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  25.     <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav>  <ul class="sidebar-links"><li><a href="/blog/pages/f344d070a1031ef7/" class="sidebar-link">ECMAScript 6 简介</a></li><li><a href="/blog/pages/c1edd70a6b7c7872/" class="sidebar-link">let 和 const 命令</a></li><li><a href="/blog/pages/b1ab10a62f7564da/" class="sidebar-link">变量的解构赋值</a></li><li><a href="/blog/pages/ca89eca8adeba5f4/" class="sidebar-link">字符串的扩展</a></li><li><a href="/blog/pages/a650b4a0ebfc9350/" class="sidebar-link">字符串的新增方法</a></li><li><a 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href="/blog/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/blog/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/blog/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/blog/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/blog/pages/83f8c3a0cd87dd83/" aria-current="page" class="active sidebar-link">Class 的继承</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/83f8c3a0cd87dd83/#简介" class="sidebar-link">简介</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/83f8c3a0cd87dd83/#object-getprototypeof" class="sidebar-link">Object.getPrototypeOf()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/83f8c3a0cd87dd83/#super-关键字" class="sidebar-link">super 关键字</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/83f8c3a0cd87dd83/#类的-prototype-属性和-proto-属性" class="sidebar-link">类的 prototype 属性和__proto__属性</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/83f8c3a0cd87dd83/#实例的-proto-属性" class="sidebar-link">实例的 __proto__ 属性</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/83f8c3a0cd87dd83/#原生构造函数的继承" class="sidebar-link">原生构造函数的继承</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/83f8c3a0cd87dd83/#mixin-模式的实现" class="sidebar-link">Mixin 模式的实现</a></li></ul></li><li><a href="/blog/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/blog/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">Class 的继承<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="class-的继承"><a href="#class-的继承" class="header-anchor">#</a> Class 的继承</h1> <h2 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h2> <p>Class 可以通过<code>extends</code>关键字实现继承，这比 ES5 的通过修改原型链实现继承，要清晰和方便很多。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    26. 

  26. <span class="token punctuation">}</span>
    27. 

  27. 

  28. <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    29. 

  29. <span class="token punctuation">}</span>
    30. 

  30. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div> <p>上面代码定义了一个<code>ColorPoint</code>类，该类通过<code>extends</code>关键字，继承了<code>Point</code>类的所有属性和方法。但是由于没有部署任何代码，所以这两个类完全一样，等于复制了一个<code>Point</code>类。下面，我们在<code>ColorPoint</code>内部加上代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    31. 

  31.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> color</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    32. 

  32.     <span class="token keyword">super</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 调用父类的constructor(x, y)</span>
    33. 

  33.     <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">=</span> color<span class="token punctuation">;</span>
    34. 

  34.   <span class="token punctuation">}</span>
    35. 

  35. 

  36.   <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    37. 

  37.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">+</span> <span class="token string">' '</span> <span class="token operator">+</span> <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 调用父类的toString()</span>
    38. 

  38.   <span class="token punctuation">}</span>
    39. 

  39. <span class="token punctuation">}</span>
    40. 

  40. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>constructor</code>方法和<code>toString</code>方法之中，都出现了<code>super</code>关键字，它在这里表示父类的构造函数，用来新建父类的<code>this</code>对象。</p> <p>**子类必须在<code>constructor</code>方法中调用<code>super</code>方法，否则新建实例时会报错。**这是因为子类自己的<code>this</code>对象，必须先通过父类的构造函数完成塑造，得到与父类同样的实例属性和方法，然后再对其进行加工，加上子类自己的实例属性和方法。<strong>如果不调用<code>super</code>方法，子类就得不到<code>this</code>对象。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span> <span class="token comment">/* ... */</span> <span class="token punctuation">}</span>
    41. 

  41. 

  42. <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    43. 

  43.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    44. 

  44.   <span class="token punctuation">}</span>
    45. 

  45. <span class="token punctuation">}</span>
    46. 

  46. 

  47. <span class="token keyword">let</span> cp <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ColorPoint</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    48. 

  48. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>ColorPoint</code>继承了父类<code>Point</code>，但是它的构造函数没有调用<code>super</code>方法，导致新建实例时报错。</p> <p>ES5 的继承，实质是先创造子类的实例对象<code>this</code>，然后再将父类的方法添加到<code>this</code>上面（<code>Parent.apply(this)</code>）。ES6 的继承机制完全不同，实质是先将父类实例对象的属性和方法，加到<code>this</code>上面（所以必须先调用<code>super</code>方法），然后再用子类的构造函数修改<code>this</code>。</p> <p>如果子类没有定义<code>constructor</code>方法，这个方法会被默认添加，代码如下。也就是说，不管有没有显式定义，任何一个子类都有<code>constructor</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    49. 

  49. <span class="token punctuation">}</span>
    50. 

  50. 

  51. <span class="token comment">// 等同于</span>
    52. 

  52. <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    53. 

  53.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    54. 

  54.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
    55. 

  55.   <span class="token punctuation">}</span>
    56. 

