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---
title: extends
slug: Web/JavaScript/Reference/Classes/extends
translation_of: Web/JavaScript/Reference/Classes/extends
---
<div>{{jsSidebar("Classes")}}</div>
<p><strong><code>extends</code></strong> 關鍵字被使用於<a href="/en-US/docs/Web/JavaScript/Reference/Statements/class">類別（class）宣告</a>或<a href="/en-US/docs/Web/JavaScript/Reference/Operators/class">類別（class）表達式</a>中來建立擴展的子類別 。</p>
<div>{{EmbedInteractiveExample("pages/js/classes-extends.html", "taller")}}</div>
<h2 id="語法">語法</h2>
<pre class="syntaxbox">class ChildClass extends ParentClass { ... }
</pre>
<h2 id="解釋">解釋</h2>
<p><code>extends</code> 關鍵字可用於建立一個自訂類別或內建類別的子類別。</p>
<p>其繼承之原型 <code>.prototype</code> 必須是 {{jsxref("Object")}} 或 {{jsxref("null")}}。</p>
<h2 id="範例">範例</h2>
<h3 id="使用_extends">使用 <code>extends</code></h3>
<p>第一個範例是根據 <code>Polygon</code>類別建立一個名為 <code>Square</code> 的子類別。此範例提取自<a href="https://googlechrome.github.io/samples/classes-es6/index.html">線上示例</a>。</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_於內建類別">使用 <code>extends</code> 於內建類別</h3>
<p>這個範例擴展了內建的 {{jsxref("Date")}} 類別。此範例提取自<a href="https://googlechrome.github.io/samples/classes-es6/index.html">線上範例</a>。</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="擴展_null">擴展 <code>null</code></h3>
<p>像擴展普通類別一樣擴展 {{jsxref("null")}}，但新對象的原型不會繼承 {{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="標準">標準</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="瀏覽器相容性">瀏覽器相容性</h2>
<p>{{Compat("javascript.classes.extends")}}</p>
<h2 id="參見">參見</h2>
<ul>
<li><a href="/zh-TW/docs/Web/JavaScript/Reference/Classes">Classes</a></li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/super">super</a></li>
</ul>
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