Web/JavaScript/Reference/statements/class-2

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

Specification
ECMAScript (ECMA-262)The definition of 'Class definitions' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`class`	Chrome Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required. No support 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.	Edge Full support 13	Firefox Full support 45	IE No support No	Opera Full support 36 Full support 36 No support 29 — 36 Notes' Notes' Strict mode is required. No support 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`).	Safari Full support 10.1	WebView Android Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required.	Chrome Android Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required. No support 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.	Firefox Android Full support 45	Opera Android Full support 36 Full support 36 No support 29 — 36 Notes' Notes' Strict mode is required. No support 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`).	Safari iOS Full support 10.3	Samsung Internet Android Full support 5.0 Full support 5.0 No support 4.0 — 5.0 Notes' Notes' Strict mode is required.	nodejs Full support 6.0.0

Legend

Full support: Full support
No support: No support
See implementation notes.': See implementation notes.
User must explicitly enable this feature.': User must explicitly enable this feature.