extends 關鍵字被使用於類別(class)宣告類別(class)表達式中來建立擴展的子類別 。


class ChildClass extends ParentClass { ... }


extends 關鍵字可用於建立一個自訂類別或內建類別的子類別。

其繼承之原型 .prototype 必須是 Object 或 null


使用 extends

第一個範例是根據 Polygon類別建立一個名為 Square 的子類別。此範例提取自線上示例

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 於內建類別

這個範例擴展了內建的 Date 類別。此範例提取自線上範例

class myDate extends Date {
  constructor() {

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

擴展 null

像擴展普通類別一樣擴展 null,但新對象的原型不會繼承 Object.prototype

class nullExtends extends null {
  constructor() {}

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

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


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'extends' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'extends' in that specification.


FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support4911345 No369
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support ? Yes1345 ?9 Yes

1. From Chrome 42 to 48 strict mode is required. Non-strict mode support can be enabled using the flag "Enable Experimental JavaScript".

2. From version 4.0.0: this feature is behind the --use_strict runtime flag.

3. From version 5.0.0: this feature is behind the --harmony runtime flag.



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