  56. <span class="token punctuation">}</span>
    57. 

  57. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>另一个需要注意的地方是，**在子类的构造函数中，只有调用<code>super</code>之后，才可以使用<code>this</code>关键字，否则会报错。**这是因为子类实例的构建，基于父类实例，只有<code>super</code>方法才能调用父类实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    58. 

  58.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    59. 

  59.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> x<span class="token punctuation">;</span>
    60. 

  60.     <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">=</span> y<span class="token punctuation">;</span>
    61. 

  61.   <span class="token punctuation">}</span>
    62. 

  62. <span class="token punctuation">}</span>
    63. 

  63. 

  64. <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    65. 

  65.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> color</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    66. 

  66.     <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">=</span> color<span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    67. 

  67.     <span class="token keyword">super</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span><span class="token punctuation">;</span>
    68. 

  68.     <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">=</span> color<span class="token punctuation">;</span> <span class="token comment">// 正确</span>
    69. 

  69.   <span class="token punctuation">}</span>
    70. 

  70. <span class="token punctuation">}</span>
    71. 

  71. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，子类的<code>constructor</code>方法没有调用<code>super</code>之前，就使用<code>this</code>关键字，结果报错，而放在<code>super</code>方法之后就是正确的。</p> <p>下面是生成子类实例的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> cp <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ColorPoint</span><span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">,</span> <span class="token string">'green'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    72. 

  72. 

  73. cp <span class="token keyword">instanceof</span> <span class="token class-name">ColorPoint</span> <span class="token comment">// true</span>
    74. 

  74. cp <span class="token keyword">instanceof</span> <span class="token class-name">Point</span> <span class="token comment">// true</span>
    75. 

  75. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，实例对象<code>cp</code>同时是<code>ColorPoint</code>和<code>Point</code>两个类的实例，这与 ES5 的行为完全一致。</p> <p>最后，父类的静态方法，也会被子类继承。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    76. 

  76.   <span class="token keyword">static</span> <span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    77. 

  77.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    78. 

  78.   <span class="token punctuation">}</span>
    79. 

  79. <span class="token punctuation">}</span>
    80. 

  80. 

  81. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    82. 

  82. <span class="token punctuation">}</span>
    83. 

  83. 

  84. <span class="token constant">B</span><span class="token punctuation">.</span><span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// hello world</span>
    85. 

  85. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>hello()</code>是<code>A</code>类的静态方法，<code>B</code>继承<code>A</code>，也继承了<code>A</code>的静态方法。</p> <h2 id="object-getprototypeof"><a href="#object-getprototypeof" class="header-anchor">#</a> Object.getPrototypeOf()</h2> <p><code>Object.getPrototypeOf</code>方法可以用来从子类上获取父类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>ColorPoint<span class="token punctuation">)</span> <span class="token operator">===</span> Point
    86. 

  86. <span class="token comment">// true</span>
    87. 

  87. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>因此，可以使用这个方法判断，一个类是否继承了另一个类。</p> <h2 id="super-关键字"><a href="#super-关键字" class="header-anchor">#</a> super 关键字</h2> <p><strong><code>super</code>这个关键字，既可以当作函数使用，也可以当作对象使用</strong>。在这两种情况下，它的用法完全不同。</p> <p>第一种情况，<code>super</code>作为函数调用时，代表父类的构造函数。ES6 要求，子类的构造函数必须执行一次<code>super</code>函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    88. 

  88. 

  89. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    90. 

  90.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    91. 

  91.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    92. 

  92.   <span class="token punctuation">}</span>
    93. 

  93. <span class="token punctuation">}</span>
    94. 

  94. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，子类<code>B</code>的构造函数之中的<code>super()</code>，代表调用父类的构造函数。这是必须的，否则 JavaScript 引擎会报错。</p> <p>注意，<strong><code>super</code>虽然代表了父类<code>A</code>的构造函数，但是返回的是子类<code>B</code>的实例</strong>，即<code>super</code>内部的<code>this</code>指的是<code>B</code>的实例，因此<code>super()</code>在这里相当于<code>A.prototype.constructor.call(this)</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    95. 

  95.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    96. 

  96.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">new</span><span class="token punctuation">.</span>target<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
    97. 

  97.   <span class="token punctuation">}</span>
    98. 

  98. <span class="token punctuation">}</span>
    99. 

  99. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    100. 

  100.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    101. 

  101.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    102. 

  102.   <span class="token punctuation">}</span>
    103. 

  103. <span class="token punctuation">}</span>
    104. 

  104. <span class="token keyword">new</span> <span class="token class-name">A</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// A</span>
    105. 

  105. <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// B</span>
    106. 

  106. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，<code>new.target</code>指向当前正在执行的函数。可以看到，在<code>super()</code>执行时，它指向的是子类<code>B</code>的构造函数，而不是父类<code>A</code>的构造函数。也就是说，<code>super()</code>内部的<code>this</code>指向的是<code>B</code>。</p> <p><strong>作为函数时，<code>super()</code>只能用在子类的构造函数之中，用在其他地方就会报错。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    107. 

