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class

The class declaration creates a new class with a given name using prototype-based inheritance.

You can also define a class using a class expression. But unlike a class expression, a class declaration doesn't allow an existing class to be declared again and will throw a SyntaxError if attempted.

Syntax

class name [extends otherName] {
  // class body
}

Description

The class body of a class declaration is executed in strict mode. The constructor method is optional.

Class declarations are not hoisted (unlike function declarations).

Examples

A simple class declaration

In the following example, we first define a class named Polygon, then extend it to create a class named Square.

Note that super(), used in the constructor, can only be used in constructors, and must be called before the this keyword can be used.

class Polygon {
  constructor(height, width) {
    this.name = 'Polygon';
    this.height = height;
    this.width = width;
  }
}

class Square extends Polygon {
  constructor(length) {
    super(length, length);
    this.name = 'Square';
  }
}

Attempting to declare a class twice

Re-declaring a class using the class declaration throws a SyntaxError.

class Foo {};
class Foo {}; // Uncaught SyntaxError: Identifier 'Foo' has already been declared

The same error is thrown when a class has been defined before using the class expression.

let Foo = class {};
class Foo {}; // Uncaught SyntaxError: Identifier 'Foo' has already been declared

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
class 49
49
42 — 49
Strict mode is required.
42 — 49
Disabled
Disabled From version 42 until version 49 (exclusive): this feature is behind the Experimental JavaScript preference (needs to be set to Enabled). To change preferences in Chrome, visit chrome://flags.
13 45 No 36
36
29 — 36
Strict mode is required.
29 — 36
Disabled
Disabled From version 29 until version 36 (exclusive): this feature is behind the Experimental JavaScript preference (needs to be set to Enabled).
10.1
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
class 49
49
42 — 49
Strict mode is required.
49
49
42 — 49
Strict mode is required.
42 — 49
Disabled
Disabled From version 42 until version 49 (exclusive): this feature is behind the Experimental JavaScript preference (needs to be set to Enabled). To change preferences in Chrome, visit chrome://flags.
45 36
36
29 — 36
Strict mode is required.
29 — 36
Disabled
Disabled From version 29 until version 36 (exclusive): this feature is behind the Experimental JavaScript preference (needs to be set to Enabled).
10.3 5.0
5.0
4.0 — 5.0
Strict mode is required.
Server
Node.js
class 6.0.0

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class