the Record {[[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.
It is not necessary to treat export defaultAssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in 16.2, an implementation may extend Script syntax, Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an implementation-defined extension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in 16.2, an implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined behaviour instead of throwing an error (such as ReferenceError).
An implementation may define behaviour other than throwing RangeError for toFixed, toExponential, and toPrecision when the fractionDigits or precision argument is outside the specified range.
HostReportErrors is an implementation-defined abstract operation that allows host environments to report parsing errors, early errors, and runtime errors.
An implementation of HostReportErrors must complete normally in all cases. The default implementation of HostReportErrors is to do nothing.
Note
errorList will be a List of ECMAScript language values. If the errors are parsing errors or early errors, these will always be SyntaxError or ReferenceError objects. Runtime errors, however, can be any ECMAScript value.
An implementation must not extend this specification in the following ways:
Other than as defined in this specification, ECMAScript Function objects defined using syntactic constructors in strict mode code must not be created with own properties named "caller" or "arguments" other than those that are created by applying the AddRestrictedFunctionProperties abstract operation to the function. Such own properties also must not be created for function objects defined using an ArrowFunction, MethodDefinition, GeneratorDeclaration, GeneratorExpression, ClassDeclaration, or ClassExpression regardless of whether the definition is contained in strict mode code. Built-in functions, strict mode functions created using the Function constructor, generator functions created using the Generator constructor, and functions created using the bind method also must not be created with such own properties.
If an implementation extends non-strict or built-in function objects with an own property named "caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function object. If it is an accessor property, the function that is the value of the property's [[Get]] attribute must never return a strict function when called.
The behaviour of the following methods must not be extended except as specified in ECMA-402: Object.prototype.toLocaleString, Array.prototype.toLocaleString, Number.prototype.toLocaleString, Date.prototype.toLocaleDateString, Date.prototype.toLocaleString, Date.prototype.toLocaleTimeString, String.prototype.localeCompare, %TypedArray%.prototype.toLocaleString.
The RegExp pattern grammars in 21.2.1 and B.1.4 must not be extended to recognize any of the source characters A-Z or a-z as IdentityEscape[U] when the U grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediate follow source text that matches the BindingIdentifier nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the lexical environment of the executing program. Others are accessible as initial properties of the global object or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in Function object with the characteristics described in 9.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value true. Every built-in Function object has a [[Realm]] internal slot whose value is the Realm Record of the realm for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “this value” and “NewTarget” have the meanings given in 9.3.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (19.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (19.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.
Unless otherwise specified, each built-in function defined in this specification is created as if by calling the CreateBuiltinFunction abstract operation (9.3.3).
Every built-in Function object, including constructors, has a length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description. Optional parameters (which are indicated with brackets: []) or rest parameters (which are shown using the form «...name») are not included in the default argument count.
Note 2
For example, the function object that is the initial value of the map property of the Array prototype object is described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the length property of that Function object is 1.
Unless otherwise specified, the length property of a built-in Function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every built-in Function object, including constructors, that is not identified as an anonymous function has a name property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. For functions that are specified as properties of objects, the name value is the property name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have "get " or "set " prepended to the property name string. The value of the name property is explicitly specified for each built-in functions whose property key is a Symbol value.
Unless otherwise specified, the name property of a built-in Function object, if it exists, has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every other data property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.
Every accessor property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value, undefined. If only a set accessor is described the get accessor is the default value, undefined.
The global object does not have a [[Construct]] internal method; it is not possible to use the global object as a constructor with the new operator.
The global object does not have a [[Call]] internal method; it is not possible to invoke the global object as a function.
The value of the [[Prototype]] internal slot of the global object is implementation-dependent.
In addition to the properties defined in this specification the global object may have additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.
The value of undefined is undefined (see 6.1.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Let script be the ECMAScript code that is the result of parsing x, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, for the goal symbol Script. If the parse fails, throw a SyntaxError exception. If any early errors are detected, throw a SyntaxError or a ReferenceError exception, depending on the type of the error (but see also clause 16). Parsing and early error detection may be interweaved in an implementation dependent manner.
If script Contains ScriptBody is false, return undefined.
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if the calling context is evaluating formal parameter initializers or if either the code of the calling context or the eval code is strict code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.
Let vnDefinable be ? varEnvRec.CanDeclareGlobalVar(vn).
If vnDefinable is false, throw a TypeError exception.
If vn is not an element of declaredVarNames, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step unless varEnvRec is a global Environment Record and the global object is a Proxy exotic object.
Let lexDeclarations be the LexicallyScopedDeclarations of body.
For each element d in lexDeclarations do
NOTE Lexically declared names are only instantiated here but not initialized.
The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:
A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The result will be true if and only if X is a NaN.
The parseFloat function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.
The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:
Let trimmedString be a substring of inputString consisting of the leftmost code unit that is not a StrWhiteSpaceChar and all code units to the right of that code unit. (In other words, remove leading white space.) If inputString does not contain any such code units, let trimmedString be the empty string.
If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see 7.1.3.1), return NaN.
Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a StrDecimalLiteral.
Let mathFloat be MV of numberString.
If mathFloat=0, then
If the first code unit of trimmedString is "-", return -0.
Return +0.
Return the Number value for mathFloat.
Note
parseFloat may interpret only a leading portion of string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of an decimal literal, and no indication is given that any such code units were ignored.
The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the code unit pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.
The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:
Let S be a newly created substring of inputString consisting of the first code unit that is not a StrWhiteSpaceChar and all code units following that code unit. (In other words, remove leading white space.) If inputString does not contain any such code unit, let S be the empty string.
Let sign be 1.
If S is not empty and the first code unit of S is 0x002D (HYPHEN-MINUS), let sign be -1.
If S is not empty and the first code unit of S is 0x002B (PLUS SIGN) or 0x002D (HYPHEN-MINUS), remove the first code unit from S.
If the length of S is at least 2 and the first two code units of S are either "0x" or "0X", remove the first two code units from S and let R be 16.
If S contains a code unit that is not a radix-R digit, let Z be the substring of S consisting of all code units before the first such code unit; otherwise, let Z be S.
If Z is empty, return NaN.
Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
If mathInt = 0, then
If sign = -1, return -0.
Return +0.
Let number be the Number value for mathInt.
Return sign × number.
Note
parseInt may interpret only a leading portion of string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an integer, and no indication is given that any such code units were ignored.
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 18.2.6.2, 18.2.6.3, 18.2.6.4 and 18.2.6.5
Note
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.
A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme:First/Second;Third?Fourth
where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form "%xx".
The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.
Let strLen be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat
If k equals strLen, return R.
Let C be the code unit at index k within string.
If C is not "%", then
Let S be the String containing only the code unit C.
Else C is "%",
Let start be k.
If k + 2 is greater than or equal to strLen, throw a URIError exception.
If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
Increment k by 2.
If the most significant bit in B is 0, then
Let C be the code unit with code unit value B.
If C is not in reservedSet, then
Let S be the String containing only the code unit C.
Else C is in reservedSet,
Let S be the substring of string from index start to index k inclusive.
Else the most significant bit in B is 1,
Let n be the smallest nonnegative integer such that (B << n) & 0x80 is equal to 0.
If n equals 1 or n is greater than 4, throw a URIError exception.
Let Octets be an array of 8-bit integers of size n.
Put B into Octets at index 0.
If k + (3 × (n - 1)) is greater than or equal to strLen, throw a URIError exception.
Let j be 1.
Repeat, while j < n
Increment k by 1.
If the code unit at index k within string is not "%", throw a URIError exception.
If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
If the two most significant bits in B are not 10, throw a URIError exception.
Increment k by 2.
Put B into Octets at index j.
Increment j by 1.
Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an array of octets into a 21-bit value. If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a URIError exception.
If V < 0x10000, then
Let C be the code unit V.
If C is not in reservedSet, then
Let S be the String containing only the code unit C.
Else C is in reservedSet,
Let S be the substring of string from index start to index k inclusive.
Else V ≥ 0x10000,
Let L be (((V - 0x10000) & 0x3FF) + 0xDC00).
Let H be ((((V - 0x10000) >> 10) & 0x3FF) + 0xD800).
Let S be the String containing the two code units H and L.
Let R be a new String value computed by concatenating the previous value of R and S.
Increase k by 1.
Note
This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 44.
Table 44 (Informative): UTF-8 Encodings
Code Unit Value
Representation
1st Octet
2nd Octet
3rd Octet
4th Octet
0x0000 - 0x007F
00000000 0zzzzzzz
0zzzzzzz
0x0080 - 0x07FF
00000yyy yyzzzzzz
110yyyyy
10zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 - 0xDFFF
110110vv vvwwwwxx
followed by
110111yy yyzzzzzz
11110uuu
10uuwwww
10xxyyyy
10zzzzzz
0xD800 - 0xDBFF
not followed by
0xDC00 - 0xDFFF
causes URIError
0xDC00 - 0xDFFF
causes URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in Surrogates, section 3.8, of the Unicode Standard.
The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.
The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.
The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:
The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.
The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:
Let componentString be ? ToString(encodedURIComponent).
The encodeURI function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code points.
The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:
The encodeURIComponent function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.
The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:
The Object constructor is the %Object% intrinsic object and the initial value of the Object property of the global object. When called as a constructor it creates a new ordinary object. When Object is called as a function rather than as a constructor, it performs a type conversion.
The Object constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition.
The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:
19.1.2.3Object.defineProperties ( O, Properties )#
The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:
19.1.2.4Object.defineProperty ( O, P, Attributes )#
The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:
If Type(O) is not Object, throw a TypeError exception.
19.1.2.8.1Runtime Semantics: GetOwnPropertyKeys ( O, Type )#
The abstract operation GetOwnPropertyKeys is called with arguments O and Type where O is an Object and Type is one of the ECMAScript specification types String or Symbol. The following steps are taken:
If an implementation defines a specific order of enumeration for the for-in statement, the same order must be used for the elements of the array returned in step 3.
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.
19.1.3.4Object.prototype.propertyIsEnumerable ( V )#
When the propertyIsEnumerable method is called with argument V, the following steps are taken:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocalString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.
Note 1
This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and Typed Arrays provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
If Type(tag) is not String, let tag be builtinTag.
Return the String that is the result of concatenating "[object ", tag, and "]".
This function is the %ObjProto_toString% intrinsic object.
Note
Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.
The Function constructor is the %Function% intrinsic object and the initial value of the Function property of the global object. When Function is called as a function rather than as a constructor, it creates and initializes a new Function object. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
The Function constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Function constructor to create and initialize a subclass instances with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in Generator Function subclass.
The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction is called with arguments constructor, newTarget, kind, and args. constructor is the constructor function that is performing this action, newTarget is the constructor that new was initially applied to, kind is either "normal" or "generator", and args is a List containing the actual argument values that were passed to constructor. The following steps are taken:
If newTarget is undefined, let newTarget be constructor.
Let parameters be the result of parsing P, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, using parameterGoal as the goal symbol. Throw a SyntaxError exception if the parse fails.
Let body be the result of parsing bodyText, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, using goal as the goal symbol. Throw a SyntaxError exception if the parse fails.
If bodyText is strict mode code, then let strict be true, else let strict be false.
If any static semantics errors are detected for parameters or body, throw a SyntaxError or a ReferenceError exception, depending on the type of the error. If strict is true, the Early Error rules for StrictFormalParameters:FormalParameters are applied. Parsing and early error detection may be interweaved in an implementation dependent manner.
If ContainsUseStrict of body is true and IsSimpleParameterList of parameters is false, throw a SyntaxError exception.
If any element of the BoundNames of parameters also occurs in the LexicallyDeclaredNames of body, throw a SyntaxError exception.
If body Contains SuperCall is true, throw a SyntaxError exception.
If parameters Contains SuperCall is true, throw a SyntaxError exception.
If body Contains SuperProperty is true, throw a SyntaxError exception.
If parameters Contains SuperProperty is true, throw a SyntaxError exception.
If kind is "generator", then
If parameters Contains YieldExpression is true, throw a SyntaxError exception.
If strict is true, then
If BoundNames of parameters contains any duplicate elements, throw a SyntaxError exception.
A prototype property is automatically created for every function created using CreateDynamicFunction, to provide for the possibility that the function will be used as a constructor.
The Function constructor is itself a built-in function object. The value of the [[Prototype]] internal slot of the Function constructor is the intrinsic object %FunctionPrototype%.
The value of the [[Extensible]] internal slot of the Function constructor is true.
The Function constructor has the following properties:
The value of Function.prototype is %FunctionPrototype%, the intrinsic Function prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.2.3Properties of the Function Prototype Object#
The Function prototype object is the intrinsic object %FunctionPrototype%. The Function prototype object is itself a built-in function object. When invoked, it accepts any arguments and returns undefined. It does not have a [[Construct]] internal method so it is not a constructor.
Note
The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The value of the [[Prototype]] internal slot of the Function prototype object is the intrinsic object %ObjectPrototype%. The initial value of the [[Extensible]] internal slot of the Function prototype object is true.
The Function prototype object does not have a prototype property.
The value of the length property of the Function prototype object is 0.
The value of the name property of the Function prototype object is the empty String.
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is an arrow function or a bound function then the thisArg will be ignored by the function [[Call]] in step 5.
If this method was called with more than one argument, then in left to right order, starting with the second argument, append each argument as the last element of argList.
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is an arrow function or a bound function then the thisArg will be ignored by the function [[Call]] in step 5.
When the toString method is called on an object func, the following steps are taken:
If func is a Bound Function exotic object, then
Return an implementation-dependent String source code representation of func. The representation must conform to the rules below. It is implementation dependent whether the representation includes bound function information or information about the target function.
If Type(func) is Object and is either a built-in function object or has an [[ECMAScriptCode]] internal slot, then
Return an implementation-dependent String source code representation of func. The representation must conform to the rules below.
The use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.
If the object was defined using ECMAScript code and the returned string representation is not in the form of a MethodDefinition or GeneratorMethod then the representation must be such that if the string is evaluated, using eval in a lexical context that is equivalent to the lexical context used to create the original object, it will result in a new functionally equivalent object. In that case the returned source code must not mention freely any variables that were not mentioned freely by the original function's source code, even if these “extra” names were originally in scope.
If the implementation cannot produce a source code string that meets these criteria then it must return a string for which eval will throw a SyntaxError exception.
19.2.3.6Function.prototype [ @@hasInstance ] ( V )#
When the @@hasInstance method of an object F is called with value V, the following steps are taken:
The value of the name property of this function is "[Symbol.hasInstance]".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Note
This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specific constructor. An expression such as
v instanceof F
evaluates as
F[@@hasInstance](v)
A constructor function can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.
Every function instance is an ECMAScript function object and has the internal slots listed in Table 27. Function instances created using the Function.prototype.bind method (19.2.3.2) have the internal slots listed in Table 28.
The Function instances have the following properties:
The value of the length property is an integer that indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its length property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
The value of the name property is an String that is descriptive of the function. The name has no semantic significance but is typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript code. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Anonymous functions objects that do not have a contextual name associated with them by this specification do not have a name own property but inherit the name property of %FunctionPrototype%.
Function instances that can be used as a constructor have a prototype property. Whenever such a function instance is created another ordinary object is also created and is the initial value of the function's prototype property. Unless otherwise specified, the value of the prototype property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a constructor.