  107. 

  108. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    109. 

  109.   <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    110. 

  110.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错</span>
    111. 

  111.   <span class="token punctuation">}</span>
    112. 

  112. <span class="token punctuation">}</span>
    113. 

  113. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>super()</code>用在<code>B</code>类的<code>m</code>方法之中，就会造成语法错误。</p> <p>第二种情况，<strong><code>super</code>作为对象时，在普通方法中，指向父类的原型对象；在静态方法中，指向父类。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    114. 

  114.   <span class="token function">p</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    115. 

  115.     <span class="token keyword">return</span> <span class="token number">2</span><span class="token punctuation">;</span>
    116. 

  116.   <span class="token punctuation">}</span>
    117. 

  117. <span class="token punctuation">}</span>
    118. 

  118. 

  119. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    120. 

  120.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    121. 

  121.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    122. 

  122.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">p</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 2</span>
    123. 

  123.   <span class="token punctuation">}</span>
    124. 

  124. <span class="token punctuation">}</span>
    125. 

  125. 

  126. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    127. 

  127. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，子类<code>B</code>当中的<code>super.p()</code>，就是将<code>super</code>当作一个对象使用。这时，<code>super</code>在普通方法之中，指向<code>A.prototype</code>，所以<code>super.p()</code>就相当于<code>A.prototype.p()</code>。</p> <p>这里需要注意，由于<code>super</code>指向父类的原型对象，所以定义在父类实例上的方法或属性，是无法通过<code>super</code>调用的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    128. 

  128.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    129. 

  129.     <span class="token keyword">this</span><span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// 这是定于在实例上的属性</span>
    130. 

  130.   <span class="token punctuation">}</span>
    131. 

  131. <span class="token punctuation">}</span>
    132. 

  132. 

  133. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    134. 

  134.   <span class="token keyword">get</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    135. 

  135.     <span class="token keyword">return</span> <span class="token keyword">super</span><span class="token punctuation">.</span>p<span class="token punctuation">;</span>
    136. 

  136.   <span class="token punctuation">}</span>
    137. 

  137. <span class="token punctuation">}</span>
    138. 

  138. 

  139. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    140. 

  140. b<span class="token punctuation">.</span>m <span class="token comment">// undefined</span>
    141. 

  141. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，<code>p</code>是父类<code>A</code>实例的属性，<code>super.p</code>就引用不到它。</p> <p>如果属性定义在父类的原型对象上，<code>super</code>就可以取到。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    142. 

  142. <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    143. 

  143. 

  144. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    145. 

  145.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    146. 

  146.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    147. 

  147.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">super</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span> <span class="token comment">// 2</span>
    148. 

  148.   <span class="token punctuation">}</span>
    149. 

  149. <span class="token punctuation">}</span>
    150. 

  150. 

  151. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    152. 

  152. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，属性<code>x</code>是定义在<code>A.prototype</code>上面的，所以<code>super.x</code>可以取到它的值。</p> <p>ES6 规定，<strong>在子类普通方法中通过<code>super</code>调用父类的方法时，方法内部的<code>this</code>指向当前的子类实例</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    153. 

  153.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    154. 

  154.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    155. 

  155.   <span class="token punctuation">}</span>
    156. 

  156.   <span class="token function">print</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    157. 

  157.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    158. 

  158.   <span class="token punctuation">}</span>
    159. 

  159. <span class="token punctuation">}</span>
    160. 

  160. 

  161. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    162. 

  162.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    163. 

  163.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    164. 

  164.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    165. 

  165.   <span class="token punctuation">}</span>
    166. 

  166.   <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    167. 

  167.     <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    168. 

  168.   <span class="token punctuation">}</span>
    169. 

  169. <span class="token punctuation">}</span>
    170. 

  170. 

  171. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    172. 

  172. b<span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    173. 

  173. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>上面代码中，<code>super.print()</code>虽然调用的是<code>A.prototype.print()</code>，但是<code>A.prototype.print()</code>内部的<code>this</code>指向子类<code>B</code>的实例，导致输出的是<code>2</code>，而不是<code>1</code>。也就是说，实际上执行的是<code>super.print.call(this)</code>。</p> <p>由于<code>this</code>指向子类实例，所以如果通过<code>super</code>对某个属性赋值，这时<code>super</code>就是<code>this</code>，赋值的属性会变成子类实例的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    174. 

  174.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    175. 

  175.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    176. 

  176.   <span class="token punctuation">}</span>
    177. 

  177. <span class="token punctuation">}</span>
    178. 

  178. 

  179. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    180. 

  180.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    181. 

  181.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    182. 

  182.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    183. 

  183.     <span class="token keyword">super</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    184. 

  184.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">super</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
    185. 

  185.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    186. 

  186.   <span class="token punctuation">}</span>
    187. 

  187. <span class="token punctuation">}</span>
    188. 

  188. 

  189. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    190. 

