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;
  } 
}

Another example

Take a look at this code snippet

class Polygon {
    constructor() {
        this.name = "Polygon";
    }
}

class Square extends Polygon {
    constructor() {
        super();
    }
}

class Rectangle {}

Object.setPrototypeOf(Square.prototype, Rectangle.prototype);

console.log(Object.getPrototypeOf(Square.prototype) === Polygon.prototype); //false
console.log(Object.getPrototypeOf(Square.prototype) === Rectangle.prototype); //true

let newInstance = new Square();
console.log(newInstance.name); //Polygon

Here the prototype of Square class is changed but still the constructor of the previous base class Polygon is called when new instance of Square will be created.

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

Browser compatibility

See also

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