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Function objects created using Function.prototype.bind, or by evaluating a MethodDefinition (that are not a GeneratorMethod) or an ArrowFunction grammar production do not have a prototype property.
The Boolean constructor is the %Boolean% intrinsic object and the initial value of the Boolean property of the global object. When called as a constructor it creates and initializes a new Boolean object. When Boolean is called as a function rather than as a constructor, it performs a type conversion.
The Boolean constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.
The Boolean prototype object is the intrinsic object %BooleanPrototype%. The Boolean prototype object is an ordinary object. The Boolean prototype is itself a Boolean object; it has a [[BooleanData]] internal slot with the value false.
The value of the [[Prototype]] internal slot of the Boolean prototype object is the intrinsic object %ObjectPrototype%.
Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.
The Symbol constructor is the %Symbol% intrinsic object and the initial value of the Symbol property of the global object. When Symbol is called as a function, it returns a new Symbol value.
The Symbol constructor is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the Symbol constructor will cause an exception.
For each element e of the GlobalSymbolRegistry List,
If SameValue(e.[[Key]], stringKey) is true, return e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistry List.
Return newSymbol.
The GlobalSymbolRegistry is a List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized as a new empty List. Elements of the GlobalSymbolRegistry are Records with the structure defined in Table 45.
The Symbol prototype object is the intrinsic object %SymbolPrototype%. The Symbol prototype object is an ordinary object. It is not a Symbol instance and does not have a [[SymbolData]] internal slot.
The value of the [[Prototype]] internal slot of the Symbol prototype object is the intrinsic object %ObjectPrototype%.
If Type(s) is not Object, throw a TypeError exception.
If s does not have a [[SymbolData]] internal slot, throw a TypeError exception.
Return the value of s's [[SymbolData]] internal slot.
19.4.3.4Symbol.prototype [ @@toPrimitive ] ( hint )#
This function is called by ECMAScript language operators to convert a Symbol object to a primitive value. The allowed values for hint are "default", "number", and "string".
When the @@toPrimitive method is called with argument hint, the following steps are taken:
Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.
The Error constructor is the %Error% intrinsic object and the initial value of the Error property of the global object. When Error is called as a function rather than as a constructor, it creates and initializes a new Error object. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
The Error constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.
The Error prototype object is the intrinsic object %ErrorPrototype%. The Error prototype object is an ordinary object. It is not an Error instance and does not have an [[ErrorData]] internal slot.
The value of the [[Prototype]] internal slot of the Error prototype object is the intrinsic object %ObjectPrototype%.
Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified uses of [[ErrorData]] is to identify Error and NativeError instances as Error objects within Object.prototype.toString.
A new instance of one of the NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in 19.5.6.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 19.5.5. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in the name property of the prototype object, and in the implementation-defined message property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 19.5.5.
When a NativeError constructor is called as a function rather than as a constructor, it creates and initializes a new NativeError object. A call of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
Each NativeError constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeError constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.
The actual value of the string passed in step 2 is either "%EvalErrorPrototype%", "%RangeErrorPrototype%", "%ReferenceErrorPrototype%", "%SyntaxErrorPrototype%", "%TypeErrorPrototype%", or "%URIErrorPrototype%" corresponding to which NativeError constructor is being defined.
19.5.6.2Properties of the NativeError Constructors#
The value of the [[Prototype]] internal slot of a NativeError constructor is the intrinsic object %Error%.
Each NativeError constructor has a name property whose value is the String value `"NativeError"`.
Each NativeError constructor has the following properties:
The initial value of the constructor property of the prototype for a given NativeError constructor is the corresponding intrinsic object %NativeError% (19.5.6.1).
The initial value of the name property of the prototype for a given NativeError constructor is a string consisting of the name of the constructor (the name used instead of NativeError).
NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (19.1.3.6) to identify Error or NativeError instances.
The Number constructor is the %Number% intrinsic object and the initial value of the Number property of the global object. When called as a constructor, it creates and initializes a new Number object. When Number is called as a function rather than as a constructor, it performs a type conversion.
The Number constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.
The value of Number.EPSILON is the difference between 1 and the smallest value greater than 1 that is representable as a Number value, which is approximately 2.2204460492503130808472633361816 x 10-16.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
This function differs from the global isNaN function (18.2.3) in that it does not convert its argument to a Number before determining whether it is NaN.
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10-324.
In the IEEE 754-2008 double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of the Number.parseFloat data property is the same built-in function object that is the value of the parseFloat property of the global object defined in 18.2.4.
The value of the Number.parseInt data property is the same built-in function object that is the value of the parseInt property of the global object defined in 18.2.5.
The Number prototype object is the intrinsic object %NumberPrototype%. The Number prototype object is an ordinary object. The Number prototype is itself a Number object; it has a [[NumberData]] internal slot with the value +0.
The value of the [[Prototype]] internal slot of the Number prototype object is the intrinsic object %ObjectPrototype%.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot that has been initialized to a Number value.
The abstract operation thisNumberValue(value) performs the following steps:
If Type(value) is Object and value has a [[NumberData]] internal slot, then
Assert: value's [[NumberData]] internal slot is a Number value.
Return the value of value's [[NumberData]] internal slot.
Throw a TypeError exception.
The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract operation thisNumberValue with the this value of the method invocation passed as the argument.
Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:
Return the concatenation of the Strings s and "Infinity".
If f < 0 or f > 20, throw a RangeError exception. However, an implementation is permitted to extend the behaviour of toExponential for values of f less than 0 or greater than 20. In this case toExponential would not necessarily throw RangeError for such values.
If x = 0, then
Let m be the String consisting of f+1 occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else x ≠ 0,
If fractionDigits is not undefined, then
Let e and n be integers such that 10f ≤ n < 10f+1 and for which the exact mathematical value of n × 10e-f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e-f is larger.
Else fractionDigits is undefined,
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the Number value for n × 10e-f is x, and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let a be the first element of m, and let b be the remaining f elements of m.
Let m be the concatenation of the three Strings a, ".", and b.
If e = 0, then
Let c be "+".
Let d be "0".
Else,
If e > 0, let c be "+".
Else e ≤ 0,
Let c be "-".
Let e be -e.
Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Let m be the concatenation of the four Strings m, "e", c, and d.
Return the concatenation of the Strings s and m.
If the toExponential method is called with more than one argument, then the behaviour is undefined (see clause 17).
Note
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 10.b.i be used as a guideline:
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the Number value for n × 10e-f is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n × 10e-f is closest in value to x. If there are two such possible values of n, choose the one that is even.
toFixed returns a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.
The following steps are performed:
Let x be ? thisNumberValue(this value).
Let f be ? ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0.)
If f < 0 or f > 20, throw a RangeError exception. However, an implementation is permitted to extend the behaviour of toFixed for values of f less than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such values.
Let n be an integer for which the exact mathematical value of n ÷ 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let k be the number of elements in m.
If k ≤ f, then
Let z be the String consisting of f+1-k occurrences of the code unit 0x0030 (DIGIT ZERO).
Let m be the concatenation of Strings z and m.
Let k be f + 1.
Let a be the first k-f elements of m, and let b be the remaining f elements of m.
Let m be the concatenation of the three Strings a, ".", and b.
Return the concatenation of the Strings s and m.
If the toFixed method is called with more than one argument, then the behaviour is undefined (see clause 17).
Note 2
The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example,
(1000000000000000128).toString() returns "1000000000000000100", while
(1000000000000000128).toFixed(0) returns "1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Produces a String value that represents this Number value formatted according to the conventions of the host environment's current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision-1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, call ToString instead. Specifically, perform the following steps:
Return the String that is the concatenation of s and "Infinity".
If p < 1 or p > 21, throw a RangeError exception. However, an implementation is permitted to extend the behaviour of toPrecision for values of p less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such values.
If x = 0, then
Let m be the String consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else x ≠ 0,
Let e and n be integers such that 10p-1 ≤ n < 10p and for which the exact mathematical value of n × 10e-p+1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e-p+1 is larger.
Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If e < -6 or e ≥ p, then
Assert: e ≠ 0.
Let a be the first element of m, and let b be the remaining p-1 elements of m.
Let m be the concatenation of a, ".", and b.
If e > 0, then
Let c be code unit 0x002B (PLUS SIGN).
Else e < 0,
Let c be code unit 0x002D (HYPHEN-MINUS).
Let e be -e.
Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Return the concatenation of s, m, code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
If e = p-1, return the concatenation of the Strings s and m.
If e ≥ 0, then
Let m be the concatenation of the first e+1 elements of m, the code unit 0x002E (FULL STOP), and the remaining p- (e+1) elements of m.
Else e < 0,
Let m be the String formed by the concatenation of code unit 0x0030 (DIGIT ZERO), code unit 0x002E (FULL STOP), -(e+1) occurrences of code unit 0x0030 (DIGIT ZERO), and the String m.
Return the String that is the concatenation of s and m.
If the toPrecision method is called with more than one argument, then the behaviour is undefined (see clause 17).
The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined the Number 10 is used as the value of radix.
The following steps are performed:
Let x be ? thisNumberValue(this value).
If radix is not present, let radixNumber be 10.
Else if radix is undefined, let radixNumber be 10.
Return the String representation of this Number value using the radix specified by radixNumber. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent, however the algorithm should be a generalization of that specified in 7.1.12.1.
The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.
The Math object is the %Math% intrinsic object and the initial value of the Math property of the global object. The Math object is a single ordinary object.
The value of the [[Prototype]] internal slot of the Math object is the intrinsic object %ObjectPrototype%.
The Math object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use the Math object as a constructor with the new operator. The Math object also does not have a [[Call]] internal method; it is not possible to invoke the Math object as a function.
Note
In this specification, the phrase “the Number value for x” has a technical meaning defined in 6.1.6.
Each of the following Math object functions applies the ToNumber abstract operation to each of its arguments (in left-to-right order if there is more than one). If ToNumber returns an abrupt completion, that Completion Record is immediately returned. Otherwise, the function performs a computation on the resulting Number value(s). The value returned by each function is a Number.
In the function descriptions below, the symbols NaN, -0, +0, -∞ and +∞ refer to the Number values described in 6.1.6.
Note
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log,log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2008 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π.
If either x or y is NaN, the result is NaN.
If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
If y>0 and x is -0, the result is an implementation-dependent approximation to +π/2.
If y is +0 and x>0, the result is +0.
If y is +0 and x is +0, the result is +0.
If y is +0 and x is -0, the result is an implementation-dependent approximation to +π.
If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
If y is -0 and x>0, the result is -0.
If y is -0 and x is +0, the result is -0.
If y is -0 and x is -0, the result is an implementation-dependent approximation to -π.
If y is -0 and x<0, the result is an implementation-dependent approximation to -π.
If y<0 and x is +0, the result is an implementation-dependent approximation to -π/2.
If y<0 and x is -0, the result is an implementation-dependent approximation to -π/2.
If y>0 and y is finite and x is +∞, the result is +0.
If y>0 and y is finite and x is -∞, the result is an implementation-dependent approximation to +π.
If y<0 and y is finite and x is +∞, the result is -0.
If y<0 and y is finite and x is -∞, the result is an implementation-dependent approximation to -π.
If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
If y is -∞ and x is finite, the result is an implementation-dependent approximation to -π/2.
If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
If y is +∞ and x is -∞, the result is an implementation-dependent approximation to +3π/4.
If y is -∞ and x is +∞, the result is an implementation-dependent approximation to -π/4.
If y is -∞ and x is -∞, the result is an implementation-dependent approximation to -3π/4.
Returns the smallest (closest to -∞) Number value that is not less than x and is equal to a mathematical integer. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is less than 0 but greater than -1, the result is -0.
The value of Math.ceil(x) is the same as the value of -Math.floor(-x).
Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).
Returns an implementation-dependent approximation to subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close 0.
Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical integer. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 1, the result is +0.
Note
The value of Math.floor(x) is the same as the value of -Math.ceil(-x).
Math.hypot returns an implementation-dependent approximation of the square root of the sum of squares of its arguments.
If no arguments are passed, the result is +0.
If any argument is +∞, the result is +∞.
If any argument is -∞, the result is +∞.
If no argument is +∞ or -∞, and any argument is NaN, the result is NaN.
If all arguments are either +0 or -0, the result is +0.
Note
Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.
Returns an implementation-dependent approximation to the natural logarithm of 1 + x. The result is computed in a way that is accurate even when the value of x is close to zero.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.
If no arguments are given, the result is -∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the largest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.
If no arguments are given, the result is +∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the smallest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.
Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.
Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.
Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 0.5, the result is +0.
If x is less than 0 but greater than or equal to -0.5, the result is -0.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of Math.floor(x+0.5). When x is -0 or is less than 0 but greater than or equal to -0.5, Math.round(x) returns -0, but Math.floor(x+0.5) returns +0. Math.round(x) may also differ from the value of Math.floor(x+0.5)because of internal rounding when computing x+0.5.
20.3.1Overview of Date Objects and Definitions of Abstract Operations#
The following functions are abstract operations that operate on time values (defined in 20.3.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.
A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called a time value. A time value may also be NaN, indicating that the Date object does not represent a specific instant of time.
Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly -100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.
ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by
All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:
A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time valuet to a date number is defined by:
An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the local standard time. Daylight saving time is not reflected by LocalTZA.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database http://www.iana.org/time-zones/.
An implementation dependent algorithm using best available information on time zones to determine the local daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds. An implementation of ECMAScript is expected to make its best effort to determine the local daylight saving time adjustment.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database http://www.iana.org/time-zones/.
The abstract operation MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:
If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
Let t be h*msPerHour+m*msPerMinute+s*msPerSecond+milli, performing the arithmetic according to IEEE 754-2008 rules (that is, as if using the ECMAScript operators * and +).
The abstract operation MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:
If year is not finite or month is not finite or date is not finite, return NaN.
Find a value t such that YearFromTime(t) is ym and MonthFromTime(t) is mn and DateFromTime(t) is 1; but if this is not possible (because some argument is out of range), return NaN.
The abstract operation MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:
If day is not finite or time is not finite, return NaN.
The abstract operation TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:
The point of step 4 is that an implementation is permitted a choice of internal representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal representation may or may not distinguish -0 and +0.
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the fields are as follows:
YYYY
is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year from 01 (January) to 12 (December).
DD
is the day of the month from 01 to 31.
T
"T" appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the time zone offset specified as "Z" (for UTC) or either "+" or "-" followed by a time expression HH:mm
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional time zone offset appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
All numbers must be base 10. If the MM or DD fields are absent "01" is used as the value. If the HH, mm, or ss fields are absent "00" is used as the value and the value of an absent sss field is "000". When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies numeric representations of date and time.
ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,426 years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year representation shall have 2 extra year digits and is always prefixed with a + or - sign. The year 0 is considered positive and hence prefixed with a + sign.
The Date constructor is the %Date% intrinsic object and the initial value of the Date property of the global object. When called as a constructor it creates and initializes a new Date object. When Date is called as a function rather than as a constructor, it returns a String representing the current time (UTC).
The Date constructor is a single function whose behaviour is overloaded based upon the number and types of its arguments.
The Date constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
The length property of the Date constructor function is 7.
Let tv be the result of parsing v as a date, in exactly the same manner as for the parse method (20.3.3.2). If the parse resulted in an abrupt completion, tv is the Completion Record.