  190. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，<code>super.x</code>赋值为<code>3</code>，这时等同于对<code>this.x</code>赋值为<code>3</code>。而当读取<code>super.x</code>的时候，读的是<code>A.prototype.x</code>，所以返回<code>undefined</code>。</p> <p><strong>如果<code>super</code>作为对象，用在静态方法之中，这时<code>super</code>将指向父类，而不是父类的原型对象。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Parent</span> <span class="token punctuation">{</span>
    191. 

  191.   <span class="token keyword">static</span> <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    192. 

  192.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'static'</span><span class="token punctuation">,</span> msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
    193. 

  193.   <span class="token punctuation">}</span>
    194. 

  194. 

  195.   <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    196. 

  196.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'instance'</span><span class="token punctuation">,</span> msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
    197. 

  197.   <span class="token punctuation">}</span>
    198. 

  198. <span class="token punctuation">}</span>
    199. 

  199. 

  200. <span class="token keyword">class</span> <span class="token class-name">Child</span> <span class="token keyword">extends</span> <span class="token class-name">Parent</span> <span class="token punctuation">{</span>
    201. 

  201.   <span class="token keyword">static</span> <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    202. 

  202.     <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">myMethod</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
    203. 

  203.   <span class="token punctuation">}</span>
    204. 

  204. 

  205.   <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    206. 

  206.     <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">myMethod</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
    207. 

  207.   <span class="token punctuation">}</span>
    208. 

  208. <span class="token punctuation">}</span>
    209. 

  209. 

  210. Child<span class="token punctuation">.</span><span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// static 1</span>
    211. 

  211. 

  212. <span class="token keyword">var</span> child <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Child</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    213. 

  213. child<span class="token punctuation">.</span><span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// instance 2</span>
    214. 

  214. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br></div></div><p>上面代码中，<code>super</code>在静态方法之中指向父类，在普通方法之中指向父类的原型对象。</p> <p>另外，<strong>在子类的静态方法中通过<code>super</code>调用父类的方法时，方法内部的<code>this</code>指向当前的子类，而不是子类的实例。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    215. 

  215.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    216. 

  216.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    217. 

  217.   <span class="token punctuation">}</span>
    218. 

  218.   <span class="token keyword">static</span> <span class="token function">print</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    219. 

  219.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    220. 

  220.   <span class="token punctuation">}</span>
    221. 

  221. <span class="token punctuation">}</span>
    222. 

  222. 

  223. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    224. 

  224.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    225. 

  225.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    226. 

  226.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    227. 

  227.   <span class="token punctuation">}</span>
    228. 

  228.   <span class="token keyword">static</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    229. 

  229.     <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    230. 

  230.   <span class="token punctuation">}</span>
    231. 

  231. <span class="token punctuation">}</span>
    232. 

  232. 

  233. <span class="token constant">B</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    234. 

  234. <span class="token constant">B</span><span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    235. 

  235. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>上面代码中，静态方法<code>B.m</code>里面，<code>super.print</code>指向父类的静态方法。这个方法里面的<code>this</code>指向的是<code>B</code>，而不是<code>B</code>的实例。</p> <p>注意，使用<code>super</code>的时候，必须显式指定是作为函数、还是作为对象使用，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    236. 

  236. 

  237. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    238. 

  238.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    239. 

  239.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    240. 

  240.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">super</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错</span>
    241. 

  241.   <span class="token punctuation">}</span>
    242. 

  242. <span class="token punctuation">}</span>
    243. 

  243. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>console.log(super)</code>当中的<code>super</code>，无法看出是作为函数使用，还是作为对象使用，所以 JavaScript 引擎解析代码的时候就会报错。这时，如果能清晰地表明<code>super</code>的数据类型，就不会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    244. 

  244. 

  245. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    246. 

  246.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    247. 

  247.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    248. 

  248.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">instanceof</span> <span class="token class-name">B</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    249. 

  249.   <span class="token punctuation">}</span>
    250. 

  250. <span class="token punctuation">}</span>
    251. 

  251. 

  252. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    253. 

  253. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>super.valueOf()</code>表明<code>super</code>是一个对象，因此就不会报错。同时，由于<code>super</code>使得<code>this</code>指向<code>B</code>的实例，所以<code>super.valueOf()</code>返回的是一个<code>B</code>的实例。</p> <p>最后，由于对象总是继承其他对象的，所以可以在任意一个对象中，使用<code>super</code>关键字。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    254. 

  254.   <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    255. 

  255.     <span class="token keyword">return</span> <span class="token string">&quot;MyObject: &quot;</span> <span class="token operator">+</span> <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    256. 

  256.   <span class="token punctuation">}</span>
    257. 

  257. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    258. 

  258. 

  259. obj<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// MyObject: [object Object]</span>
    260. 

