extends

The extends keyword is used in class declarations or class expressions to create a class which 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 {
  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();
  }
}

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

Specifications

Browser compatibility

1. From Chrome 42 to 48 strict mode is required. Non-strict mode support can be enabled using the flag "Enable Experimental JavaScript".

See also

• Classes
• super