The parse function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately returned. Otherwise, parse interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the format of the String according to the rules (including extended years) called out in Date Time String Format (20.3.1.16). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognizable Strings or dates containing illegal element values in the format String shall cause Date.parse to return NaN.
If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:
is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (20.3.1.16) and that could not be produced in that implementation by the toString or toUTCString method.
When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent. When the UTC function is called with two to seven arguments, it computes the date from year, month and (optionally) date, hours, minutes, seconds and ms. The following steps are taken:
The UTC function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
The Date prototype object is the intrinsic object %DatePrototype%. The Date prototype object is itself an ordinary object. It is not a Date instance and does not have a [[DateValue]] internal slot.
The value of the [[Prototype]] internal slot of the Date prototype object is the intrinsic object %ObjectPrototype%.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.
The abstract operation thisTimeValue(value) performs the following steps:
If Type(value) is Object and value has a [[DateValue]] internal slot, then
Return the value of value's [[DateValue]] internal slot.
Throw a TypeError exception.
In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the this value for the invocation of the function. If the Type of the this value is not Object, a TypeError exception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operation thisTimeValue with the this value of the method invocation passed as the argument.
Set the [[DateValue]] internal slot of this Date object to u.
Return u.
The length property of the setFullYear method is 3.
Note
If month is not specified, this method behaves as if month were specified with the value getMonth(). If date is not specified, it behaves as if date were specified with the value getDate().
20.3.4.22Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )#
Set the [[DateValue]] internal slot of this Date object to u.
Return u.
The length property of the setHours method is 4.
Note
If min is not specified, this method behaves as if min were specified with the value getMinutes(). If sec is not specified, it behaves as if sec were specified with the value getSeconds(). If ms is not specified, it behaves as if ms were specified with the value getMilliseconds().
Set the [[DateValue]] internal slot of this Date object to u.
Return u.
The length property of the setMinutes method is 3.
Note
If sec is not specified, this method behaves as if sec were specified with the value getSeconds(). If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().
20.3.4.25Date.prototype.setMonth ( month [ , date ] )#
Set the [[DateValue]] internal slot of this Date object to v.
Return v.
The length property of the setUTCFullYear method is 3.
Note
If month is not specified, this method behaves as if month were specified with the value getUTCMonth(). If date is not specified, it behaves as if date were specified with the value getUTCDate().
20.3.4.30Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )#
Set the [[DateValue]] internal slot of this Date object to v.
Return v.
The length property of the setUTCHours method is 4.
Note
If min is not specified, this method behaves as if min were specified with the value getUTCMinutes(). If sec is not specified, it behaves as if sec were specified with the value getUTCSeconds(). If ms is not specified, it behaves as if ms were specified with the value getUTCMilliseconds().
20.3.4.31Date.prototype.setUTCMilliseconds ( ms )#
Set the [[DateValue]] internal slot of this Date object to v.
Return v.
The length property of the setUTCMinutes method is 3.
Note
If sec is not specified, this method behaves as if sec were specified with the value getUTCSeconds(). If ms is not specified, it function behaves as if ms were specified with the value return by getUTCMilliseconds().
20.3.4.33Date.prototype.setUTCMonth ( month [ , date ] )#
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form.
This function returns a String value representing the instance in time corresponding to this time value. The format of the String is the Date Time string format defined in 20.3.1.16. All fields are present in the String. The time zone is always UTC, denoted by the suffix Z. If this time value is not a finite Number or if the year is not a value that can be represented in that format (if necessary using extended year format), a RangeError exception is thrown.
The toJSON function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
For any Date object d whose milliseconds amount is zero, the result of Date.parse(d.toString()) is equal to d.valueOf(). See 20.3.3.2.
Note 2
The toString function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method.
Return an implementation-dependent String value that represents tv as a date and time in the current time zone using a convenient, human-readable form.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent this time value in a convenient, human-readable form in UTC.
Note
The intent is to produce a String representation of a date that is more readable than the format specified in 20.3.1.16. It is not essential that the chosen format be unambiguous or easily machine parsable. If an implementation does not have a preferred human-readable format it is recommended to use the format defined in 20.3.1.16 but with a space rather than a "T" used to separate the date and time elements.
20.3.4.45Date.prototype [ @@toPrimitive ] ( hint )#
This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are "default", "number", and "string". Date objects, are unique among built-in ECMAScript object in that they treat "default" as being equivalent to "string", All other built-in ECMAScript objects treat "default" as being equivalent to "number".
When the @@toPrimitive method is called with argument hint, the following steps are taken:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If hint is the String value "string" or the String value "default", then
Let tryFirst be "string".
Else if hint is the String value "number", then
Let tryFirst be "number".
Else, throw a TypeError exception.
Return ? OrdinaryToPrimitive(O, tryFirst).
The value of the name property of this function is "[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is the time value represented by this Date object.
The String constructor is the %String% intrinsic object and the initial value of the String property of the global object. When called as a constructor it creates and initializes a new String object. When String is called as a function rather than as a constructor, it performs a type conversion.
The String constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.
The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the Listsubstitutions. The following steps are taken:
Let substitutions be a List consisting of all of the arguments passed to this function, starting with the second argument. If fewer than two arguments were passed, the List is empty.
Let numberOfSubstitutions be the number of elements in substitutions.
Append in order the code unit elements of nextSeg to the end of stringElements.
If nextIndex + 1 = literalSegments, then
Return the String value whose code units are, in order, the elements in the ListstringElements. If stringElements has no elements, the empty string is returned.
If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
Append in order the code unit elements of nextSub to the end of stringElements.
Let nextIndex be nextIndex + 1.
Note
String.raw is intended for use as a tag function of a Tagged Template (12.3.7). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.
The String prototype object is the intrinsic object %StringPrototype%. The String prototype object is an ordinary object. The String prototype is itself a String object; it has a [[StringData]] internal slot with the value "".
The value of the [[Prototype]] internal slot of the String prototype object is the intrinsic object %ObjectPrototype%.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the this value passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.
The abstract operation thisStringValue(value) performs the following steps:
If Type(value) is Object and value has a [[StringData]] internal slot, then
Assert: value's [[StringData]] internal slot is a String value.
Return the value of value's [[StringData]] internal slot.
Throw a TypeError exception.
The phrase “this String value” within the specification of a method refers to the result returned by calling the abstract operation thisStringValue with the this value of the method invocation passed as the argument.
Returns a single element String containing the code unit at index pos in the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.
If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos+1).
When the charAt method is called with one argument pos, the following steps are taken:
If position < 0 or position ≥ size, return the empty String.
Return a String of length 1, containing one code unit from S, namely the code unit at index position.
Note 2
The charAt function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Returns a Number (a nonnegative integer less than 216) that is the code unit value of the string element at index pos in the String resulting from converting this object to a String. If there is no element at that index, the result is NaN.
When the charCodeAt method is called with one argument pos, the following steps are taken:
Return a value of Number type, whose value is the code unit value of the element at index position in the String S.
Note 2
The charCodeAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns a nonnegative integer Number less than 1114112 (0x110000) that is the code point value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos in the String resulting from converting this object to a String. If there is no element at that index, the result is undefined. If a valid UTF-16 surrogate pair does not begin at pos, the result is the code unit at pos.
When the codePointAt method is called with one argument pos, the following steps are taken:
The codePointAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
When the concat method is called it returns a String consisting of the code units of the this object (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.
When the concat method is called with zero or more arguments, the following steps are taken:
Let R be the String value consisting of the code units of the previous value of R followed by the code units of nextString.
Return R.
The length property of the concat method is 1.
Note 2
The concat function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Let searchLength be the number of elements in searchStr.
Let start be end - searchLength.
If start is less than 0, return false.
If the sequence of elements of S starting at start of length searchLength is the same as the full element sequence of searchStr, return true.
Otherwise, return false.
Note 1
Returns true if the sequence of elements of searchString converted to a String is the same as the corresponding elements of this object (converted to a String) starting at endPosition - length(this). Otherwise returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The endsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.7String.prototype.includes ( searchString [ , position ] )#
The includes method takes two arguments, searchString and position, and performs the following steps:
Let searchLen be the number of elements in searchStr.
If there exists any integer k not smaller than start such that k + searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j of S is the same as the code unit at index j of searchStr, return true; but if there is no such integer k, return false.
Note 1
If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, return true; otherwise, returns false. If position is undefined, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The includes function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.8String.prototype.indexOf ( searchString [ , position ] )#
Note 1
If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.
The indexOf method takes two arguments, searchString and position, and performs the following steps:
Let searchLen be the number of elements in searchStr.
Return the smallest possible integer k not smaller than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j of S is the same as the code unit at index j of searchStr; but if there is no such integer k, return the value -1.
Note 2
The indexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.9String.prototype.lastIndexOf ( searchString [ , position ] )#
Note 1
If searchString appears as a substring of the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise, -1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.
The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:
Let searchLen be the number of elements in searchStr.
Return the largest possible nonnegative integer k not larger than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j of S is the same as the code unit at index j of searchStr; but if there is no such integer k, return the value -1.
Note 2
The lastIndexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.
When the localeCompare method is called with argument that, it returns a Number other than NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by a host default locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.
Before performing the comparisons, the following steps are performed to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.
The localeCompare method, if considered as a function of two arguments this and that, is a consistent comparison function (as defined in 22.1.3.25) on the set of all Strings.
The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically equivalent according to the Unicode standard as identical and must return 0 when comparing Strings that are considered canonically equivalent.
Note 1
The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.
Note 2
This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment's current locale. However, regardless of the host provided comparison capabilities, this function must treat Strings that are canonically equivalent according to the Unicode standard as identical. It is recommended that this function should not honour Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (http://www.unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (http://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (http://www.unicode.org/reports/tr10/).
Note 3
The localeCompare function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The match function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The normalize function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Let T be a String value that is made from n copies of S appended together. If n is 0, T is the empty String.
Return T.
Note 1
This method creates a String consisting of the code units of the this object (converted to String) repeated count times.
Note 2
The repeat function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Search string for the first occurrence of searchString and let pos be the index within string of the first code unit of the matched substring and let matched be searchString. If no occurrences of searchString were found, return string.
If functionalReplace is true, then
Let replValue be ? Call(replaceValue, undefined, « matched, pos, string »).
Let replStr be GetSubstitution(matched, string, pos, captures, replaceValue).
Let tailPos be pos + the number of code units in matched.
Let newString be the String formed by concatenating the first pos code units of string, replStr, and the trailing substring of string starting at index tailPos. If pos is 0, the first element of the concatenation will be the empty String.
Return newString.
Note
The replace function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let result be a String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified in Table 46. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements.
Return result.
Table 46: Replacement Text Symbol Substitutions
Code units
Unicode Characters
Replacement text
0x0024, 0x0024
$$
$
0x0024, 0x0026
$&
matched
0x0024, 0x0060
$`
If position is 0, the replacement is the empty String. Otherwise the replacement is the substring of str that starts at index 0 and whose last code unit is at index position - 1.
0x0024, 0x0027
$'
If tailPos ≥ stringLength, the replacement is the empty String. Otherwise the replacement is the substring of str that starts at index tailPos and continues to the end of str.
0x0024, N
Where
0x0031 ≤ N ≤ 0x0039
$n where
n is one of 1 2 3 4 5 6 7 8 9 and $n is not followed by a decimal digit
The nth element of captures, where n is a single digit in the range 1 to 9. If n≤m and the nth element of captures is undefined, use the empty String instead. If n>m, the result is implementation-defined.
0x0024, N, N
Where
0x0030 ≤ N ≤ 0x0039
$nn where
n is one of 0 1 2 3 4 5 6 7 8 9
The nnth element of captures, where nn is a two-digit decimal number in the range 01 to 99. If nn≤m and the nnth element of captures is undefined, use the empty String instead. If nn is 00 or nn>m, the result is implementation-defined.
0x0024
$ in any context that does not match any of the above.
The search function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength+end where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:
Return a String value containing span consecutive elements from S beginning with the element at index from.
Note
The slice function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as an RegExp, that has a @@split method.
When the split method is called, the following steps are taken:
The value of separator may be an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.
If the this object is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.
Note 2
The split function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.17.1Runtime Semantics: SplitMatch ( S, q, R )#
The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String R, and performs the following steps in order to return either false or the end index of a match:
If there exists an integer i between 0 (inclusive) and r (exclusive) such that the code unit at index q+i of S is different from the code unit at index i of R, return false.
Return q+r.
21.1.3.18String.prototype.startsWith ( searchString [ , position ] )#
Let searchLength be the number of elements in searchStr.
If searchLength+start is greater than len, return false.
If the sequence of elements of S starting at start of length searchLength is the same as the full element sequence of searchStr, return true.
Otherwise, return false.
Note 1
This method returns true if the sequence of elements of searchString converted to a String is the same as the corresponding elements of this object (converted to a String) starting at index position. Otherwise returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The startsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.19String.prototype.substring ( start, end )#
The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end is undefined). The result is a String value, not a String object.
If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.
Return a String whose length is to - from, containing code units from S, namely the code units with indices from through to - 1, in ascending order.
Note
The substring function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let cpList be a List containing in order the code points as defined in 6.1.4 of S, starting at the first element of S.
For each code point c in cpList, if the Unicode Character Database provides a language insensitive lower case equivalent of c, then replace c in cpList with that equivalent code point(s).
For each code point c in cpList, in order, append to cuList the elements of the UTF16Encoding of c.
Let L be a String whose elements are, in order, the elements of cuList.
Return L.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().
Note 2
The toLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This function behaves in exactly the same way as String.prototype.toLowerCase, except that code points are mapped to their uppercase equivalents as specified in the Unicode Character Database.
Note
The toUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let T be a String value that is a copy of S with both leading and trailing white space removed. The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code point is in Unicode general category “Zs”, code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.
Return T.
Note
The trim function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
When the @@iterator method is called it returns an Iterator object (25.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following steps are taken:
String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot.
String instances have a length property, and a set of enumerable properties with integer indexed names.
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
An String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.
Several methods of String objects return Iterator objects. The abstract operation CreateStringIterator with argument string is used to create such iterator objects. It performs the following steps:
All String Iterator Objects inherit properties from the %StringIteratorPrototype% intrinsic object. The %StringIteratorPrototype% object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %StringIteratorPrototype% has the following properties:
String Iterator instances are ordinary objects that inherit properties from the %StringIteratorPrototype% intrinsic object. String Iterator instances are initially created with the internal slots listed in Table 47.
Table 47: Internal Slots of String Iterator Instances
Internal Slot
Description
[[IteratedString]]
The String value whose elements are being iterated.
[[StringIteratorNextIndex]]
The integer index of the next string index to be examined by this iteration.
The RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion of Pattern.
A regular expression pattern is converted into an internal procedure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The internal procedure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
A Pattern is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a "u". A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of Pattern is defined as if the source code for the Pattern was a List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character) List consisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two element List consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern UTF16Decode must be used in producing a List consisting of a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.
The descriptions below use the following variables:
Input is a List consisting of all of the characters, in order, of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the nth character of Input, where n can range between 0 (inclusive) and InputLength (exclusive).
InputLength is the number of characters in Input.
NcapturingParens is the total number of left capturing parentheses (i.e. the total number of times the Atom::(Disjunction) production is expanded) in the pattern. A left capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom::(Disjunction) production.
IgnoreCase is true if the RegExp object's [[OriginalFlags]] internal slot contains "i" and otherwise is false.
