How to get an array of function property names from own enumerable properties of an object in JavaScript ?

Enumerable Properties: All the properties of an object that can be iterated using for..in loop or Object.keys() method are known as enumerable properties.

• Using Object.keys() method, we can get an array of all enumerable properties (containing functions and data members) of an Object.

  Syntax:

  Object.keys(object_name)

• Then we can filter the above obtained array using Array filter() method so that it contains only enumerable function names (eliminating all data members).

  Syntax:

  new_arr = arr_name.filter( (element)=> {.....} )

In the following implementation for creating a non-enumerable property, we have used Object.defineProperty() method.

<script>

    // Person object has name, age, salary
    // and print properties 
    // Except salary, all properties are enumerable
    let Person = {
        name: "Mahesh",
        age: 25,
        print: function () {
            console.log(`${this.name} ${(this, age)}`);
        },
    };

    // salary- non enumerable property
    Object.defineProperty(Person, "salary", {
        value: "Rs. 50,000",
        enumerable: false,
    });

    // greeting- non enumerable function
    Object.defineProperty(Person, "greeting", {
        value: function () {
            console.log("Welcome to GFG");
        },
        enumerable: false,
    });

    // arr contains all the enumerable
    // properties of Person
    let arr = Object.keys(Person);

    // functionsArr contains enumerable
    // function properties of Person.
    // typeof returns an string representing
    // data type.
    // Using filter() method to filter the array
    let functionsArr = arr.filter((key) => {
        return typeof Person[key] === "function";
    });

    console.log(arr);
    console.log(functionsArr);
</script>

["name", "age", "print"]
["print"]

In the above code, we have an object named Person with 3 data members (name, age and salary) and 2 functions ('print' and 'greeting')  out of which salary data-member and greeting function are non-enumerable properties.

Here 'print' is the only function which is enumerable. Using Object.keys(Person) we get an array containing all enumerable properties i.e. ["name", "age", "print"] .

Then we used Array.filter() property to filter out all variables so that array has only function property names, i.e. ["print"].