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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>