  260. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h2 id="类的-prototype-属性和-proto-属性"><a href="#类的-prototype-属性和-proto-属性" class="header-anchor">#</a> 类的 prototype 属性和__proto__属性</h2> <p>大多数浏览器的 ES5 实现之中，每一个对象都有<code>__proto__</code>属性，指向对应的构造函数的<code>prototype</code>属性。Class 作为构造函数的语法糖，同时有<code>prototype</code>属性和<code>__proto__</code>属性，因此同时存在两条继承链。</p> <p>（1）子类的<code>__proto__</code>属性，表示构造函数的继承，总是指向父类。</p> <p>（2）子类<code>prototype</code>属性的<code>__proto__</code>属性，表示方法的继承，总是指向父类的<code>prototype</code>属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    261. 

  261. <span class="token punctuation">}</span>
    262. 

  262. 

  263. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    264. 

  264. <span class="token punctuation">}</span>
    265. 

  265. 

  266. <span class="token constant">B</span><span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token constant">A</span> <span class="token comment">// true</span>
    267. 

  267. <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
    268. 

  268. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，子类<code>B</code>的<code>__proto__</code>属性指向父类<code>A</code>，子类<code>B</code>的<code>prototype</code>属性的<code>__proto__</code>属性指向父类<code>A</code>的<code>prototype</code>属性。</p> <p>这样的结果是因为，类的继承是按照下面的模式实现的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    269. 

  269. <span class="token punctuation">}</span>
    270. 

  270. 

  271. <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token punctuation">{</span>
    272. 

  272. <span class="token punctuation">}</span>
    273. 

  273. 

  274. <span class="token comment">// B 的实例继承 A 的实例</span>
    275. 

  275. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
    276. 

  276. 

  277. <span class="token comment">// B 继承 A 的静态属性</span>
    278. 

  278. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token constant">B</span><span class="token punctuation">,</span> <span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    279. 

  279. 

  280. <span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    281. 

  281. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>《对象的扩展》一章给出过<code>Object.setPrototypeOf</code>方法的实现。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function-variable function">setPrototypeOf</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> proto</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    282. 

  282.   obj<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> proto<span class="token punctuation">;</span>
    283. 

  283.   <span class="token keyword">return</span> obj<span class="token punctuation">;</span>
    284. 

  284. <span class="token punctuation">}</span>
    285. 

  285. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>因此，就得到了上面的结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
    286. 

  286. <span class="token comment">// 等同于</span>
    287. 

  287. <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">;</span>
    288. 

  288. 

  289. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token constant">B</span><span class="token punctuation">,</span> <span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    290. 

  290. <span class="token comment">// 等同于</span>
    291. 

  291. <span class="token constant">B</span><span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> <span class="token constant">A</span><span class="token punctuation">;</span>
    292. 

  292. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>这两条继承链，可以这样理解：作为一个对象，子类（<code>B</code>）的原型（<code>__proto__</code>属性）是父类（<code>A</code>）；作为一个构造函数，子类（<code>B</code>）的原型对象（<code>prototype</code>属性）是父类的原型对象（<code>prototype</code>属性）的实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token class-name">B</span><span class="token punctuation">.</span>prototype <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
    293. 

  293. <span class="token comment">// 等同于</span>
    294. 

  294. <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">;</span>
    295. 

  295. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>extends</code>关键字后面可以跟多种类型的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    296. 

  296. <span class="token punctuation">}</span>
    297. 

  297. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码的<code>A</code>，只要是一个有<code>prototype</code>属性的函数，就能被<code>B</code>继承。由于函数都有<code>prototype</code>属性（除了<code>Function.prototype</code>函数），因此<code>A</code>可以是任意函数。</p> <p>下面，讨论两种情况。第一种，子类继承<code>Object</code>类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token keyword">extends</span> <span class="token class-name">Object</span> <span class="token punctuation">{</span>
    298. 

  298. <span class="token punctuation">}</span>
    299. 

  299. 

  300. <span class="token constant">A</span><span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> Object <span class="token comment">// true</span>
    301. 

  301. <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
    302. 

  302. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>这种情况下，<code>A</code>其实就是构造函数<code>Object</code>的复制，<code>A</code>的实例就是<code>Object</code>的实例。</p> <p>第二种情况，不存在任何继承。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
    303. 

  303. <span class="token punctuation">}</span>
    304. 

  304. 

  305. <span class="token constant">A</span><span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">Function</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
    306. 

  306. <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
    307. 

  307. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>这种情况下，<code>A</code>作为一个基类（即不存在任何继承），就是一个普通函数，所以直接继承<code>Function.prototype</code>。但是，<code>A</code>调用后返回一个空对象（即<code>Object</code>实例），所以<code>A.prototype.__proto__</code>指向构造函数（<code>Object</code>）的<code>prototype</code>属性。</p> <h3 id="实例的-proto-属性"><a href="#实例的-proto-属性" class="header-anchor">#</a> 实例的 __proto__ 属性</h3> <p>子类实例的<code>__proto__</code>属性的<code>__proto__</code>属性，指向父类实例的<code>__proto__</code>属性。也就是说，子类的原型的原型，是父类的原型。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Point</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    308. 

  308. <span class="token keyword">var</span> p2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ColorPoint</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token string">'red'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    309. 

  309. 

  310. p2<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> p1<span class="token punctuation">.</span>__proto__ <span class="token comment">// false</span>
    311. 

