constructor

The constructor method is a special method for creating and initializing an object created with a class.

Syntax

constructor([arguments]) { ... }

Description

There can only be one special method with the name "constructor" in a class. A SyntaxError will be thrown, if the class contains more than one occurrence of a constructor method.

A constructor can use the super keyword to call the constructor of a parent class.

If you don't specify a constructor method, a default constructor is used.

Examples

Using the constructor method

This code snippet is taken from the classes sample (live demo).

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;
  } 
}

Default constructors

If you don't specify a constructor method, a default constructor is used. For base classes, the default constructor is:

constructor() {}

For derived classes, the default constructor is:

constructor(...args) {
  super(...args);
}

Specifications

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Constructor Method' in that specification.	Standard	Initial definition.
ECMAScript 2017 Draft (ECMA-262) The definition of 'Constructor Method' in that specification.	Draft

Browser compatibility

See also

• super()
• class expression
• class declaration
• Classes