Please note, this is a STATIC archive of website developer.mozilla.org from 03 Nov 2016, cach3.com does not collect or store any user information, there is no "phishing" involved.

class

Nasi wolontariusze nie przetłumaczyli jeszcze tego artykułu na język Polski. Dołącz do nas i pomóż go przetłumaczyć!

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 the class expression, the class declaration doesn't allow an existing class to be declared again and will throw a type error if attempted. 

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';
  }
}

Attempting to declare a class twice

Re-declaring a class using the class declartion throws a type error.

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

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

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

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Class definitions' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Class definitions' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 42.0 45 (45) ? ? ?
Array subclassing 43.0 No support ? ? ?
Allowed in sloppy mode 49.0        
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
Array subclassing No support 43.0 No support ? ? ? 43.0
Allowed in sloppy mode No support 49.0         49.0

See also

Autorzy i etykiety dokumentu

 Autorzy tej strony: deveedutta, jpmedley, fscholz
 Ostatnia aktualizacja: deveedutta,