class

class 声明创建一个基于原型继承的具有给定名称的新类。

你也可以使用类表达式定义类。但是不同于类表达式,类声明不允许再次声明已经存在的类,否则将会抛出一个类型错误。

语法

class name [extends] {
  // class body
}

描述

和类表达式一样,类声明体在严格模式下运行。构造函数是可选的。

类声明不可以提升(这与函数声明不同)。

示例

声明一个类

在下面的例子中,我们首先定义一个名为Polygon的类,然后继承它来创建一个名为Square的类。注意,构造函数中使用的 super() 只能在构造函数中使用,并且必须在使用 this 关键字前调用。

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';
  }
}

重复定义类

重复声明一个类会引起类型错误。

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

若之前使用类表达式定义了一个类,则再次声明这个类同样会引起类型错误。

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

规范

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
Class definitions
Standard Initial definition.
ECMAScript 2016 (ECMA-262)
Class definitions
Standard  
ECMAScript 2017 (ECMA-262)
Class definitions
Standard  
ECMAScript (ECMA-262)
Class definitions
Living Standard  

浏览器兼容

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
classChrome 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 13Firefox Full support 45IE No support NoOpera 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 10.1WebView 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 45Opera 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 10.3Samsung 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.

See also