Multiline is true if the RegExp object's [[OriginalFlags]] internal slot contains "m" and otherwise is false.
Unicode is true if the RegExp object's [[OriginalFlags]] internal slot contains "u" and otherwise is false.
Furthermore, the descriptions below use the following internal data structures:
A CharSet is a mathematical set of characters, either code units or code points depending up the state of the Unicode flag. “All characters” means either all code unit values or all code point values also depending upon the state if Unicode.
A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is a List of NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either a List that represents the value obtained by the nth set of capturing parentheses or undefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a State or the special token failure that indicates that the match failed.
A Continuation procedure is an internal closure (i.e. an internal procedure with some arguments already bound to values) that takes one State argument and returns a MatchResult result. If an internal closure references variables which are bound in the function that creates the closure, the closure uses the values that these variables had at the time the closure was created. The Continuation attempts to match the remaining portion (specified by the closure's already-bound arguments) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returns failure.
A Matcher procedure is an internal closure that takes two arguments — a State and a Continuation — and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's already-bound arguments) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
An AssertionTester procedure is an internal closure that takes a State argument and returns a Boolean result. The assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current place in Input and returns true if the condition matched or false if not.
An EscapeValue is either a character or an integer. An EscapeValue is used to denote the interpretation of a DecimalEscape escape sequence: a character ch means that the escape sequence is interpreted as the character ch, while an integer n means that the escape sequence is interpreted as a backreference to the nth set of capturing parentheses.
Return an internal closure that takes two arguments, a String str and an integer index, and performs the following steps:
Assert: index ≤ the number of elements in str.
If Unicode is true, let Input be a List consisting of the sequence of code points of str interpreted as a UTF-16 encoded (6.1.4) Unicode string. Otherwise, let Input be a List consisting of the sequence of code units that are the elements of str. Input will be used throughout the algorithms in 21.2.2. Each element of Input is considered to be a character.
Let InputLength be the number of characters contained in Input. This variable will be used throughout the algorithms in 21.2.2.
Let listIndex be the index into Input of the character that was obtained from element index of str.
Let c be a Continuation that always returns its State argument as a successful MatchResult.
Let cap be a List of NcapturingParensundefined values, indexed 1 through NcapturingParens.
Let x be the State (listIndex, cap).
Call m(x, c) and return its result.
Note
A Pattern evaluates (“compiles”) to an internal procedure value. RegExpBuiltinExec can then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 21.2.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting internal procedure to find a match in a String cannot throw an exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Call m1(x, c) and let r be its result.
If r is not failure, return r.
Call m2(x, c) and return its result.
Note
The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of Strings. Thus, for example,
The production Alternative::[empty] evaluates by returning a Matcher that takes two arguments, a State x and a Continuation c, and returns the result of calling c(x).
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Create a Continuation d that takes a State argument y and returns the result of calling m2(y, c).
Call m1(x, d) and return its result.
Note
Consecutive Terms try to simultaneously match consecutive portions of Input. If the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative.
The production Term::Assertion evaluates by returning an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Evaluate Assertion to obtain an AssertionTester t.
Call t(x) and let r be the resulting Boolean value.
Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and Boolean greedy.
If max is finite and less than min, throw a SyntaxError exception.
Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's Term. This is the total number of times the Atom::(Disjunction) production is expanded prior to this production's Term plus the total number of Atom::(Disjunction) productions enclosing this Term.
Let parenCount be the number of left capturing parentheses in the expansion of this production's Atom. This is the total number of Atom::(Disjunction) productions enclosed by this production's Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its result.
The abstract operation RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer (or ∞) max, a Boolean greedy, a State x, a Continuation c, an integer parenIndex, and an integer parenCount, and performs the following steps:
If max is zero, return c(x).
Create an internal Continuation closure d that takes one State argument y and performs the following steps when evaluated:
If min is zero and y's endIndex is equal to x's endIndex, return failure.
If min is zero, let min2 be zero; otherwise let min2 be min-1.
If max is ∞, let max2 be ∞; otherwise let max2 be max-1.
Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its result.
For every integer k that satisfies parenIndex < k and k ≤ parenIndex+parenCount, set cap[k] to undefined.
Let e be x's endIndex.
Let xr be the State (e, cap).
If min is not zero, return m(xr, d).
If greedy is false, then
Call c(x) and let z be its result.
If z is not failure, return z.
Call m(xr, d) and return its result.
Call m(xr, d) and let z be its result.
If z is not failure, return z.
Call c(x) and return its result.
Note 1
An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the Atom can match different input substrings.
Note 2
If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (nth) repetition of Atom are tried before moving on to the next choice in the next-to-last (n-1)st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n-1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 1 of the RepeatMatcher's d closure states that, once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:
The production Assertion::^ evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
If e is zero, return true.
If Multiline is false, return false.
If the character Input[e-1] is one of LineTerminator, return true.
Return false.
Note
Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if Multiline is true) at the beginning of a line.
The production Assertion::$ evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
If e is equal to InputLength, return true.
If Multiline is false, return false.
If the character Input[e] is one of LineTerminator, return true.
Return false.
The production Assertion::\b evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
Call IsWordChar(e-1) and let a be the Boolean result.
Call IsWordChar(e) and let b be the Boolean result.
If a is true and b is false, return true.
If a is false and b is true, return true.
Return false.
The production Assertion::\B evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
Call IsWordChar(e-1) and let a be the Boolean result.
Call IsWordChar(e) and let b be the Boolean result.
Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's initial left parenthesis. This is the total number of times the Atom::(Disjunction) production is expanded prior to this production's Atom plus the total number of Atom::(Disjunction) productions enclosing this Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
Create an internal Continuation closure d that takes one State argument y and performs the following steps:
The abstract operation Canonicalize takes a character parameter ch and performs the following steps:
If IgnoreCase is false, return ch.
If Unicode is true, then
If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
Else, return ch.
Else,
Assert: ch is a UTF-16 code unit.
Let s be the ECMAScript String value consisting of the single code unit ch.
Let u be the same result produced as if by performing the algorithm for String.prototype.toUpperCase using s as the this value.
Assert: u is a String value.
If u does not consist of a single code unit, return ch.
Let cu be u's single code unit element.
If ch's code unit value ≥ 128 and cu's code unit value < 128, return ch.
Return cu.
Note 1
Parentheses of the form (Disjunction) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching internal procedure. To inhibit the capturing behaviour of parentheses, use the form (?:Disjunction) instead.
Note 2
The form (?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
Note 3
The form (?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
Note 4
In case-insignificant matches when Unicode is true, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, "ß" (U+00DF) to "SS". It may however map a code point outside the Basic Latin range to a character within, for example, "ſ" (U+017F) to "s". Such characters are not mapped if Unicode is false. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as /[a-z]/i, but they will match /[a-z]/ui.
If there exists an integer i between 0 (inclusive) and len (exclusive) such that Canonicalize(s[i]) is not the same character value as Canonicalize(Input[e+i]), return failure.
An escape sequence of the form \ followed by a nonzero decimal number n matches the result of the nth set of capturing parentheses (see 0). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one is undefined because it has not captured anything, then the backreference always succeeds.
If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left capturing parentheses in the entire regular expression. \0 represents the <NUL> character and cannot be followed by a decimal digit.
The production CharacterClassEscape::s evaluates by returning the set of characters containing the characters that are on the right-hand side of the WhiteSpace or LineTerminator productions.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.
Note 1
ClassRanges can expand into a single ClassAtom and/or ranges of two ClassAtom separated by dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's character value is greater than the second ClassAtom's character value.
Note 2
Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.
Note 3
A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.
A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes an error.
The RegExp constructor is the %RegExp% intrinsic object and the initial value of the RegExp property of the global object. When RegExp is called as a function rather than as a constructor, it creates and initializes a new RegExp object. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.
The RegExp constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExp constructor to create and initialize subclass instances with the necessary internal slots.
If pattern is supplied using a StringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral are formed.
21.2.3.2Abstract Operations for the RegExp Constructor#
If F contains any code unit other than "g", "i", "m", "u", or "y" or if it contains the same code unit more than once, throw a SyntaxError exception.
If F contains "u", let BMP be false; else let BMP be true.
If BMP is true, then
Parse P using the grammars in 21.2.1 and interpreting each of its 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements. The goal symbol for the parse is Pattern. Throw a SyntaxError exception if P did not conform to the grammar, if any elements of P were not matched by the parse, or if any Early Error conditions exist.
Let patternCharacters be a List whose elements are the code unit elements of P.
Else,
Parse P using the grammars in 21.2.1 and interpreting P as UTF-16 encoded Unicode code points (6.1.4). The goal symbol for the parse is Pattern[U]. Throw a SyntaxError exception if P did not conform to the grammar, if any elements of P were not matched by the parse, or if any Early Error conditions exist.
Let patternCharacters be a List whose elements are the code points resulting from applying UTF-16 decoding to P's sequence of elements.
Set the value of obj's [[OriginalSource]] internal slot to P.
Set the value of obj's [[OriginalFlags]] internal slot to F.
Set obj's [[RegExpMatcher]] internal slot to the internal procedure that evaluates the above parse of P by applying the semantics provided in 21.2.2 using patternCharacters as the pattern's List of SourceCharacter values and F as the flag parameters.
21.2.3.2.4Runtime Semantics: EscapeRegExpPattern ( P, F )#
When the abstract operation EscapeRegExpPattern with arguments P and F is called, the following occurs:
Let S be a String in the form of a Pattern (Pattern[U] if F contains "u") equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not be identical to P; however, the internal procedure that would result from evaluating S as a Pattern (Pattern[U] if F contains "u") must behave identically to the internal procedure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
The code points / or any LineTerminator occurring in the pattern shall be escaped in S as necessary to ensure that the String value formed by concatenating the Strings "/", S, "/", and F can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is "/", then S could be "\/" or "\u002F", among other possibilities, but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be "(?:)".
RegExp[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
RegExp prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
The RegExp prototype object is the intrinsic object %RegExpPrototype%. The RegExp prototype object is an ordinary object. It is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
The value of the [[Prototype]] internal slot of the RegExp prototype object is the intrinsic object %ObjectPrototype%.
Note
The RegExp prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype object.
Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, or null if string did not match.
The String ToString(string) is searched for an occurrence of the regular expression pattern as follows:
Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have a [[RegExpMatcher]] internal slot, throw a TypeError exception.
If a callable exec property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of exec.
21.2.5.2.2Runtime Semantics: RegExpBuiltinExec ( R, S )#
The abstract operation RegExpBuiltinExec with arguments R and S performs the following steps:
e is an index into the Input character list, derived from S, matched by matcher. Let eUTF be the smallest index into S that corresponds to the character at element e of Input. If e is greater than or equal to the length of Input, then eUTF is the number of code units in S.
The value of the name property of this function is "[Symbol.match]".
Note
The @@match property is used by the IsRegExp abstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a @@match property or the existence of such a property whose value does not Boolean coerce to true indicates that the object is not intended to be used as a regular expression object.
Let replacement be GetSubstitution(matched, S, position, captures, replaceValue).
If position ≥ nextSourcePosition, then
NOTE position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
Let accumulatedResult be the String formed by concatenating the code units of the current value of accumulatedResult with the substring of S consisting of the code units from nextSourcePosition (inclusive) up to position (exclusive) and with the code units of replacement.
Let nextSourcePosition be position + matchLength.
If nextSourcePosition ≥ lengthS, return accumulatedResult.
Return the String formed by concatenating the code units of accumulatedResult with the substring of S consisting of the code units from nextSourcePosition (inclusive) up through the final code unit of S (inclusive).
The value of the name property of this function is "[Symbol.replace]".
Returns an Array object into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of the this value regular expression; these occurrences are not part of any substring in the returned array, but serve to divide up the String value.
The this value may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-empty-substring match at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a","b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If the string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,
RegExp instances are ordinary objects that inherit properties from the RegExp prototype object. RegExp instances have internal slots [[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value of the [[RegExpMatcher]] internal slot is an implementation dependent representation of the Pattern of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties source, global, ignoreCase, and multiline. Those properties are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
The value of the lastIndex property specifies the String index at which to start the next match. It is coerced to an integer when used (see 21.2.5.2.2). This property shall have the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Array objects are exotic objects that give special treatment to a certain class of property names. See 9.4.2 for a definition of this special treatment.
The Array constructor is the %Array% intrinsic object and the initial value of the Array property of the global object. When called as a constructor it creates and initializes a new exotic Array object. When Array is called as a function rather than as a constructor, it also creates and initializes a new Array object. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
The Array constructor is a single function whose behaviour is overloaded based upon the number and types of its arguments.
The Array constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Array constructor to initialize subclass instances that are exotic Array objects. However, most of the Array.prototype methods are generic methods that are not dependent upon their this value being an exotic Array object.
The length property of the Array constructor function is 1.
The from function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.
The items argument is assumed to be a well-formed rest argument value.
Note 2
The of function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.
Array[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Array prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
The Array prototype object is the intrinsic object %ArrayPrototype%. The Array prototype object is an Array exotic objects and has the internal methods specified for such objects. It has a length property whose initial value is 0 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
The value of the [[Prototype]] internal slot of the Array prototype object is the intrinsic object %ObjectPrototype%.
Note
The Array prototype object is specified to be an Array exotic object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
When the concat method is called with zero or more arguments, it returns an array containing the array elements of the object followed by the array elements of each argument in order.
Let items be a List whose first element is O and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
Repeat, while items is not empty
Remove the first element from items and let E be the value of the element.
The explicit setting of the length property in step 6 is necessary to ensure that its value is correct in situations where the trailing elements of the result Array are not present.
Note 2
The concat function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.1.1Runtime Semantics: IsConcatSpreadable ( O )#
The abstract operation IsConcatSpreadable with argument O performs the following steps:
The initial value of Array.prototype.constructor is the intrinsic object %Array%.
22.1.3.3Array.prototype.copyWithin (target, start [ , end ] )#
The copyWithin method takes up to three arguments target, start and end.
Note 1
The end argument is optional with the length of the this object as its default value. If target is negative, it is treated as length+target where length is the length of the array. If start is negative, it is treated as length+start. If end is negative, it is treated as length+end.
The copyWithin function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns false. If such an element is found, every immediately returns false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.
When the every method is called with one or two arguments, the following steps are taken:
Let testResult be ToBoolean(? Call(callbackfn, T, « kValue, k, O »)).
If testResult is false, return false.
Increase k by 1.
Return true.
Note 2
The every function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The fill method takes up to three arguments value, start and end.
Note 1
The start and end arguments are optional with default values of 0 and the length of the this object. If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end.
The fill function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.
When the filter method is called with one or two arguments, the following steps are taken:
The filter function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The find method is called with one or two arguments, predicate and thisArg.
Note 1
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. find calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, find immediately returns that element value. Otherwise, find returns undefined.
If a thisArg parameter is provided, it will be used as the this value for each invocation of predicate. If it is not provided, undefined is used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
find does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by find is set before the first call to callbackfn. Elements that are appended to the array after the call to find begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that find visits them.
When the find method is called, the following steps are taken:
Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
If testResult is true, return kValue.
Increase k by 1.
Return undefined.
Note 2
The find function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
predicate should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.
If a thisArg parameter is provided, it will be used as the this value for each invocation of predicate. If it is not provided, undefined is used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by findIndex is set before the first call to callbackfn. Elements that are appended to the array after the call to findIndex begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findIndex visits them.
