The extends keyword is used in a class declarations or class expressions to create a class with a child of another class.


class ChildClass extends ParentClass { ... }


The extends keyword can be used to subclass custom classes as well as built-in objects.

The .prototype of the extension must be an Object or null.


Using extends

The first example creates a class called Square from a class called Polygon. This example is extracted from this live demo (source).

class Square extends Polygon {
  constructor(length) {
    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);
    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error. = 'Square';

  get area() {
    return this.height * this.width;

  set area(value) {
    this.height = this.width = Math.sqrt(value);
    this.area = value;

Using extends with built-in objects

This example extends the built-in Date object. This example is extracted from this live demo (source).

class myDate extends Date {
  constructor() {

  getFormattedDate() {
    var months = ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'];
    return this.getDate() + "-" + months[this.getMonth()] + "-" + this.getFullYear();

Extending null

Extending from null works like with a normal class, except that the prototype object does not inherit from Object.prototype.

class nullExtends extends null {
  constructor() {}

Object.getPrototypeOf(nullExtends); // Function.prototype
Object.getPrototypeOf(nullExtends.prototype) // null


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'extends' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'extends' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 42.0 45 (45) ? ? ?
Array subclassing 43.0 No support ? ? ?
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support 45.0 (45) ? ? ? 42.0
Array subclassing No support No support ? ? ? 43.0

See also

Document Tags and Contributors

 Contributors to this page: stevenkaspar, fscholz, rwaldron, devinivy, jpmedley, claudepache,
 Last updated by: stevenkaspar,