Subscript is fast, tiny & extensible expression evaluator / microlanguage with standard syntax.

• templates (eg. sprae)
• expressions evaluators, calculators
• subsets of languages (eg. justin)
• sandboxes, playgrounds, safe eval
• custom DSL (eg. piezo)
• preprocessors (eg. prepr)

Subscript has 3.5kb footprint (compare to 11.4kb jsep + 4.5kb expression-eval), best performance and extensive test coverage.

import subscript from './subscript.js'

// parse expression
const fn = subscript('a.b + Math.sqrt(c - 1)')

// evaluate with context
fn({ a: { b:1 }, c: 5, Math })
// 3

Subscript supports common syntax (JavaScript, C, C++, Java, C#, PHP, Swift, Objective-C, Kotlin, Perl etc.):

• a.b, a[b], a(b)
• a++, a--, ++a, --a
• a * b, a / b, a % b
• +a, -a, a + b, a - b
• a < b, a <= b, a > b, a >= b, a == b, a != b
• ~a, a & b, a ^ b, a | b, a << b, a >> b
• !a, a && b, a || b
• a = b, a += b, a -= b, a *= b, a /= b, a %= b, , a <<= b, a >>= b
• (a, (b)), a; b;
• "abc", 'abc'
• 0.1, 1.2e+3

Just-in is no-keywords JS subset, JSON + expressions (see thread).
It extends subscript with:

• a === b, a !== b
• a ** b, a **= b
• a ?? b, a ??= b
• a ||= b, a &&= b
• a >>> b, a >>>= b
• a ? b : c, a?.b
• ...a
• [a, b]
• {a: b}
• (a, b) => c
• // foo, /* bar */
• true, false, null, NaN, undefined
• a in b

import jstin from './justin.js'

let xy = jstin('{ x: 1, "y": 2+2 }["x"]')
xy()  // 1

Subscript exposes parse to build AST and compile to create evaluators.

import { parse, compile } from 'subscript'

// parse expression
let tree = parse('a.b + c - 1')
tree // ['-', ['+', ['.', 'a', 'b'], 'c'], [,1]]

// compile tree to evaluable function
fn = compile(tree)
fn({ a: {b: 1}, c: 2 }) // 3

AST has simplified lispy tree structure (inspired by frisk / nisp), opposed to ESTree:

• not limited to particular language (JS), can be compiled to different targets;
• reflects execution sequence, rather than code layout;
• has minimal overhead, directly maps to operators;
• simplifies manual evaluation and debugging;
• has conventional form and one-liner docs:

import { compile } from 'subscript.js'

const fn = compile(['+', ['*', 'min', [,60]], [,'sec']])
fn({min: 5}) // min*60 + "sec" == "300sec"

// node kinds
['+', a];       // unary operator `+a`
['+', a, b];    // binary operator `a + b`
['+', a, b, c]; // n-ary operator `a + b + c`
['()', a];      // group operator `(a)`
['()', a, b];   // access operator `a(b)`
[, a];          // literal value `'a'`
a;              // variable (from scope)
null;           // placeholder

// eg.
['()', 'a'] // (a)
['()', 'a', null] // a()

To convert tree back to code, there's codegenerator function:

import { stringify } from 'subscript.js'

stringify(['+', ['*', 'min', [,60]], [,'sec']])
// 'min*60 + "sec" == "300sec"'

Subscript provides premade language features and API to customize syntax:

• unary(str, precedence, postfix=false) − register unary operator, either prefix ⚬a or postfix a⚬.
• binary(str, precedence, rassoc=false) − register binary operator a ⚬ b, optionally right-associative.
• nary(str, precedence) − register n-ary (sequence) operator like a; b; or a, b, allows missing args.
• group(str, precedence) - register group, like [a], {a}, (a) etc.
• access(str, precedence) - register access operator, like a[b], a(b) etc.
• token(str, precedence, lnode => node) − register custom token or literal. Callback takes left-side node and returns complete expression node.
• operator(str, (a, b) => ctx => value) − register evaluator for an operator. Callback takes node arguments and returns evaluator function.

Longer operators should be registered after shorter ones, eg. first |, then ||, then ||=.

import script, { compile, operator, unary, binary, token } from './subscript.js'

// enable objects/arrays syntax
import 'subscript/feature/array.js';
import 'subscript/feature/object.js';

// add identity operators (precedence of comparison)
binary('===', 9), binary('!==', 9)
operator('===', (a, b) => (a = compile(a), b = compile(b), ctx => a(ctx)===b(ctx)))
operator('===', (a, b) => (a = compile(a), b = compile(b), ctx => a(ctx)!==b(ctx)))

// add nullish coalescing (precedence of logical or)
binary('??', 3)
operator('??', (a, b) => b && (a = compile(a), b = compile(b), ctx => a(ctx) ?? b(ctx)))

// add JS literals
token('undefined', 20, a => a ? err() : [, undefined])
token('NaN', 20, a => a ? err() : [, NaN])

See ./feature/* or ./justin.js for examples.

Subscript shows good performance within other evaluators. Example expression:

1 + (a * b / c % d) - 2.0 + -3e-3 * +4.4e4 / f.g[0] - i.j(+k == 1)(0)

Parse 30k times:

subscript: ~150 ms 🥇
justin: ~183 ms
jsep: ~270 ms 🥈
jexpr: ~297 ms 🥉
mr-parser: ~420 ms
expr-eval: ~480 ms
math-parser: ~570 ms
math-expression-evaluator: ~900ms
jexl: ~1056 ms
mathjs: ~1200 ms
new Function: ~1154 ms

Eval 30k times:

new Function: ~7 ms 🥇
subscript: ~15 ms 🥈
justin: ~17 ms
jexpr: ~23 ms 🥉
jsep (expression-eval): ~30 ms
math-expression-evaluator: ~50ms
expr-eval: ~72 ms
jexl: ~110 ms
mathjs: ~119 ms
mr-parser: -
math-parser: -

jexpr, jsep, jexl, mozjexl, expr-eval, expression-eval, string-math, nerdamer, math-codegen, math-parser, math.js, nx-compile, built-in-math-eval

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