The extends keyword is used in class declarations or class expressions to create a class which is 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.
    this.name = '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

new nullExtends(); //ReferenceError: this is not defined


Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 42.0 (Yes) 45 (45) ? ? ?
Array subclassing 43.0 (Yes) 53 (53) ? ? ?
Feature Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support (Yes) 45.0 (45) ? ? ? 42.0
Array subclassing No support (Yes) No support ? ? ? 43.0

See also

© 2005–2017 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.