  311. p2<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> p1<span class="token punctuation">.</span>__proto__ <span class="token comment">// true</span>
    312. 

  312. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>ColorPoint</code>继承了<code>Point</code>，导致前者原型的原型是后者的原型。</p> <p>因此，通过子类实例的<code>__proto__.__proto__</code>属性，可以修改父类实例的行为。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>p2<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span><span class="token function-variable function">printName</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    313. 

  313.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Ha'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    314. 

  314. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    315. 

  315. 

  316. p1<span class="token punctuation">.</span><span class="token function">printName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Ha&quot;</span>
    317. 

  317. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码在<code>ColorPoint</code>的实例<code>p2</code>上向<code>Point</code>类添加方法，结果影响到了<code>Point</code>的实例<code>p1</code>。</p> <h2 id="原生构造函数的继承"><a href="#原生构造函数的继承" class="header-anchor">#</a> 原生构造函数的继承</h2> <p>原生构造函数是指语言内置的构造函数，通常用来生成数据结构。ECMAScript 的原生构造函数大致有下面这些。</p> <ul><li>Boolean()</li> <li>Number()</li> <li>String()</li> <li>Array()</li> <li>Date()</li> <li>Function()</li> <li>RegExp()</li> <li>Error()</li> <li>Object()</li></ul> <p>以前，这些原生构造函数是无法继承的，比如，不能自己定义一个<code>Array</code>的子类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">MyArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    318. 

  318.   <span class="token function">Array</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
    319. 

  319. <span class="token punctuation">}</span>
    320. 

  320. 

  321. <span class="token class-name">MyArray</span><span class="token punctuation">.</span>prototype <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    322. 

  322.   <span class="token literal-property property">constructor</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    323. 

  323.     <span class="token literal-property property">value</span><span class="token operator">:</span> MyArray<span class="token punctuation">,</span>
    324. 

  324.     <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    325. 

  325.     <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    326. 

  326.     <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span>
    327. 

  327.   <span class="token punctuation">}</span>
    328. 

  328. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    329. 

  329. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码定义了一个继承 Array 的<code>MyArray</code>类。但是，这个类的行为与<code>Array</code>完全不一致。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> colors <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    330. 

  330. colors<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token string">&quot;red&quot;</span><span class="token punctuation">;</span>
    331. 

  331. colors<span class="token punctuation">.</span>length  <span class="token comment">// 0</span>
    332. 

  332. 

  333. colors<span class="token punctuation">.</span>length <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    334. 

  334. colors<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span>  <span class="token comment">// &quot;red&quot;</span>
    335. 

  335. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>之所以会发生这种情况，是因为子类无法获得原生构造函数的内部属性，通过<code>Array.apply()</code>或者分配给原型对象都不行。原生构造函数会忽略<code>apply</code>方法传入的<code>this</code>，也就是说，原生构造函数的<code>this</code>无法绑定，导致拿不到内部属性。</p> <p>ES5 是先新建子类的实例对象<code>this</code>，再将父类的属性添加到子类上，由于父类的内部属性无法获取，导致无法继承原生的构造函数。比如，<code>Array</code>构造函数有一个内部属性<code>[[DefineOwnProperty]]</code>，用来定义新属性时，更新<code>length</code>属性，这个内部属性无法在子类获取，导致子类的<code>length</code>属性行为不正常。</p> <p>下面的例子中，我们想让一个普通对象继承<code>Error</code>对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> e <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    336. 

  336. 

  337. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span><span class="token function">Error</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span>
    338. 

  338. <span class="token comment">// [ 'stack' ]</span>
    339. 

  339. 

  340. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span>
    341. 

  341. <span class="token comment">// []</span>
    342. 

  342. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，我们想通过<code>Error.call(e)</code>这种写法，让普通对象<code>e</code>具有<code>Error</code>对象的实例属性。但是，<code>Error.call()</code>完全忽略传入的第一个参数，而是返回一个新对象，<code>e</code>本身没有任何变化。这证明了<code>Error.call(e)</code>这种写法，无法继承原生构造函数。</p> <p>ES6 允许继承原生构造函数定义子类，因为 ES6 是先新建父类的实例对象<code>this</code>，然后再用子类的构造函数修饰<code>this</code>，使得父类的所有行为都可以继承。下面是一个继承<code>Array</code>的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">MyArray</span> <span class="token keyword">extends</span> <span class="token class-name">Array</span> <span class="token punctuation">{</span>
    343. 

  343.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    344. 

  344.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
    345. 

  345.   <span class="token punctuation">}</span>
    346. 

  346. <span class="token punctuation">}</span>
    347. 

  347. 

  348. <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    349. 

  349. arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">12</span><span class="token punctuation">;</span>
    350. 

  350. arr<span class="token punctuation">.</span>length <span class="token comment">// 1</span>
    351. 

  351. 

  352. arr<span class="token punctuation">.</span>length <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    353. 

  353. arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token comment">// undefined</span>
    354. 

