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
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | Safari on iOS | Samsung Internet | Node.js
`class` | Chrome Full support 42 | Edge Full support Yes | Firefox Full support 45 | IE No support No | Opera Full support 36 | Safari Full support 10.1 | WebView Android Full support 42 | Chrome Android Full support 42 | Edge Mobile Full support Yes | Firefox Android Full support 45 | Opera Android ? | Safari iOS Full support 10.1 | Samsung Internet Android Full support 4.0 | nodejs Full support 6.0.0
Array subclassing | Chrome Full support 43 | Edge ? | Firefox ? | IE No support No | Opera Full support 36 | Safari ? | WebView Android Full support 43 | Chrome Android Full support 43 | Edge Mobile ? | Firefox Android ? | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support 4.0 | nodejs ?
Allowed in sloppy mode | Chrome Full support 49 | Edge ? | Firefox ? | IE No support No | Opera Full support 36 | Safari ? | WebView Android Full support 49 | Chrome Android Full support 49 | Edge Mobile ? | Firefox Android ? | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support 5.0 | nodejs ?

### Legend

Full support
  Full support
No support
  No support
Compatibility unknown
  Compatibility unknown

## See also

- `function` declaration
- `class` expression
- Classes