When the findIndex method is called with one or two arguments, the following steps are taken:
Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
If testResult is true, return k.
Increase k by 1.
Return -1.
Note 2
The findIndex function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
When the forEach method is called with one or two arguments, the following steps are taken:
The forEach function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
includes compares searchElement to the elements of the array, in ascending order, using the SameValueZero algorithm, and if found at any position, returns true; otherwise, false is returned.
The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, false is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.
When the includes method is called, the following steps are taken:
Let n be ? ToInteger(fromIndex). (If fromIndex is undefined, this step produces the value 0.)
If n ≥ 0, then
Let k be n.
Else n < 0,
Let k be len + n.
If k < 0, let k be 0.
Repeat, while k < len
Let elementK be the result of ? Get(O, ! ToString(k)).
If SameValueZero(searchElement, elementK) is true, return true.
Increase k by 1.
Return false.
Note 2
The includes function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
The includes method intentionally differs from the similar indexOf method in two ways. First, it uses the SameValueZero algorithm, instead of Strict Equality Comparison, allowing it to detect NaN array elements. Second, it does not skip missing array elements, instead treating them as undefined.
indexOf compares searchElement to the elements of the array, in ascending order, using the Strict Equality Comparison algorithm, and if found at one or more indices, returns the smallest such index; otherwise, -1 is returned.
The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.
When the indexOf method is called with one or two arguments, the following steps are taken:
The indexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.
The join method takes one argument, separator, and performs the following steps:
If element is undefined or null, let next be the empty String; otherwise, let next be ? ToString(element).
Let R be a String value produced by concatenating S and next.
Increase k by 1.
Return R.
Note 2
The join function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
lastIndexOf compares searchElement to the elements of the array in descending order using the Strict Equality Comparison algorithm, and if found at one or more indices, returns the largest such index; otherwise, -1 is returned.
The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, -1 is returned.
When the lastIndexOf method is called with one or two arguments, the following steps are taken:
The lastIndexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.
When the map method is called with one or two arguments, the following steps are taken:
The map function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The pop function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.
When the push method is called with zero or more arguments, the following steps are taken:
The push function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element present in the array, in ascending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.
When the reduce method is called with one or two arguments, the following steps are taken:
Let accumulator be ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
Increase k by 1.
Return accumulator.
Note 2
The reduce function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element present in the array, in descending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.
When the reduceRight method is called with one or two arguments, the following steps are taken:
Let accumulator be ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
Decrease k by 1.
Return accumulator.
Note 2
The reduceRight function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The reverse function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
The shift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end where length is the length of the array.
The explicit setting of the length property of the result Array in step 11 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting length became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.
Note 3
The slice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.
When the some method is called with one or two arguments, the following steps are taken:
Let testResult be ToBoolean(? Call(callbackfn, T, « kValue, k, O »)).
If testResult is true, return true.
Increase k by 1.
Return false.
Note 2
The some function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not necessarily remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.
Upon entry, the following steps are performed to initialize evaluation of the sort function:
Within this specification of the sort method, an object, obj, is said to be sparse if the following algorithm returns true:
For each integer i in the range 0≤i< len
Let elem be obj.[[GetOwnProperty]](! ToString(i)).
If elem is undefined, return true.
Return false.
The sort order is the ordering, after completion of this function, of the integer indexed property values of obj whose integer indexes are less than len. The result of the sort function is then determined as follows:
If comparefn is not undefined and is not a consistent comparison function for the elements of this array (see below), the sort order is implementation-defined. The sort order is also implementation-defined if comparefn is undefined and SortCompare does not act as a consistent comparison function.
Let proto be obj.[[GetPrototypeOf]](). If proto is not null and there exists an integer j such that all of the conditions below are satisfied then the sort order is implementation-defined:
Any integer index property of obj whose name is a nonnegative integer less than len is a data property whose [[Configurable]] attribute is false.
The sort order is also implementation defined if any of the following conditions are true:
If obj is an exotic object (including Proxy exotic objects) whose behaviour for [[Get]], [[Set]], [[Delete]], and [[GetOwnProperty]] is not the ordinary object implementation of these internal methods.
If any index property of obj whose name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute is false.
If comparefn is undefined and the application of ToString to any value passed as an argument to SortCompare modifies obj or any object on obj's prototype chain.
If comparefn is undefined and all applications of ToString, to any specific value passed as an argument to SortCompare, do not produce the same result.
The following steps are taken:
Perform an implementation-dependent sequence of calls to the [[Get]] and [[Set]] internal methods of obj, to the DeletePropertyOrThrow and HasOwnProperty abstract operation with obj as the first argument, and to SortCompare (described below), such that:
The property key argument for each call to [[Get]], [[Set]], HasOwnProperty, or DeletePropertyOrThrow is the string representation of a nonnegative integer less than len.
The arguments for calls to SortCompare are values returned by a previous call to the [[Get]] internal method, unless the properties accessed by those previous calls did not exist according to HasOwnProperty. If both perspective arguments to SortCompare correspond to non-existent properties, use +0 instead of calling SortCompare. If only the first perspective argument is non-existent use +1. If only the second perspective argument is non-existent use -1.
If any [[Set]] call returns false a TypeError exception is thrown.
If an abrupt completion is returned from any of these operations, it is immediately returned as the value of this function.
Return obj.
Unless the sort order is specified above to be implementation-defined, the returned object must have the following two characteristics:
There must be some mathematical permutation π of the nonnegative integers less than len, such that for every nonnegative integer j less than len, if property old[j] existed, then new[π(j)] is exactly the same value as old[j]. But if property old[j] did not exist, then new[π(j)] does not exist.
Then for all nonnegative integers j and k, each less than len, if SortCompare(old[j], old[k]) < 0 (see SortCompare below), then new[π(j)] < new[π(k)].
Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j after this function has been executed.
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CFb means comparefn(a, b) < 0; a =CFb means comparefn(a, b) = 0 (of either sign); and a >CFb means comparefn(a, b) > 0.
Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <CFb, a =CFb, and a >CFb will be true for a given pair of a and b.
Calling comparefn(a, b) does not modify obj or any object on obj's prototype chain.
a =CFa (reflexivity)
If a =CFb, then b =CFa (symmetry)
If a =CFb and b =CFc, then a =CFc (transitivity of =CF)
If a <CFb and b <CFc, then a <CFc (transitivity of <CF)
If a >CFb and b >CFc, then a >CFc (transitivity of >CF)
Note 1
The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.
Note 2
The sort function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.25.1Runtime Semantics: SortCompare( x, y )#
The SortCompare abstract operation is called with two arguments x and y. It also has access to the comparefn argument passed to the current invocation of the sort method. The following steps are taken:
If x and y are both undefined, return +0.
If x is undefined, return 1.
If y is undefined, return -1.
If the argument comparefn is not undefined, then
Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.
Note 2
Method calls performed by the ToString abstract operations in steps 5 and 7 have the potential to cause SortCompare to not behave as a consistent comparison function.
When the splice method is called with two or more arguments start, deleteCount and zero or more items, the deleteCount elements of the array starting at integer index start are replaced by the arguments items. An Array object containing the deleted elements (if any) is returned.
Let items be a List whose elements are, in left to right order, the portion of the actual argument list starting with the third argument. The list is empty if fewer than three arguments were passed.
Perform ? Set(O, "length", len - actualDeleteCount + itemCount, true).
Return A.
Note 2
The explicit setting of the length property of the result Array in step 19 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting length became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.
Note 3
The splice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for the Array.prototype.toLocaleString method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Let separator be the String value for the list-separator String appropriate for the host environment's current locale (this is derived in an implementation-defined way).
Let R be ? ToString(? Invoke(nextElement, "toLocaleString")).
Let R be a String value produced by concatenating S and R.
Increase k by 1.
Return R.
Note 2
The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.
Note 3
The toLocaleString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The toString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.
When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:
The unshift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Note
The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.
Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from the Array prototype object.
Array instances have a length property, and a set of enumerable properties with array index names.
The length property of an Array instance is a data property whose value is always numerically greater than the name of every configurable own property whose name is an array index.
The length property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Reducing the value of the length property has the side-effect of deleting own array elements whose array index is between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric own property name of an existing non-configurable array indexed property of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric own property name. See 9.4.2.1.
An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a named constructor for Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.
Several methods of Array objects return Iterator objects. The abstract operation CreateArrayIterator with arguments array and kind is used to create such iterator objects. It performs the following steps:
All Array Iterator Objects inherit properties from the %ArrayIteratorPrototype% intrinsic object. The %ArrayIteratorPrototype% object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %ArrayIteratorPrototype% has the following properties:
Array Iterator instances are ordinary objects that inherit properties from the %ArrayIteratorPrototype% intrinsic object. Array Iterator instances are initially created with the internal slots listed in Table 49.
Table 49: Internal Slots of Array Iterator Instances
Internal Slot
Description
[[IteratedObject]]
The object whose array elements are being iterated.
[[ArrayIteratorNextIndex]]
The integer index of the next integer index to be examined by this iteration.
[[ArrayIterationKind]]
A String value that identifies what is returned for each element of the iteration. The possible values are: "key", "value", "key+value".
TypedArray objects present an array-like view of an underlying binary data buffer (24.1). Each element of a TypedArray instance has the same underlying binary scalar data type. There is a distinct TypedArray constructor, listed in Table 50, for each of the nine supported element types. Each constructor in Table 50 has a corresponding distinct prototype object.
In the definitions below, references to TypedArray should be replaced with the appropriate constructor name from the above table. The phrase “the element size in bytes” refers to the value in the Element Size column of the table in the row corresponding to the constructor. The phrase “element Type” refers to the value in the Element Type column for that row.
The %TypedArray% intrinsic object is a constructor function object that all of the TypedArray constructor object inherit from. %TypedArray% and its corresponding prototype object provide common properties that are inherited by all TypedArray constructors and their instances. The %TypedArray% intrinsic does not have a global name or appear as a property of the global object.
The %TypedArray% intrinsic function object acts as the abstract superclass of the various TypedArray constructors. Because it is an abstract class constructor it will throw an error when invoked. The TypeArray constructors do not perform a super call to it.
%TypedArray%[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
%TypedArrayPrototype% methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
22.2.3Properties of the %TypedArrayPrototype% Object#
The value of the [[Prototype]] internal slot of the %TypedArrayPrototype% object is the intrinsic object %ObjectPrototype%. The %TypedArrayPrototype% object is an ordinary object. It does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
%TypedArray%.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
22.2.3.5%TypedArray%.prototype.copyWithin (target, start [ , end ] )#
%TypedArray%.prototype.copyWithin is a distinct function that implements the same algorithm as Array.prototype.copyWithin as defined in 22.1.3.3 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length" and the actual copying of values in step 12 must be performed in a manner that preserves the bit-level encoding of the source data
The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
22.2.3.5.1Runtime Semantics: ValidateTypedArray ( O )#
When called with argument O, the following steps are taken:
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
%TypedArray%.prototype.every is a distinct function that implements the same algorithm as Array.prototype.every as defined in 22.1.3.5 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.fill is a distinct function that implements the same algorithm as Array.prototype.fill as defined in 22.1.3.6 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.find is a distinct function that implements the same algorithm as Array.prototype.find as defined in 22.1.3.8 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to predicate may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.findIndex is a distinct function that implements the same algorithm as Array.prototype.findIndex as defined in 22.1.3.9 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to predicate may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.forEach is a distinct function that implements the same algorithm as Array.prototype.forEach as defined in 22.1.3.10 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.indexOf is a distinct function that implements the same algorithm as Array.prototype.indexOf as defined in 22.1.3.12 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.includes is a distinct function that implements the same algorithm as Array.prototype.includes as defined in 22.1.3.11 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.join is a distinct function that implements the same algorithm as Array.prototype.join as defined in 22.1.3.13 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.lastIndexOf is a distinct function that implements the same algorithm as Array.prototype.lastIndexOf as defined in 22.1.3.15 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.length is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.reduce is a distinct function that implements the same algorithm as Array.prototype.reduce as defined in 22.1.3.19 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reduceRight is a distinct function that implements the same algorithm as Array.prototype.reduceRight as defined in 22.1.3.20 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reverse is a distinct function that implements the same algorithm as Array.prototype.reverse as defined in 22.1.3.21 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
Sets multiple values in this TypedArray, reading the values from the object array. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
Assert: array is any ECMAScript language value other than an Object with a [[TypedArrayName]] internal slot. If it is such an Object, the definition in 22.2.3.23.2 applies.
Let target be the this value.
If Type(target) is not Object, throw a TypeError exception.
If target does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: target has a [[ViewedArrayBuffer]] internal slot.
Sets multiple values in this TypedArray, reading the values from the typedArray argument object. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
Assert: typedArray has a [[TypedArrayName]] internal slot. If it does not, the definition in 22.2.3.23.1 applies.
Let target be the this value.
If Type(target) is not Object, throw a TypeError exception.
If target does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: target has a [[ViewedArrayBuffer]] internal slot.
22.2.3.24%TypedArray%.prototype.slice ( start, end )#
The interpretation and use of the arguments of %TypedArray%.prototype.slice are the same as for Array.prototype.slice as defined in 22.1.3.23. The following steps are taken:
%TypedArray%.prototype.some is a distinct function that implements the same algorithm as Array.prototype.some as defined in 22.1.3.24 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.sort is a distinct function that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in 22.1.3.25. The implementation of the %TypedArray%.prototype.sort specification may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. The only internal methods of the this object that the algorithm may call are [[Get]] and [[Set]].
This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
Upon entry, the following steps are performed to initialize evaluation of the sort function. These steps are used instead of the entry steps in 22.1.3.25:
Let len be the value of obj's [[ArrayLength]] internal slot.
The implementation defined sort order condition for exotic objects is not applied by %TypedArray%.prototype.sort.
The following version of SortCompare is used by %TypedArray%.prototype.sort. It performs a numeric comparison rather than the string comparison used in 22.1.3.25. SortCompare has access to the comparefn and buffer values of the current invocation of the sort method.
When the TypedArray SortCompare abstract operation is called with two arguments x and y, the following steps are taken:
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
If v is NaN, return +0.
Return v.
If x and y are both NaN, return +0.
If x is NaN, return 1.
If y is NaN, return -1.
If x < y, return -1.
If x > y, return 1.
If x is -0 and y is +0, return -1.
If x is +0 and y is -0, return 1.
Return +0.
Note
Because NaN always compares greater than any other value, NaN property values always sort to the end of the result when comparefn is not provided.
22.2.3.27%TypedArray%.prototype.subarray( begin, end )#
Returns a new TypedArray object whose element type is the same as this TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this TypedArray, referencing the elements at begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
Let srcLength be the value of O's [[ArrayLength]] internal slot.
%TypedArray%.prototype.toLocaleString is a distinct function that implements the same algorithm as Array.prototype.toLocaleString as defined in 22.1.3.27 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.
The initial value of the %TypedArray%.prototype.toString data property is the same built-in function object as the Array.prototype.toString method defined in 22.1.3.28.
%TypedArray%.prototype[@@toStringTag] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Each of the TypedArray constructor objects is an intrinsic object that has the structure described below, differing only in the name used as the constructor name instead of TypedArray, in Table 50.
The TypedArray intrinsic constructor functions are single functions whose behaviour is overloaded based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
The TypedArray constructors are not intended to be called as a function and will throw an exception when called in that manner.