  354. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码定义了一个<code>MyArray</code>类，继承了<code>Array</code>构造函数，因此就可以从<code>MyArray</code>生成数组的实例。这意味着，ES6 可以自定义原生数据结构（比如<code>Array</code>、<code>String</code>等）的子类，这是 ES5 无法做到的。</p> <p>上面这个例子也说明，<code>extends</code>关键字不仅可以用来继承类，还可以用来继承原生的构造函数。因此可以在原生数据结构的基础上，定义自己的数据结构。下面就是定义了一个带版本功能的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">VersionedArray</span> <span class="token keyword">extends</span> <span class="token class-name">Array</span> <span class="token punctuation">{</span>
    355. 

  355.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    356. 

  356.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    357. 

  357.     <span class="token keyword">this</span><span class="token punctuation">.</span>history <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    358. 

  358.   <span class="token punctuation">}</span>
    359. 

  359.   <span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    360. 

  360.     <span class="token keyword">this</span><span class="token punctuation">.</span>history<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    361. 

  361.   <span class="token punctuation">}</span>
    362. 

  362.   <span class="token function">revert</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    363. 

  363.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">.</span>length<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token keyword">this</span><span class="token punctuation">.</span>history<span class="token punctuation">[</span><span class="token keyword">this</span><span class="token punctuation">.</span>history<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    364. 

  364.   <span class="token punctuation">}</span>
    365. 

  365. <span class="token punctuation">}</span>
    366. 

  366. 

  367. <span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">VersionedArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    368. 

  368. 

  369. x<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    370. 

  370. x<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    371. 

  371. x <span class="token comment">// [1, 2]</span>
    372. 

  372. x<span class="token punctuation">.</span>history <span class="token comment">// [[]]</span>
    373. 

  373. 

  374. x<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    375. 

  375. x<span class="token punctuation">.</span>history <span class="token comment">// [[], [1, 2]]</span>
    376. 

  376. 

  377. x<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    378. 

  378. x <span class="token comment">// [1, 2, 3]</span>
    379. 

  379. x<span class="token punctuation">.</span>history <span class="token comment">// [[], [1, 2]]</span>
    380. 

  380. 

  381. x<span class="token punctuation">.</span><span class="token function">revert</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    382. 

  382. x <span class="token comment">// [1, 2]</span>
    383. 

  383. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br></div></div><p>上面代码中，<code>VersionedArray</code>会通过<code>commit</code>方法，将自己的当前状态生成一个版本快照，存入<code>history</code>属性。<code>revert</code>方法用来将数组重置为最新一次保存的版本。除此之外，<code>VersionedArray</code>依然是一个普通数组，所有原生的数组方法都可以在它上面调用。</p> <p>下面是一个自定义<code>Error</code>子类的例子，可以用来定制报错时的行为。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">ExtendableError</span> <span class="token keyword">extends</span> <span class="token class-name">Error</span> <span class="token punctuation">{</span>
    384. 

  384.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">message</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    385. 

  385.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    386. 

  386.     <span class="token keyword">this</span><span class="token punctuation">.</span>message <span class="token operator">=</span> message<span class="token punctuation">;</span>
    387. 

  387.     <span class="token keyword">this</span><span class="token punctuation">.</span>stack <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span>stack<span class="token punctuation">;</span>
    388. 

  388.     <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>constructor<span class="token punctuation">.</span>name<span class="token punctuation">;</span>
    389. 

  389.   <span class="token punctuation">}</span>
    390. 

  390. <span class="token punctuation">}</span>
    391. 

  391. 

  392. <span class="token keyword">class</span> <span class="token class-name">MyError</span> <span class="token keyword">extends</span> <span class="token class-name">ExtendableError</span> <span class="token punctuation">{</span>
    393. 

  393.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">m</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    394. 

  394.     <span class="token keyword">super</span><span class="token punctuation">(</span>m<span class="token punctuation">)</span><span class="token punctuation">;</span>
    395. 

  395.   <span class="token punctuation">}</span>
    396. 

  396. <span class="token punctuation">}</span>
    397. 

  397. 

  398. <span class="token keyword">var</span> myerror <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyError</span><span class="token punctuation">(</span><span class="token string">'ll'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    399. 

  399. myerror<span class="token punctuation">.</span>message <span class="token comment">// &quot;ll&quot;</span>
    400. 

  400. myerror <span class="token keyword">instanceof</span> <span class="token class-name">Error</span> <span class="token comment">// true</span>
    401. 

  401. myerror<span class="token punctuation">.</span>name <span class="token comment">// &quot;MyError&quot;</span>
    402. 

  402. myerror<span class="token punctuation">.</span>stack
    403. 

  403. <span class="token comment">// Error</span>
    404. 

  404. <span class="token comment">//     at MyError.ExtendableError</span>
    405. 

  405. <span class="token comment">//     ...</span>
    406. 

  406. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><p>注意，继承<code>Object</code>的子类，有一个<a href="http://stackoverflow.com/questions/36203614/super-does-not-pass-arguments-when-instantiating-a-class-extended-from-object" target="_blank" rel="noopener noreferrer">行为差异<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">NewObj</span> <span class="token keyword">extends</span> <span class="token class-name">Object</span><span class="token punctuation">{</span>
    407. 

