Web/JavaScript/Reference/Classes/extends

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

Syntax

class ChildClass extends ParentClass { ... }

Description

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.

Examples

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;
  }
}

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 {

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

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'extends' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`extends`	Chrome Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required. No support 42 — 49 Disabled' Disabled' From version 42 until version 49 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`). To change preferences in Chrome, visit chrome://flags.	Edge Full support 13	Firefox Full support 45	IE No support No	Opera Full support 36 Full support 36 No support 29 — 36 Notes' Notes' Strict mode is required. No support 29 — 36 Disabled' Disabled' From version 29 until version 36 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`).	Safari Full support 9	WebView Android Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required.	Chrome Android Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required. No support 42 — 49 Disabled' Disabled' From version 42 until version 49 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`). To change preferences in Chrome, visit chrome://flags.	Firefox Android Full support 45	Opera Android Full support 36 Full support 36 No support 29 — 36 Notes' Notes' Strict mode is required. No support 29 — 36 Disabled' Disabled' From version 29 until version 36 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`).	Safari iOS Full support 9	Samsung Internet Android Full support 5.0 Full support 5.0 No support 4.0 — 5.0 Notes' Notes' Strict mode is required.	nodejs Full support 6.0.0 Full support 6.0.0 Full support 4.0.0 Disabled' Disabled' From version 4.0.0: this feature is behind the `--use_strict` runtime flag. Full support 5.0.0 Disabled' Disabled' From version 5.0.0: this feature is behind the `--harmony` runtime flag.

Legend

Full support: Full support
No support: No support
See implementation notes.': See implementation notes.
User must explicitly enable this feature.': User must explicitly enable this feature.