The TypedArray constructors are designed to be subclassable. They may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArray constructor to create and initialize the subclass instance with the internal state necessary to support the %TypedArray%.prototype built-in methods.
The length property of the TypedArray constructor function is 3.
The abstract operation AllocateTypedArray with arguments constructorName, newTarget, defaultProto and optional argument length is used to validate and create an instance of a TypedArray constructor. constructorName is required to be the name of a TypedArray constructor in Table 50. If the length argument is passed an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by all of the TypedArray overloads. AllocateTypedArray performs the following steps:
22.2.4.2.2Runtime Semantics: AllocateTypedArrayBuffer ( O, length )#
The abstract operation AllocateTypedArrayBuffer with arguments O and length allocates and associates an ArrayBuffer with the TypedArray instance O. It performs the following steps:
Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
Assert: The [[ViewedArrayBuffer]] internal slot of O is undefined.
Assert: length ≥ 0.
Let constructorName be the String value of O's [[TypedArrayName]] internal slot.
Let elementSize be the Element Size value in Table 50 for constructorName.
This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object has a [[TypedArrayName]] internal slot.
TypedArray called with argument typedArray performs the following steps:
Assert: Type(typedArray) is Object and typedArray has a [[TypedArrayName]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 50 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArrayPrototype%").
Let srcArray be typedArray.
Let srcData be the value of srcArray's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(srcData) is true, throw a TypeError exception.
Let constructorName be the String value of O's [[TypedArrayName]] internal slot.
Let elementType be the String value of the Element Type value in Table 50 for constructorName.
Let elementLength be the value of srcArray's [[ArrayLength]] internal slot.
Let srcName be the String value of srcArray's [[TypedArrayName]] internal slot.
Let srcType be the String value of the Element Type value in Table 50 for srcName.
Let srcElementSize be the Element Size value in Table 50 for srcName.
Let srcByteOffset be the value of srcArray's [[ByteOffset]] internal slot.
Let elementSize be the Element Size value in Table 50 for constructorName.
This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
TypedArray called with argument object performs the following steps:
Assert: Type(object) is Object and object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 50 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArrayPrototype%").
This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object has an [[ArrayBufferData]] internal slot.
TypedArray called with arguments buffer, byteOffset, and length performs the following steps:
Assert: Type(buffer) is Object and buffer has an [[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 50 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArrayPrototype%").
Let constructorName be the String value of O's [[TypedArrayName]] internal slot.
Let elementSize be the Number value of the Element Size value in Table 50 for constructorName.
The abstract operation TypedArrayCreate with arguments constructor and argumentList is used to specify the creation of a new TypedArray object using a constructor function. It performs the following steps:
Let newTypedArray be ? Construct(constructor, argumentList).
The abstract operation TypedArraySpeciesCreate with arguments exemplar and argumentList is used to specify the creation of a new TypedArray object using a constructor function that is derived from exemplar. It performs the following steps:
Assert: exemplar is an Object that has a [[TypedArrayName]] internal slot.
Let defaultConstructor be the intrinsic object listed in column one of Table 50 for the value of exemplar's [[TypedArrayName]] internal slot.
The value of the [[Prototype]] internal slot of a TypedArray prototype object is the intrinsic object %TypedArrayPrototype%. A TypedArray prototype object is an ordinary object. It does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
TypedArray instances are Integer Indexed exotic objects. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using the SameValueZero comparison algorithm.
Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.
The Map constructor is the %Map% intrinsic object and the initial value of the Map property of the global object. When called as a constructor it creates and initializes a new Map object. Map is not intended to be called as a function and will throw an exception when called in that manner.
The Map constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Map constructor to create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.
If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.
Map[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.
The Map prototype object is the intrinsic object %MapPrototype%. The value of the [[Prototype]] internal slot of the Map prototype object is the intrinsic object %ObjectPrototype%. The Map prototype object is an ordinary object. It does not have a [[MapData]] internal slot.
If Type(M) is not Object, throw a TypeError exception.
If M does not have a [[MapData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of M's [[MapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} p that is an element of entries,
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return undefined.
Note
The existing [[MapData]] List is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that List.
If Type(M) is not Object, throw a TypeError exception.
If M does not have a [[MapData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of M's [[MapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} p that is an element of entries,
If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
When the forEach method is called with one or two arguments, the following steps are taken:
Let M be the this value.
If Type(M) is not Object, throw a TypeError exception.
If M does not have a [[MapData]] internal slot, throw a TypeError exception.
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let entries be the List that is the value of M's [[MapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} e that is an element of entries, in original key insertion order
If e.[[Key]] is not empty, then
Perform ? Call(callbackfn, T, « e.[[Value]], e.[[Key]], M »).
Return undefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the map object, in key insertion order. callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named constructor for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.
Several methods of Map objects return Iterator objects. The abstract operation CreateMapIterator with arguments map and kind is used to create such iterator objects. It performs the following steps:
If Type(map) is not Object, throw a TypeError exception.
If map does not have a [[MapData]] internal slot, throw a TypeError exception.
All Map Iterator Objects inherit properties from the %MapIteratorPrototype% intrinsic object. The %MapIteratorPrototype% intrinsic object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %MapIteratorPrototype% has the following properties:
Let entries be the List that is the value of the [[MapData]] internal slot of m.
Repeat while index is less than the total number of elements of entries. The number of elements must be redetermined each time this method is evaluated.
Let e be the Record {[[Key]], [[Value]]} that is the value of entries[index].
Set index to index+1.
Set the [[MapNextIndex]] internal slot of O to index.
If e.[[Key]] is not empty, then
If itemKind is "key", let result be e.[[Key]].
Else if itemKind is "value", let result be e.[[Value]].
Map Iterator instances are ordinary objects that inherit properties from the %MapIteratorPrototype% intrinsic object. Map Iterator instances are initially created with the internal slots described in Table 51.
Table 51: Internal Slots of Map Iterator Instances
Internal Slot
Description
[[Map]]
The Map object that is being iterated.
[[MapNextIndex]]
The integer index of the next Map data element to be examined by this iterator.
[[MapIterationKind]]
A String value that identifies what is to be returned for each element of the iteration. The possible values are: "key", "value", "key+value".
Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using the SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Set objects specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.
The Set constructor is the %Set% intrinsic object and the initial value of the Set property of the global object. When called as a constructor it creates and initializes a new Set object. Set is not intended to be called as a function and will throw an exception when called in that manner.
The Set constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Set constructor to create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.
Set[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.
The Set prototype object is the intrinsic object %SetPrototype%. The value of the [[Prototype]] internal slot of the Set prototype object is the intrinsic object %ObjectPrototype%. The Set prototype object is an ordinary object. It does not have a [[SetData]] internal slot.
If Type(S) is not Object, throw a TypeError exception.
If S does not have a [[SetData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of S's [[SetData]] internal slot.
Repeat for each e that is an element of entries,
Replace the element of entries whose value is e with an element whose value is empty.
Return undefined.
Note
The existing [[SetData]] List is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that List.
If Type(S) is not Object, throw a TypeError exception.
If S does not have a [[SetData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of S's [[SetData]] internal slot.
Repeat for each e that is an element of entries,
If e is not empty and SameValueZero(e, value) is true, then
Replace the element of entries whose value is e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.
The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.
A Set Iterator is an ordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.
Several methods of Set objects return Iterator objects. The abstract operation CreateSetIterator with arguments set and kind is used to create such iterator objects. It performs the following steps:
If Type(set) is not Object, throw a TypeError exception.
If set does not have a [[SetData]] internal slot, throw a TypeError exception.
All Set Iterator Objects inherit properties from the %SetIteratorPrototype% intrinsic object. The %SetIteratorPrototype% intrinsic object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %SetIteratorPrototype% has the following properties:
Let entries be the List that is the value of the [[SetData]] internal slot of s.
Repeat while index is less than the total number of elements of entries. The number of elements must be redetermined each time this method is evaluated.
Let e be entries[index].
Set index to index+1.
Set the [[SetNextIndex]] internal slot of O to index.
Set Iterator instances are ordinary objects that inherit properties from the %SetIteratorPrototype% intrinsic object. Set Iterator instances are initially created with the internal slots specified in Table 52.
Table 52: Internal Slots of Set Iterator Instances
Internal Slot
Description
[[IteratedSet]]
The Set object that is being iterated.
[[SetNextIndex]]
The integer index of the next Set data element to be examined by this iterator
[[SetIterationKind]]
A String value that identifies what is to be returned for each element of the iteration. The possible values are: "key", "value", "key+value". "key" and "value" have the same meaning.
WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. If an object that is being used as the key of a WeakMap key/value pair is only reachable by following a chain of references that start within that WeakMap, then that key/value pair is inaccessible and is automatically removed from the WeakMap. WeakMap implementations must detect and remove such key/value pairs and any associated resources.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended to be a viable implementation model.
Note
WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
The WeakMap constructor is the %WeakMap% intrinsic object and the initial value of the WeakMap property of the global object. When called as a constructor it creates and initializes a new WeakMap object. WeakMap is not intended to be called as a function and will throw an exception when called in that manner.
The WeakMap constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.
If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.
The WeakMap prototype object is the intrinsic object %WeakMapPrototype%. The value of the [[Prototype]] internal slot of the WeakMap prototype object is the intrinsic object %ObjectPrototype%. The WeakMap prototype object is an ordinary object. It does not have a [[WeakMapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} p that is an element of entries,
If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. If an object that is contained by a WeakSet is only reachable by following a chain of references that start within that WeakSet, then that object is inaccessible and is automatically removed from the WeakSet. WeakSet implementations must detect and remove such objects and any associated resources.
An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.
WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable implementation model.
The WeakSet constructor is the %WeakSet% intrinsic object and the initial value of the WeakSet property of the global object. When called as a constructor it creates and initializes a new WeakSet object. WeakSet is not intended to be called as a function and will throw an exception when called in that manner.
The WeakSet constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.
The WeakSet prototype object is the intrinsic object %WeakSetPrototype%. The value of the [[Prototype]] internal slot of the WeakSet prototype object is the intrinsic object %ObjectPrototype%. The WeakSet prototype object is an ordinary object. It does not have a [[WeakSetData]] internal slot.
Let entries be the List that is the value of S's [[WeakSetData]] internal slot.
Repeat for each e that is an element of entries,
If e is not empty and SameValue(e, value) is true, then
Replace the element of entries whose value is e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
The abstract operation AllocateArrayBuffer with arguments constructor and byteLength is used to create an ArrayBuffer object. It performs the following steps:
Let obj be ? OrdinaryCreateFromConstructor(constructor, "%ArrayBufferPrototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScript implementation or host environment may define such operations.
The abstract operation CloneArrayBuffer takes three parameters, an ArrayBuffer srcBuffer, an integer srcByteOffset and optionally a constructor function cloneConstructor. It creates a new ArrayBuffer whose data is a copy of srcBuffer's data starting at srcByteOffset. This operation performs the following steps:
Assert: Type(srcBuffer) is Object and it has an [[ArrayBufferData]] internal slot.
24.1.1.5GetValueFromBuffer ( arrayBuffer, byteIndex, type [ , isLittleEndian ] )#
The abstract operation GetValueFromBuffer takes four parameters, an ArrayBuffer arrayBuffer, an integer byteIndex, a String type, and optionally a Boolean isLittleEndian. This operation performs the following steps:
Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
Assert: byteIndex is an integer value ≥ 0.
Let block be arrayBuffer's [[ArrayBufferData]] internal slot.
Let elementSize be the Number value of the Element Size value specified in Table 50 for Element Type type.
Let rawValue be a List of elementSize containing, in order, the elementSize sequence of bytes starting with block[byteIndex].
If isLittleEndian is not present, set isLittleEndian to either true or false. The choice is implementation dependent and should be the alternative that is most efficient for the implementation. An implementation must use the same value each time this step is executed and the same value must be used for the corresponding step in the SetValueInBuffer abstract operation.
If isLittleEndian is false, reverse the order of the elements of rawValue.
If type is "Float32", then
Let value be the byte elements of rawValue concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary32 value.
If value is an IEEE 754-2008 binary32 NaN value, return the NaN Number value.
Return the Number value that corresponds to value.
If type is "Float64", then
Let value be the byte elements of rawValue concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary64 value.
If value is an IEEE 754-2008 binary64 NaN value, return the NaN Number value.
Return the Number value that corresponds to value.
If the first code unit of type is "U", then
Let intValue be the byte elements of rawValue concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
Let intValue be the byte elements of rawValue concatenated and interpreted as a bit string encoding of a binary little-endian 2's complement number of bit length elementSize × 8.
Return the Number value that corresponds to intValue.
The abstract operation SetValueInBuffer takes five parameters, an ArrayBuffer arrayBuffer, an integer byteIndex, a String type, a Number value, and optionally a Boolean isLittleEndian. This operation performs the following steps:
Let block be arrayBuffer's [[ArrayBufferData]] internal slot.
Assert: block is not undefined.
If isLittleEndian is not present, set isLittleEndian to either true or false. The choice is implementation dependent and should be the alternative that is most efficient for the implementation. An implementation must use the same value each time this step is executed and the same value must be used for the corresponding step in the GetValueFromBuffer abstract operation.
If type is "Float32", then
Set rawBytes to a List containing the 4 bytes that are the result of converting value to IEEE 754-2008 binary32 format using “Round to nearest, ties to even” rounding mode. If isLittleEndian is false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value is NaN, rawValue may be set to any implementation chosen IEEE 754-2008 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else, if type is "Float64", then
Set rawBytes to a List containing the 8 bytes that are the IEEE 754-2008 binary64 format encoding of value. If isLittleEndian is false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value is NaN, rawValue may be set to any implementation chosen IEEE 754-2008 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else,
Let n be the Number value of the Element Size specified in Table 50 for Element Type type.
Let convOp be the abstract operation named in the Conversion Operation column in Table 50 for Element Type type.
Let intValue be convOp(value).
If intValue ≥ 0, then
Let rawBytes be a List containing the n-byte binary encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Else,
Let rawBytes be a List containing the n-byte binary 2's complement encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Store the individual bytes of rawBytes into block, in order, starting at block[byteIndex].
The ArrayBuffer constructor is the %ArrayBuffer% intrinsic object and the initial value of the ArrayBuffer property of the global object. When called as a constructor it creates and initializes a new ArrayBuffer object. ArrayBuffer is not intended to be called as a function and will throw an exception when called in that manner.
The ArrayBuffer constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.
ArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
ArrayBuffer prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
24.1.4Properties of the ArrayBuffer Prototype Object#
The ArrayBuffer prototype object is the intrinsic object %ArrayBufferPrototype%. The value of the [[Prototype]] internal slot of the ArrayBuffer prototype object is the intrinsic object %ObjectPrototype%. The ArrayBuffer prototype object is an ordinary object. It does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have an [[ArrayBufferData]] internal slot, throw a TypeError exception.
ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is null are considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.
24.2.1.1GetViewValue ( view, requestIndex, isLittleEndian, type )#
The abstract operation GetViewValue with arguments view, requestIndex, isLittleEndian, and type is used by functions on DataView instances is to retrieve values from the view's buffer. It performs the following steps:
If Type(view) is not Object, throw a TypeError exception.
If view does not have a [[DataView]] internal slot, throw a TypeError exception.
