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

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome 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 13Firefox Full support 45IE No support NoOpera Full support 36Safari Full support 9WebView Android ? Chrome Android Full support YesEdge Mobile Full support 13Firefox Android Full support 45Opera Android ? Safari iOS Full support 9Samsung Internet Android Full support Yesnodejs 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.

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最后编辑者: zhangchen,