`extends`关键字用于类声明或者类表达式中，以创建一个类，该类是另一个类的子类。

## 语法

    class ChildClass extends ParentClass { ... }

## 描述

`extends`关键字用来创建一个普通类或者内建对象的子类。

继承的`.prototype`必须是一个`Object` 或者 `null`。

## 示例

### 使用 `extends`

第一个例子是根据名为 `Polygon` 类创建一个名为`Square`的类。这个例子是从这个在线演示中提取出来的。

    class Square extends Polygon {
      constructor(length) {
        // Here, it calls the parent class' constructor with lengths
        // provided for the Polygon's width and height
        super(length, length);
        // Note: In derived classes, super() must be called before you
        // can use 'this'. Leaving this out will cause a reference error.
        this.name = 'Square';
      }

      get area() {
        return this.height * this.width;
      }
    }

### 使用 `extends`与内置对象

这个示例继承了内置的`Date`对象。这个例子是从这个在线演示中提取出来的。

    class myDate extends Date {
      constructor() {
        super();
      }

      getFormattedDate() {
        var months = ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'];
        return this.getDate() + "-" + months[this.getMonth()] + "-" + this.getFullYear();
      }
    }

### 扩展 `null`

可以像扩展普通类一样扩展`null`，但是新对象的原型将不会继承 `Object.prototype`。

    class nullExtends extends null {
      constructor() {}
    }

    Object.getPrototypeOf(nullExtends); // Function.prototype
    Object.getPrototypeOf(nullExtends.prototype) // null

    new nullExtends(); //ReferenceError: this is not defined

## 标准

Specification | Status | Comment
--- | --- | ---
ECMAScript 2015 (6th Edition, ECMA-262) extends | Standard | Initial definition.
ECMAScript Latest Draft (ECMA-262) extends | 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
`extends` | Chrome Full support 49 Notes Full support 49 Notes Notes From Chrome 42 to 48 strict mode is required. Non-strict mode support can be enabled using the flag "Enable Experimental JavaScript". | Edge Full support 13 | Firefox Full support 45 | IE No support No | Opera Full support 36 | Safari Full support 9 | WebView Android ? | Chrome Android Full support Yes | Edge Mobile Full support 13 | Firefox Android Full support 45 | Opera Android ? | Safari iOS Full support 9 | Samsung Internet Android Full support Yes | nodejs Full support 6.0.0 Full support 6.0.0 Full support 4.0.0 Disabled Disabled From version 4.0.0: this feature is behind the `--use_strict` runtime flag. Full support 5.0.0 Disabled Disabled From version 5.0.0: this feature is behind the `--harmony` runtime flag.

### Legend

Full support
  Full support
No support
  No support
Compatibility unknown
  Compatibility unknown
See implementation notes.
  See implementation notes.
User must explicitly enable this feature.
  User must explicitly enable this feature.

## 扩展阅读

- 类
- super