24.2.1.2SetViewValue ( view, requestIndex, isLittleEndian, type, value )#
The abstract operation SetViewValue with arguments view, requestIndex, isLittleEndian, type, and value is used by functions on DataView instances to store values into the view's buffer. It performs the following steps:
If Type(view) is not Object, throw a TypeError exception.
If view does not have a [[DataView]] internal slot, throw a TypeError exception.
The DataView constructor is the %DataView% intrinsic object and the initial value of the DataView property of the global object. When called as a constructor it creates and initializes a new DataView object. DataView is not intended to be called as a function and will throw an exception when called in that manner.
The DataView constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataView constructor to create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.
The initial value of DataView.prototype is the intrinsic object %DataViewPrototype%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
24.2.4Properties of the DataView Prototype Object#
The DataView prototype object is the intrinsic object %DataViewPrototype%. The value of the [[Prototype]] internal slot of the DataView prototype object is the intrinsic object %ObjectPrototype%. The DataView prototype object is an ordinary object. It does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.
DataView.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let size be the value of O's [[ByteLength]] internal slot.
DataView.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let offset be the value of O's [[ByteOffset]] internal slot.
DataView instances are ordinary objects that inherit properties from the DataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataView constructor.
The JSON object is the %JSON% intrinsic object and the initial value of the JSON property of the global object. The JSON object is a single ordinary object that contains two functions, parse and stringify, that are used to parse and construct JSON texts. The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404.
Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.
The value of the [[Prototype]] internal slot of the JSON object is the intrinsic object %ObjectPrototype%. The value of the [[Extensible]] internal slot of the JSON object is set to true.
The JSON object does not have a [[Construct]] internal method; it is not possible to use the JSON object as a constructor with the new operator.
The JSON object does not have a [[Call]] internal method; it is not possible to invoke the JSON object as a function.
The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format is a subset of the syntax for ECMAScript literals, Array Initializers and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and null.
The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns undefined then the property is deleted from the result.
Parse JText interpreted as UTF-16 encoded Unicode points (6.1.4) as a JSON text as specified in ECMA-404. Throw a SyntaxError exception if JText is not a valid JSON text as defined in that specification.
Let scriptText be the result of concatenating "(", JText, and ");".
Let completion be the result of parsing and evaluating scriptText as if it was the source text of an ECMAScript Script, but using the alternative definition of DoubleStringCharacter provided below. The extended PropertyDefinitionEvaluation semantics defined in B.3.1 must not be used during the evaluation.
Let unfiltered be completion.[[Value]].
Assert: unfiltered will be either a primitive value or an object that is defined by either an ArrayLiteral or an ObjectLiteral.
JSON allows Unicode code units 0x2028 (LINE SEPARATOR) and 0x2029 (PARAGRAPH SEPARATOR) to directly appear in String literals without using an escape sequence. This is enabled by using the following alternative definition of DoubleStringCharacter when parsing scriptText in step 4:
The syntax of a valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Hence a valid JSON text is also a valid PrimaryExpression. Step 2 above verifies that JText conforms to that subset. When scriptText is parsed and evaluated as a Script the result will be either a String, Number, Boolean, or Null primitive value or an Object defined as if by an ArrayLiteral or ObjectLiteral.
The abstract operation InternalizeJSONProperty is a recursive abstract operation that takes two parameters: a holder object and the String name of a property in that object. InternalizeJSONProperty uses the value of reviver that was originally passed to the above parse function.
It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.
Note
In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.
24.3.2JSON.stringify ( value [ , replacer [ , space ] ] )#
The stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value. It can take three parameters. The value parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number or null. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as a white list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.
The length property of the stringify function is 3.
Note 1
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw a TypeError exception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String null.
The undefined value is not rendered.
The true value is rendered in JSON text as the String true.
The false value is rendered in JSON text as the String false.
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String null.
Note 5
Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String null. In objects an unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key or property name, a U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
The abstract operation SerializeJSONProperty with arguments key, and holder has access to ReplacerFunction from the invocation of the stringify method. Its algorithm is as follows:
24.3.2.2Runtime Semantics: QuoteJSONString ( value )#
The abstract operation QuoteJSONString with argument value wraps a String value in QUOTATION MARK code units and escapes certain other code units within it.
Let product be code unit 0x0022 (QUOTATION MARK).
For each code unit C in value
If C is 0x0022 (QUOTATION MARK) or 0x005C (REVERSE SOLIDUS), then
Let product be the concatenation of product and code unit 0x005C (REVERSE SOLIDUS).
Let product be the concatenation of product and C.
Else if C is 0x0008 (BACKSPACE), 0x000C (FORM FEED), 0x000A (LINE FEED), 0x000D (CARRIAGE RETURN), or 0x0009 (CHARACTER TABULATION), then
Let product be the concatenation of product and code unit 0x005C (REVERSE SOLIDUS).
Let abbrev be the String value corresponding to the value of C as follows:
BACKSPACE
"b"
FORM FEED (FF)
"f"
LINE FEED (LF)
"n"
CARRIAGE RETURN (CR)
"r"
CHARACTER TABULATION
"t"
Let product be the concatenation of product and abbrev.
Else if C has a code unit value less than 0x0020 (SPACE), then
Let product be the concatenation of product and code unit 0x005C (REVERSE SOLIDUS).
Let product be the concatenation of product and "u".
Let hex be the string result of converting the numeric code unit value of C to a String of four hexadecimal digits. Alphabetic hexadecimal digits are presented as lowercase Latin letters.
Let product be the concatenation of product and hex.
Else,
Let product be the concatenation of product and C.
Let product be the concatenation of product and code unit 0x0022 (QUOTATION MARK).
Return product.
24.3.2.3Runtime Semantics: SerializeJSONObject ( value )#
The abstract operation SerializeJSONObject with argument value serializes an object. It has access to the stack, indent, gap, and PropertyList values of the current invocation of the stringify method.
If stack contains value, throw a TypeError exception because the structure is cyclical.
Append value to stack.
Let stepback be indent.
Let indent be the concatenation of indent and gap.
Let member be the concatenation of member and the string ":".
If gap is not the empty String, then
Let member be the concatenation of member and code unit 0x0020 (SPACE).
Let member be the concatenation of member and strP.
Append member to partial.
If partial is empty, then
Let final be "{}".
Else,
If gap is the empty String, then
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let final be the result of concatenating "{", properties, and "}".
Else gap is not the empty String
Let separator be the result of concatenating code unit 0x002C (COMMA), code unit 0x000A (LINE FEED), and indent.
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be the result of concatenating "{", code unit 0x000A (LINE FEED), indent, properties, code unit 0x000A (LINE FEED), stepback, and "}".
Remove the last element of stack.
Let indent be stepback.
Return final.
24.3.2.4Runtime Semantics: SerializeJSONArray ( value )#
The abstract operation SerializeJSONArray with argument value serializes an array. It has access to the stack, indent, and gap values of the current invocation of the stringify method.
If stack contains value, throw a TypeError exception because the structure is cyclical.
Append value to stack.
Let stepback be indent.
Let indent be the concatenation of indent and gap.
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let final be the result of concatenating "[", properties, and "]".
Else,
Let separator be the result of concatenating code unit 0x002C (COMMA), code unit 0x000A (LINE FEED), and indent.
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be the result of concatenating "[", code unit 0x000A (LINE FEED), indent, properties, code unit 0x000A (LINE FEED), stepback, and "]".
Remove the last element of stack.
Let indent be stepback.
Return final.
Note
The representation of arrays includes only the elements between zero and array.length - 1 inclusive. Properties whose keys are not array indexes are excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.
An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.
An object that implements the Iterator interface must include the property in Table 54. Such objects may also implement the properties in Table 55.
Table 54: Iterator Interface Required Properties
Property
Value
Requirements
next
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose done property is true, then all subsequent calls to the next method of that object should also return an IteratorResult object whose done property is true. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 55: Iterator Interface Optional Properties
Property
Value
Requirements
return
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a done property whose value is true, and a value property with the value passed as the argument of the return method. However, this requirement is not enforced.
throw
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a done property whose value is true.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.
The IteratorResult interface includes the properties listed in Table 56:
Table 56: IteratorResult Interface Properties
Property
Value
Requirements
done
Either true or false.
This is the result status of an iteratornext method call. If the end of the iterator was reached done is true. If the end was not reached done is false and a value is available. If a done property (either own or inherited) does not exist, it is consider to have the value false.
If done is false, this is the current iteration element value. If done is true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value, value is undefined. In that case, the value property may be absent from the conforming object if it does not inherit an explicit value property.
The value of the [[Prototype]] internal slot of the %IteratorPrototype% object is the intrinsic object %ObjectPrototype%. The %IteratorPrototype% object is an ordinary object. The initial value of the [[Extensible]] internal slot of the %IteratorPrototype% object is true.
Note
All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%.The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
The GeneratorFunction constructor is the %GeneratorFunction% intrinsic. When GeneratorFunction is called as a function rather than as a constructor, it creates and initializes a new GeneratorFunction object. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
GeneratorFunction is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.
The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.
When the GeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
25.2.2Properties of the GeneratorFunction Constructor#
The GeneratorFunction constructor is a standard built-in function object that inherits from the Function constructor. The value of the [[Prototype]] internal slot of the GeneratorFunction constructor is the intrinsic object %Function%.
The value of the [[Extensible]] internal slot of the GeneratorFunction constructor is true.
The value of the name property of the GeneratorFunction is "GeneratorFunction".
The GeneratorFunction constructor has the following properties:
The initial value of GeneratorFunction.prototype is the intrinsic object %Generator%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
25.2.3Properties of the GeneratorFunction Prototype Object#
The GeneratorFunction prototype object is an ordinary object. It is not a function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in Table 27 or Table 57. In addition to being the value of the prototype property of the %GeneratorFunction% intrinsic, it is the %Generator% intrinsic (see Figure 2).
The value of the [[Prototype]] internal slot of the GeneratorFunction prototype object is the %FunctionPrototype% intrinsic object. The initial value of the [[Extensible]] internal slot of the GeneratorFunction prototype object is true.
Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 27. The value of the [[FunctionKind]] internal slot for all such instances is "generator".
Each GeneratorFunction instance has the following own properties:
The value of the length property is an integer that indicates the typical number of arguments expected by the GeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a GeneratorFunction when invoked on a number of arguments other than the number specified by its length property depends on the function.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of the generator function's prototype property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator object when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Unlike function instances, the object that is the value of the a GeneratorFunction's prototype property does not have a constructor property whose value is the GeneratorFunction instance.
A Generator object is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.
Generator instances directly inherit properties from the object that is the value of the prototype property of the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator Prototype intrinsic, %GeneratorPrototype%.
The Generator prototype object is the %GeneratorPrototype% intrinsic. It is also the initial value of the prototype property of the %Generator% intrinsic (the GeneratorFunction.prototype).
The Generator prototype is an ordinary object. It is not a Generator instance and does not have a [[GeneratorState]] internal slot.
The value of the [[Prototype]] internal slot of the Generator prototype object is the intrinsic object %IteratorPrototype%. The initial value of the [[Extensible]] internal slot of the Generator prototype object is true.
All Generator instances indirectly inherit properties of the Generator prototype object.
Set generator's [[GeneratorState]] internal slot to "completed".
Once a generator enters the "completed" state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
If result is a normal completion, let resultValue be undefined.
Else,
If result.[[Type]] is return, let resultValue be result.[[Value]].
Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
Set generator's [[GeneratorState]] internal slot to "completed".
Once a generator enters the "completed" state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Set the code evaluation state of genContext such that when evaluation is resumed with a CompletionresumptionValue the following steps will be performed:
Return resumptionValue.
NOTE: This returns to the evaluation of the YieldExpression production that originally called this abstract operation.
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.
Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:
A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
A PromiseCapability is a Record value used to encapsulate a promise object along with the functions that are capable of resolving or rejecting that promise object. PromiseCapability records are produced by the NewPromiseCapability abstract operation.
PromiseCapability Records have the fields listed in Table 58.
The PromiseReaction is a Record value used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction records are created by the PerformPromiseThen abstract operation, and are used by a PromiseReactionJob.
PromiseReaction records have the fields listed in Table 59.
The capabilities of the promise for which this record provides a reaction handler.
[[Handler]]
A function object or a String
The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is "Identity" it is equivalent to a function that simply returns its first argument. If [[Handler]] is "Thrower" it is equivalent to a function that throws its first argument as an exception.
The abstract operation NewPromiseCapability takes a constructor function, and attempts to use that constructor function in the fashion of the built-in Promise constructor to create a Promise object and extract its resolve and reject functions. The promise plus the resolve and reject functions are used to initialize a new PromiseCapability record which is returned as the value of this abstract operation.
If IsConstructor(C) is false, throw a TypeError exception.
NOTE C is assumed to be a constructor function that supports the parameter conventions of the Promise constructor (see 25.4.3.1).
Let promiseCapability be a new PromiseCapability { [[Promise]]: undefined, [[Resolve]]: undefined, [[Reject]]: undefined }.
Let executor be a new built-in function object as defined in GetCapabilitiesExecutor Functions (25.4.1.5.1).
Set the [[Capability]] internal slot of executor to promiseCapability.
If IsCallable(promiseCapability.[[Resolve]]) is false, throw a TypeError exception.
If IsCallable(promiseCapability.[[Reject]]) is false, throw a TypeError exception.
Set promiseCapability.[[Promise]] to promise.
Return promiseCapability.
Note
This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise constructor to any subclass.
The abstract operation TriggerPromiseReactions takes a collection of PromiseReactionRecords and enqueues a new Job for each record. Each such Job processes the [[Handler]] of the PromiseReactionRecord, and if the [[Handler]] is a function calls it passing the given argument.
Repeat for each reaction in reactions, in original insertion order
HostPromiseRejectionTracker is an implementation-defined abstract operation that allows host environments to track promise rejections.
An implementation of HostPromiseRejectionTracker must complete normally in all cases. The default implementation of HostPromiseRejectionTracker is to do nothing.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its operation argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called with its operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.
Note 2
If operation is "handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is "reject", since it is expected that rejections will be rare and not on hot code paths.
The job PromiseReactionJob with parameters reaction and argument applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler.
This Job uses the supplied thenable and its then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.
The Promise constructor is the %Promise% intrinsic object and the initial value of the Promise property of the global object. When called as a constructor it creates and initializes a new Promise object. Promise is not intended to be called as a function and will throw an exception when called in that manner.
The Promise constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promise constructor to create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.
The executor argument must be a function object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise object. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.
The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the Promise constructor have the capability to actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes in customized values for resolve and reject.
The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.
Let C be the this value.
If Type(C) is not Object, throw a TypeError exception.
When the PerformPromiseAll abstract operation is called with arguments iteratorRecord, constructor, and resultCapability, the following steps are taken:
Assert: constructor is a constructor function.
Assert: resultCapability is a PromiseCapability record.
A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capabilities]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function F is called with argument x, the following steps are taken:
Let alreadyCalled be the value of F's [[AlreadyCalled]] internal slot.
If alreadyCalled.[[Value]] is true, return undefined.
Set alreadyCalled.[[Value]] to true.
Let index be the value of F's [[Index]] internal slot.
Let values be the value of F's [[Values]] internal slot.
Let promiseCapability be the value of F's [[Capabilities]] internal slot.
Let remainingElementsCount be the value of F's [[RemainingElements]] internal slot.
Set values[index] to x.
Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
Let C be the this value.
If Type(C) is not Object, throw a TypeError exception.
If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.
Note 2
The race function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor. It also expects that its this value provides a resolve method.
25.4.4.3.1Runtime Semantics: PerformPromiseRace ( iteratorRecord, promiseCapability, C )#
When the PerformPromiseRace abstract operation is called with arguments iteratorRecord, promiseCapability, and C, the following steps are taken:
Repeat
Let next be IteratorStep(iteratorRecord.[[Iterator]]).
If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this constructor.
Let C be the this value.
If Type(C) is not Object, throw a TypeError exception.
Promise[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Promise prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
The Promise prototype object is the intrinsic object %PromisePrototype%. The value of the [[Prototype]] internal slot of the Promise prototype object is the intrinsic object %ObjectPrototype%. The Promise prototype object is an ordinary object. It does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.
The abstract operation PerformPromiseThen performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. The result is resultCapability's promise.
Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic, %PromisePrototype%). Promise instances are initially created with the internal slots described in Table 60.
Table 60: Internal Slots of Promise Instances
Internal Slot
Description
[[PromiseState]]
A String value that governs how a promise will react to incoming calls to its then method. The possible values are: "pending", "fulfilled", and "rejected".
[[PromiseResult]]
The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not "pending".
[[PromiseFulfillReactions]]
A List of PromiseReaction records to be processed when/if the promise transitions from the "pending" state to the "fulfilled" state.
[[PromiseRejectReactions]]
A List of PromiseReaction records to be processed when/if the promise transitions from the "pending" state to the "rejected" state.
[[PromiseIsHandled]]
A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.
The Reflect object is the %Reflect% intrinsic object and the initial value of the Reflect property of the global object. The Reflect object is an ordinary object.
The value of the [[Prototype]] internal slot of the Reflect object is the intrinsic object %ObjectPrototype%.
The Reflect object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use the Reflect object as a constructor with the new operator. The Reflect object also does not have a [[Call]] internal method; it is not possible to invoke the Reflect object as a function.
The Proxy constructor is the %Proxy% intrinsic object and the initial value of the Proxy property of the global object. When called as a constructor it creates and initializes a new proxy exotic object. Proxy is not intended to be called as a function and will throw an exception when called in that manner.
The value of the [[Prototype]] internal slot of the Proxy constructor is the intrinsic object %FunctionPrototype%.
The Proxy constructor does not have a prototype property because proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
The Proxy constructor has the following properties:
The Proxy.revocable function is used to create a revocable Proxy object. When Proxy.revocable is called with arguments target and handler, the following steps are taken:
A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that includes a NameSpaceImport.
In addition to the properties specified in 9.4.6 each Module Namespace Object has the following own properties:
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser based ECMAScript implementations. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.
The syntax and semantics of 11.4 is extended as follows except that this extension is not allowed when parsing source code using the goal symbol Module:
The syntax of 21.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [U] parameter present on the goal symbol.
The escape function is a property of the global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.
For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:
Let char be the code unit (represented as a 16-bit unsigned integer) at index k within string.
If char is one of the code units in "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./", then
Let S be a String containing the single code unit char.
Else if char ≥ 256, then
Let S be a String containing six code units "%uwxyz" where wxyz are the code units of the four hexadecimal digits encoding the value of char.
Else, char < 256
Let S be a String containing three code units "%xy" where xy are the code units of two hexadecimal digits encoding the value of char.
Let R be a new String value computed by concatenating the previous value of R and S.
Increase k by 1.
Return R.
Note
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.
The unescape function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.
The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:
If k ≤ length-6 and the code unit at index k+1 within string is u and the four code units at indices k+2, k+3, k+4, and k+5 within string are all hexadecimal digits, then
Let c be the code unit whose value is the integer represented by the four hexadecimal digits at indices k+2, k+3, k+4, and k+5 within string.
Increase k by 5.
Else if k ≤ length-3 and the two code units at indices k+1 and k+2 within string are both hexadecimal digits, then
Let c be the code unit whose value is the integer represented by two zeroes plus the two hexadecimal digits at indices k+1 and k+2 within string.
Increase k by 2.
Let R be a new String value computed by concatenating the previous value of R and c.
Increase k by 1.
Return R.
B.2.2Additional Properties of the Object.prototype Object#
Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined as follows:
The substr method takes two arguments, start and length, and returns a substring of the result of converting the this object to a String, starting from index start and running for length code units (or through the end of the String if length is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:
If length is undefined, let end be +∞; otherwise let end be ? ToInteger(length).
Let size be the number of code units in S.
If intStart < 0, let intStart be max(size + intStart, 0).
Let resultLength be min(max(end, 0), size - intStart).
If resultLength ≤ 0, return the empty String "".
Return a String containing resultLength consecutive code units from S beginning with the code unit at index intStart.
Note
The substr function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.3.2.1Runtime Semantics: CreateHTML ( string, tag, attribute, value )#
The abstract operation CreateHTML is called with arguments string, tag, attribute, and value. The arguments tag and attribute must be String values. The following steps are taken:
Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence """.
Let p1 be the String value that is the concatenation of the following String values:
The String value of p1
Code unit 0x0020 (SPACE)
The String value of attribute
Code unit 0x003D (EQUALS SIGN)
Code unit 0x0022 (QUOTATION MARK)
The String value of escapedV
Code unit 0x0022 (QUOTATION MARK)
Let p2 be the String value that is the concatenation of p1 and ">".
Let p3 be the String value that is the concatenation of p2 and S.
Let p4 be the String value that is the concatenation of p3, "</", tag, and ">".
The property toUTCString is preferred. The toGMTString property is provided principally for compatibility with old code. It is recommended that the toUTCString property be used in new ECMAScript code.
The function object that is the initial value of Date.prototype.toGMTString is the same function object that is the initial value of Date.prototype.toUTCString.
B.2.5Additional Properties of the RegExp.prototype Object#
The compile method completely reinitializes the this object RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015, the grammar productions for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 13.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. For web browser compatibility, that rule is modified with the addition of the highlighted text:
It is a Syntax Error if any strict mode source code matches this rule.
B.3.3Block-Level Function Declarations Web Legacy Compatibility Semantics#
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript code that uses Block level function declarations is only portable among browser implementation if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:
A function is declared and only referenced within a single block
A FunctionDeclaration whose BindingIdentifier is the name f occurs exactly once within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g
A function is declared and possibly used within a single Block but also referenced by an inner function definition that is not contained within that same Block.
A FunctionDeclaration whose BindingIdentifier is the name f occurs exactly once within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g
There is at least one occurrence of f as an IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h.
All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
A FunctionDeclaration whose BindingIdentifier is the name f occurs exactly once within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g
There is at least one occurrence of f as an IdentifierReference within the function code of g that lexically follows the Block containing the declaration of f.
The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case will operate using the Block level function declarations semantics defined by clauses 9, 13, and 14 of this specification.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause 9, clause 14, clause 18.2.1 and clause 15.1.11 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create an early error, a warning message should not be produced.
B.3.3.1Changes to FunctionDeclarationInstantiation#
The above rules are only applied when parsing code that is not strict mode code. If any such code is match by one of these rules subsequent processing of that code takes places as if each matching occurrence of FunctionDeclaration[?Yield] was the sole StatementListItem of a BlockStatement occupying that position in the source code. The semantics of such a synthetic BlockStatement includes the web legacy compatibility semantics specified in B.3.3.
The Block of a Catch clause may contain var declarations that bind a name that is also bound by the CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such var declarations will assign to the corresponding catch parameter rather than the var binding. The relaxation of the normal static semantic rule does not apply to names only bound by for-of statements.
This modified behaviour also applies to var and function declarations introduced by direct eval calls contained within the Block of a Catch clause. This change is accomplished by modify the algorithm of 18.2.1.2 as follows:
Step 5.d.ii.2.a.i is replaced by:
If thisEnvRec is not the Environment Record for a Catch clause, throw a SyntaxError exception.
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global object. When a simple assignment occurs within strict mode code, its LeftHandSideExpression must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown (6.2.3.2). The LeftHandSideExpression also may not be a reference to a data property with the attribute value {[[Writable]]: false}, to an accessor property with the attribute value {[[Set]]: undefined}, nor to a non-existent property of an object whose [[Extensible]] internal slot has the value false. In these cases a TypeError exception is thrown (12.15).
Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access (9.2.7).
Arguments objects for strict mode functions do not dynamically share their array indexed property values with the corresponding formal parameter bindings of their functions. (9.4.4).
For strict mode functions, if an arguments object is created the binding of the local identifier arguments to the arguments object is immutable and hence may not be the target of an assignment expression. (9.2.12).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the eval code (18.2.1).
If this is evaluated within strict mode code, then the this value is not coerced to an object. A this value of null or undefined is not converted to the global object and primitive values are not converted to wrapper objects. The this value passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this value to an object (9.2.1.2, 19.2.3.1, 19.2.3.3).
When a delete operator occurs within strict mode code, a SyntaxError is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name (12.5.3.1).
When a delete operator occurs within strict mode code, a TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]: false } (12.5.3.2).
It is a SyntaxError if the same BindingIdentifier appears more than once in the FormalParameters of a strict mode function. An attempt to create such a function using a Function or Generator constructor is a SyntaxError (14.1.2, 19.2.1.1.1).
An implementation may not extend, beyond that defined in this specification, the meanings within strict mode functions of properties named caller or arguments of function instances. ECMAScript code may not create or modify properties with these names on function objects that correspond to strict mode functions (16.2).
DCorrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact#
8.1.1.4.15-8.1.1.4.18 Edition 5 and 5.1 used a property existence test to determine whether a global object property corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most commonly implemented by web browsers.
9.4.2.1: The 5th Edition moved the capture of the current array length prior to the integer conversion of the array index or new length value. However, the captured length value could become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible occurrence of such side-effects.
20.3.1.15: Previous editions permitted the TimeClip abstract operation to return either +0 or -0 as the representation of a 0 time value. ECMAScript 2015 specifies that +0 always returned. This means that for ECMAScript 2015 the time value of a Date object is never observably -0 and methods that return time values never return -0.
20.3.1.16: If a time zone offset is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as "z".
20.3.4.36: If the year cannot be represented using the Date Time String Format specified in 20.3.1.16 a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.
20.3.4.41: Previous editions did not specify the value returned by Date.prototype.toString when this time value is NaN. ECMAScript 2015 specifies the result to be the String value is "Invalid Date".
21.2.3.1, 21.2.3.2.4: Any LineTerminator code points in the value of the source property of an RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of "/".
21.2.5.6, 21.2.5.8: In previous editions, the specifications for String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global is flag set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by one only if the pattern matched the empty string.
22.1.3.25, 22.1.3.25.1: Previous editions did not specify how a NaN value returned by a comparefn was interpreted by Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if +0 was returned from the comparefn. ECMAScript 2015 also specifies that ToNumber is applied to the result returned by a comparefn. In previous editions, the effect of a comparefn result that is not a Number value was implementation dependent. In practice, implementations call ToNumber.
EAdditions and Changes That Introduce Incompatibilities with Prior Editions#
6.2.3: In ECMAScript 2015, Function calls are not allowed to return a Reference value.
11.6: In ECMAScript 2015, the valid code points for an IdentifierName are specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or Identifier code points were specified by enumerating various Unicode code point categories.
11.9.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change aligns the specification with the actual behaviour of most existing implementations.
12.2.6.1: In ECMAScript 2015, it is no longer an early error to have duplicate property names in Object Initializers.
12.15.1: In ECMAScript 2015, strict mode code containing an assignment to an immutable binding such as the function name of a FunctionExpression does not produce an early error. Instead it produces a runtime error.
13.2: In ECMAScript 2015, a StatementList beginning with the token let followed by the input elements LineTerminator then Identifier is the start of a LexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before the Identifier input element.
13.6.7: In ECMAScript 2015, the normal completion value of an IfStatement is never the value empty. If no Statement part is evaluated or if the evaluated Statement part produces a normal completion whose value is empty, the completion value of the IfStatement is undefined.
13.7: In ECMAScript 2015, if the ( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of a LexicalDeclaration. In previous editions such a token sequence would be the start of an Expression.
13.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence let [ then the let is treated as the start of a ForDeclaration. In previous editions such a token sequence would be the start of an LeftHandSideExpression.
13.7: Prior to ECMAScript 2015, an initialization expression could appear as part of the VariableDeclaration that precedes the in keyword. The value of that expression was always discarded. In ECMAScript 2015, the ForBinding in that same position does not allow the occurrence of such an initializer.
13.7: In ECMAScript 2015, the completion value of an IterationStatement is never the value empty. If the Statement part of an IterationStatement is not evaluated or if the final evaluation of the Statement part produces a completion whose value is empty, the completion value of the IterationStatement is undefined.
13.11.7: In ECMAScript 2015, the normal completion value of a WithStatement is never the value empty. If evaluation of the Statement part of a WithStatement produces a normal completion whose value is empty, the completion value of the WithStatement is undefined.
13.15: In ECMAScript 2015, it is an early error for a Catch clause to contain a var declaration for the same Identifier that appears as the Catch clause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment but the declaration's Initializer value would be assigned to the Catch parameter.
13.15, 18.2.1.2: In ECMAScript 2015, a runtime SyntaxError is thrown if a Catch clause evaluates a non-strict direct eval whose eval code includes a var or FunctionDeclaration declaration that binds the same Identifier that appears as the Catch clause parameter.
13.15.8: In ECMAScript 2015, the completion value of a TryStatement is never the value empty. If the Block part of a TryStatement evaluates to a normal completion whose value is empty, the completion value of the TryStatement is undefined. If the Block part of a TryStatement evaluates to a throw completion and it has a Catch part that evaluates to a normal completion whose value is empty, the completion value of the TryStatement is undefined if there is no Finally clause or if its Finally clause evalulates to an empty normal completion.
14.3.9 In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor properties in an ObjectLiteral are not constructor functions and they do not have a prototype own property. In the previous edition, they were constructors and had a prototype property.
19.1.2.5: In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.6: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.7: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.9: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.11: In ECMAScript 2015, if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.12: In ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.13: In ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.14: In ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.15: In ECMAScript 2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.17: In ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %FunctionPrototype%.
19.2.4.1: In ECMAScript 2015, the length property of function instances is configurable. In previous editions it was non-configurable.
19.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a NativeError constructor is the Error constructor. In previous editions it was the Function prototype object.
20.3.4 In ECMAScript 2015, the Date prototype object is not a Date instance. In previous editions it was a Date instance whose TimeValue was NaN.
21.1.3.10 In ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
21.1.3.22 and 21.1.3.24 In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion processing was only applied to individual code units. The only affected code points are those in the Deseret block of Unicode
21.1.3.25 In ECMAScript 2015, the String.prototype.trim method is defined to recognize white space code points that may exists outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.
21.2.3.1 In ECMAScript 2015, If the pattern argument is a RegExp instance and the flags argument is not undefined, a new RegExp instance is created just like pattern except that pattern's flags are replaced by the argument flags. In previous editions a TypeError exception was thrown when pattern was a RegExp instance and flags was not undefined.
21.2.5 In ECMAScript 2015, the RegExp prototype object is not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty string.
21.2.5 In ECMAScript 2015, source, global, ignoreCase, and multiline are accessor properties defined on the RegExp prototype object. In previous editions they were data properties defined on RegExp instances
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