class

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

Syntax

class name [extends] {
  // class body
}

Description

Just like with class expressions, the class body of a class declaration is executed in strict mode. The constructor property 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';
  }
}

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

var Foo = class {};
class Foo {}; // Uncaught TypeError: Identifier 'Foo' has already been declared

Specifications

Browser compatibility

See also

• function declaration
• class expression
• Classes