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class

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

Try it

You can also define a class using a class expression, which allows redeclarations and omitting class names. Attempting to place class declaration in the same scope, under the same name, will throw a SyntaxError.

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 Rectangle, then extend it to create a class named FilledRectangle.

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

class FilledRectangle extends Rectangle {
  constructor(height, width, color) {
    super(height, width);
    this.name = 'Filled rectangle';
    this.color = color;
  }
}

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 compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
class
49
42-49
Strict mode is required.
13
45
No
36
29-36
Strict mode is required.
10.1
49
42-49
Strict mode is required.
49
42-49
Strict mode is required.
45
36
29-36
Strict mode is required.
10.3
5.0
4.0-5.0
Strict mode is required.
1.0
6.0.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class