extends

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

Examples

The first example creates a class called Square from a class called Polygon.

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.area = value;
  } 
}

This example extends the built-in Date object.

class myDate extends Date {
  constructor() {|
    super();
  }

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

Demo

The above examples are extracted from this live demo.  (source) 

Specifications

Browser compatibility

See also

• Classes