class expression

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Class expression это способ определения класса в ECMAScript 2015 (ES6). Схожий с function expressions, class expressions может быть именованным либо не иметь имени. Если он именованный, то его имя доступно только внутри класса. JavaScript классы используют прототипно-ориентированние наследование.


var MyClass = class [className] [extends] {
  // class body


Выражение class имеет схожий сиснтаксис с объявлением class. Однако в выражениях класса можно опустить имя класса ("binding identifier"), which you can't with class statements. Additionally, class expressions allow you to redefine/re-declare classes and don't throw any type errors like class declaration. The constructor property is optional. And, typeof the classes generated using this keyword will always be "functions".

Just like with class statements, the class body of class expressions is executed in strict mode.

'use strict';
var Foo = class {}; // constructor property is optional
var Foo = class {}; // Re-declaration is allowed

typeof Foo; //returns "function"
typeof class {}; //returns "function"

Foo instanceof Object; // true
Foo instanceof Function; // true
class Foo {}; // Throws TypeError, doesn't allow re-declaration


A simple class expression

This is just a simple anonymous class expression which you can refer to using the variable "Foo".

var Foo = class {
  constructor() {}
  bar() {
    return "Hello World!";

var instance = new Foo();; // "Hello World!"; // "Foo"

Named class expressions

If you want to refer to the current class inside the class body, you can create a named class expression. This name is only visible in the scope of the class expression itself.

var Foo = class NamedFoo {
  constructor() {}
  whoIsThere() {
var bar = new Foo();
bar.whoIsThere(); // "NamedFoo"; // ReferenceError: NamedFoo is not defined; // "NamedFoo"


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
Определение 'Class definitions' в этой спецификации.
Стандарт Initial definition.
ECMAScript Latest Draft (ECMA-262)
Определение 'Class definitions' в этой спецификации.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!
Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 42.0 45 (45) ? ? ?
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support Нет 42.0 45.0 (45) ? ? ? 42.0

See also