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 Latest Draft (ECMA-262) Class definitions |
Draft |
浏览器兼容
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out
https://github.com/mdn/browser-compat-data and send us a pull request.
Update compatibility data on GitHub
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
class |
Chrome Full support 49
|
Edge Full support 13 | Firefox Full support 45 | IE No support No | Opera Full support 36 | Safari Full support 10.1 | WebView Android Full support 49
|
Chrome Android Full support 49
|
Firefox Android Full support 45 | Opera Android ? | Safari iOS Full support 10.3 | Samsung Internet Android Full support 4.0
|
nodejs Full support 6.0.0 |
Legend
- Full support
- Full support
- No support
- No support
- Compatibility unknown
- Compatibility unknown
- See implementation notes.
- See implementation notes.