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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>继承的<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>
<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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