JavaScript: Get an array of function property names from own enumerable properties of an object

JavaScript fundamental (ES6 Syntax): Exercise-48 with Solution

Array of Function Property Names

Write a JavaScript program to get an array of function property names from the own (and optionally inherited) enumerable properties of an object.

• Use Object.keys(obj) to iterate over the object's own properties.
• If inherited is true, use Object.getPrototypeOf(obj) to also get the object's inherited properties.
• Use Array.prototype.filter() to keep only those properties that are functions.
• Omit the second argument, inherited, to not include inherited properties by default.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2
// Define a function 'functions' that retrieves the names of all functions in an object.
const functions = (obj, inherited = false) =>
  // Check if 'inherited' is true.
  (inherited
    ? // If true, concatenate the keys of the object and its prototype.
      [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
    : // If false, get only the keys of the object.
      Object.keys(obj)
  )
  // Filter the keys to include only those with function values.
  .filter(key => typeof obj[key] === 'function');

// Define a constructor function 'Foo'.
function Foo() {
  // Assign arrow function expressions to properties 'a' and 'b'.
  this.a = () => 1;
  this.b = () => 2;
}

// Assign an arrow function expression to the prototype property 'c'.
Foo.prototype.c = () => 3;

// Create an instance of 'Foo' and log the functions in the instance.
console.log(functions(new Foo())); // Output: [ 'a', 'b' ]

// Create an instance of 'Foo' and include inherited functions, then log them.
console.log(functions(new Foo(), true)); // Output: [ 'a', 'b', 'c' ]

Output:

["a","b"]
["a","b","c"]

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-1-47 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript program that extracts the names of all function properties from an object, including inherited ones.
• Write a JavaScript function that returns an array of keys from an object whose values are functions.
• Write a JavaScript program that filters an object’s properties to list only those that are callable.

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