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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
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | 42.0 | 45 (45) | ? | ? | ? |
Default constructors | ? | 45 (45) | ? | ? | ? |
Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|
Basic support | No support | 42.0 | 45.0 (45) | ? | ? | ? | 42.0 |
Default constructors | ? | ? | 45.0 (45) | ? | ? | ? | ? |