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 Language Specification (ECMAScript) # sec-class-definitions

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`extends`	49 42-49 Strict mode is required. 42-49	13	45	No	36 29-36 Strict mode is required. 29-36	9	49 42-49 Strict mode is required.	49 42-49 Strict mode is required. 42-49	45	36 29-36 Strict mode is required. 29-36	9	5.0 4.0-5.0 Strict mode is required.