  407.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    408. 

  408.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token operator">...</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
    409. 

  409.   <span class="token punctuation">}</span>
    410. 

  410. <span class="token punctuation">}</span>
    411. 

  411. <span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NewObj</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">attr</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    412. 

  412. o<span class="token punctuation">.</span>attr <span class="token operator">===</span> <span class="token boolean">true</span>  <span class="token comment">// false</span>
    413. 

  413. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>NewObj</code>继承了<code>Object</code>，但是无法通过<code>super</code>方法向父类<code>Object</code>传参。这是因为 ES6 改变了<code>Object</code>构造函数的行为，一旦发现<code>Object</code>方法不是通过<code>new Object()</code>这种形式调用，ES6 规定<code>Object</code>构造函数会忽略参数。</p> <h2 id="mixin-模式的实现"><a href="#mixin-模式的实现" class="header-anchor">#</a> Mixin 模式的实现</h2> <p>Mixin 指的是多个对象合成一个新的对象，新对象具有各个组成成员的接口。它的最简单实现如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span>
    414. 

  414.   <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token string">'a'</span>
    415. 

  415. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    416. 

  416. <span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token punctuation">{</span>
    417. 

  417.   <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token string">'b'</span>
    418. 

  418. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    419. 

  419. <span class="token keyword">const</span> c <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token operator">...</span>a<span class="token punctuation">,</span> <span class="token operator">...</span>b<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// {a: 'a', b: 'b'}</span>
    420. 

  420. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>c</code>对象是<code>a</code>对象和<code>b</code>对象的合成，具有两者的接口。</p> <p>下面是一个更完备的实现，将多个类的接口“混入”（mix in）另一个类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">mix</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>mixins</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    421. 

  421.   <span class="token keyword">class</span> <span class="token class-name">Mix</span> <span class="token punctuation">{</span>
    422. 

  422.     <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    423. 

  423.       <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> mixin <span class="token keyword">of</span> mixins<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    424. 

  424.         <span class="token function">copyProperties</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">mixin</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 拷贝实例属性</span>
    425. 

  425.       <span class="token punctuation">}</span>
    426. 

  426.     <span class="token punctuation">}</span>
    427. 

  427.   <span class="token punctuation">}</span>
    428. 

  428. 

  429.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> mixin <span class="token keyword">of</span> mixins<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    430. 

  430.     <span class="token function">copyProperties</span><span class="token punctuation">(</span>Mix<span class="token punctuation">,</span> mixin<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 拷贝静态属性</span>
    431. 

  431.     <span class="token function">copyProperties</span><span class="token punctuation">(</span><span class="token class-name">Mix</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> mixin<span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 拷贝原型属性</span>
    432. 

  432.   <span class="token punctuation">}</span>
    433. 

  433. 

  434.   <span class="token keyword">return</span> Mix<span class="token punctuation">;</span>
    435. 

  435. <span class="token punctuation">}</span>
    436. 

  436. 

  437. <span class="token keyword">function</span> <span class="token function">copyProperties</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> source</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    438. 

  438.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>source<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    439. 

  439.     <span class="token keyword">if</span> <span class="token punctuation">(</span> key <span class="token operator">!==</span> <span class="token string">'constructor'</span>
    440. 

  440.       <span class="token operator">&amp;&amp;</span> key <span class="token operator">!==</span> <span class="token string">'prototype'</span>
    441. 

  441.       <span class="token operator">&amp;&amp;</span> key <span class="token operator">!==</span> <span class="token string">'name'</span>
    442. 

  442.     <span class="token punctuation">)</span> <span class="token punctuation">{</span>
    443. 

  443.       <span class="token keyword">let</span> desc <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>source<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    444. 

  444.       Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> desc<span class="token punctuation">)</span><span class="token punctuation">;</span>
    445. 

  445.     <span class="token punctuation">}</span>
    446. 

  446.   <span class="token punctuation">}</span>
    447. 

  447. <span class="token punctuation">}</span>
    448. 

  448. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br></div></div><p>上面代码的<code>mix</code>函数，可以将多个对象合成为一个类。使用的时候，只要继承这个类即可。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">DistributedEdit</span> <span class="token keyword">extends</span> <span class="token class-name">mix</span><span class="token punctuation">(</span>Loggable<span class="token punctuation">,</span> Serializable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    449. 

  449.   <span class="token comment">// ...</span>
    450. 

  450. <span class="token punctuation">}</span>
    451. 

  451. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/22.Class 的继承.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/e831e1593c82bbe0/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Class 的基本语法</div></a> <a href="/blog/pages/efe2fb04eb8ac5fb/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Module 的语法</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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  453.         <a href="/blog/pages/e831e1593c82bbe0/" class="prev">Class 的基本语法</a></span> <span class="next"><a href="/blog/pages/efe2fb04eb8ac5fb/">Module 的语法</a>→
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