From 33058f2b292b3a581333bdfb21b8f671898c5060 Mon Sep 17 00:00:00 2001
From: Peter Bengtsson <mail@peterbe.com>
Date: Tue, 8 Dec 2020 14:40:17 -0500
Subject: initial commit

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+---
+title: extends
+slug: Web/JavaScript/Reference/Classes/extends
+tags:
+  - Classes
+  - ECMAScript 2015
+  - JavaScript
+translation_of: Web/JavaScript/Reference/Classes/extends
+---
+<div>{{jsSidebar("Classes")}}</div>
+
+<p><strong><code>extends</code></strong>关键字用于<a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/class">类声明</a>或者<a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/class">类表达式</a>中，以创建一个类，该类是另一个类的子类。</p>
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox">class ChildClass extends ParentClass { ... }</pre>
+
+<h2 id="描述">描述</h2>
+
+<p><code>extends</code>关键字用来创建一个普通类或者内建对象的子类。</p>
+
+<p><font face="Open Sans, Arial, sans-serif">继承的</font><code>.prototype</code>必须是一个{{jsxref("Object")}} 或者 {{jsxref("null")}}。</p>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="使用_extends">使用 <code>extends</code></h3>
+
+<p>第一个例子是根据名为 <code style="font-style: normal;">Polygon</code> 类创建一个名为<code>Square</code>的类。这个例子是从这个<a href="https://googlechrome.github.io/samples/classes-es6/index.html">在线演示</a>中提取出来的。</p>
+
+<pre class="brush: js">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;
+  }
+}</pre>
+
+<h3 id="使用_extends与内置对象">使用 <code>extends</code>与内置对象</h3>
+
+<p>这个示例继承了内置的{{jsxref("Date")}}对象。这个例子是从这个<a href="https://googlechrome.github.io/samples/classes-es6/index.html">在线演示</a>中提取出来的。</p>
+
+<pre class="brush: js">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();
+  }
+}</pre>
+
+<h3 id="扩展_null">扩展 <code>null</code></h3>
+
+<p>可以像扩展普通类一样扩展{{jsxref("null")}}，但是新对象的原型将不会继承 {{jsxref("Object.prototype")}}。</p>
+
+<pre class="brush: js">class nullExtends extends null {
+  constructor() {}
+}
+
+Object.getPrototypeOf(nullExtends); // Function.prototype
+Object.getPrototypeOf(nullExtends.prototype) // null
+
+new nullExtends(); //ReferenceError: this is not defined</pre>
+
+<h2 id="标准">标准</h2>
+
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">Specification</th>
+   <th scope="col">Status</th>
+   <th scope="col">Comment</th>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES2015', '#sec-class-definitions', 'extends')}}</td>
+   <td>{{Spec2('ES2015')}}</td>
+   <td>Initial definition.</td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ESDraft', '#sec-class-definitions', 'extends')}}</td>
+   <td>{{Spec2('ESDraft')}}</td>
+   <td> </td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容">浏览器兼容</h2>
+
+<div class="hidden">The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out <a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
+
+<p>{{Compat("javascript.classes.extends")}}</p>
+
+<h2 id="扩展阅读">扩展阅读</h2>
+
+<ul>
+ <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes">类</a></li>
+ <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/super">super</a></li>
+</ul>
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