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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 的基本语法<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="class-的基本语法"><a href="#class-的基本语法" class="header-anchor">#</a> Class 的基本语法</h1> <h2 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h2> <h3 id="类的由来"><a href="#类的由来" class="header-anchor">#</a> 类的由来</h3> <p>JavaScript 语言中，生成实例对象的传统方法是通过构造函数。下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Point</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>
    26. 

  26.   <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> x<span class="token punctuation">;</span>
    27. 

  27.   <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">=</span> y<span class="token punctuation">;</span>
    28. 

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

  29. 

  30. <span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">toString</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>
    31. 

  31.   <span class="token keyword">return</span> <span class="token string">'('</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">+</span> <span class="token string">', '</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">+</span> <span class="token string">')'</span><span class="token punctuation">;</span>
    32. 

  32. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    33. 

  33. 

  34. <span class="token keyword">var</span> p <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">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    35. 

  35. </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>上面这种写法跟传统的面向对象语言（比如 C++ 和 Java）差异很大，很容易让新学习这门语言的程序员感到困惑。</p> <p>ES6 提供了更接近传统语言的写法，引入了 Class（类）这个概念，作为对象的模板。通过<code>class</code>关键字，可以定义类。</p> <p>基本上，ES6 的<code>class</code>可以看作只是一个语法糖，它的绝大部分功能，ES5 都可以做到，新的<code>class</code>写法只是让对象原型的写法更加清晰、更像面向对象编程的语法而已。上面的代码用 ES6 的<code>class</code>改写，就是下面这样。</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>
    36. 

  36.   <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>
    37. 

  37.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> x<span class="token punctuation">;</span>
    38. 

  38.     <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">=</span> y<span class="token punctuation">;</span>
    39. 

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

  40. 

  41.   <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    42. 

  42.     <span class="token keyword">return</span> <span class="token string">'('</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">+</span> <span class="token string">', '</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">+</span> <span class="token string">')'</span><span class="token punctuation">;</span>
    43. 

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

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

  45. </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>上面代码定义了一个“类”，可以看到里面有一个<code>constructor</code>方法，这就是构造方法，而<code>this</code>关键字则代表实例对象。也就是说，ES5 的构造函数<code>Point</code>，对应 ES6 的<code>Point</code>类的构造方法。</p> <p><code>Point</code>类除了构造方法，还定义了一个<code>toString</code>方法。注意，<strong>定义“类”的方法的时候，前面不需要加上<code>function</code>这个关键字，直接把函数定义放进去了就可以了。另外，方法之间不需要逗号分隔，加了会报错。</strong></p> <p>ES6 的类，完全可以看作构造函数的另一种写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    46. 

  46.   <span class="token comment">// ...</span>
    47. 

  47. <span class="token punctuation">}</span>
    48. 

  48. 

  49. <span class="token keyword">typeof</span> Point <span class="token comment">// &quot;function&quot;</span>
    50. 

  50. Point <span class="token operator">===</span> <span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>constructor <span class="token comment">// true</span>
    51. 

  51. </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>上面代码表明，类的数据类型就是函数，类本身就指向构造函数。</p> <p>使用的时候，也是直接对类使用<code>new</code>命令，跟构造函数的用法完全一致。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Bar</span> <span class="token punctuation">{</span>
    52. 

  52.   <span class="token function">doStuff</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    53. 

  53.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'stuff'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    54. 

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

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

  56. 

  57. <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    58. 

  58. b<span class="token punctuation">.</span><span class="token function">doStuff</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;stuff&quot;</span>
    59. 

  59. </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>构造函数的<code>prototype</code>属性，在 ES6 的“类”上面继续存在。事实上，<strong>类的所有方法都定义在类的<code>prototype</code>属性上面。</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>
    60. 

  60.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    61. 

  61.     <span class="token comment">// ...</span>
    62. 

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

  63. 

  64.   <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    65. 

  65.     <span class="token comment">// ...</span>
    66. 

  66.   <span class="token punctuation">}</span>
    67. 

  67. 

  68.   <span class="token function">toValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    69. 

  69.     <span class="token comment">// ...</span>
    70. 

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

  71. <span class="token punctuation">}</span>
    72. 

  72. 

  73. <span class="token comment">// 等同于</span>
    74. 

  74. 

  75. <span class="token class-name">Point</span><span class="token punctuation">.</span>prototype <span class="token operator">=</span> <span class="token punctuation">{</span>
    76. 

  76.   <span class="token function">constructor</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>
    77. 

  77.   <span class="token function">toString</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>
    78. 

  78.   <span class="token function">toValue</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>
    79. 

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

  80. </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>在类的实例上面调用方法，其实就是调用原型上的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    81. 

  81. <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>
    82. 

  82. 

  83. b<span class="token punctuation">.</span>constructor <span class="token operator">===</span> <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>constructor <span class="token comment">// true</span>
    84. 

  84. </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，<code>b</code>是<code>B</code>类的实例，它的<code>constructor</code>方法就是<code>B</code>类原型的<code>constructor</code>方法。</p> <p>由于类的方法都定义在<code>prototype</code>对象上面，所以类的新方法可以添加在<code>prototype</code>对象上面。<strong><code>Object.assign</code>方法可以很方便地一次向类添加多个方法。</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>
    85. 

  85.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    86. 

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

  87.   <span class="token punctuation">}</span>
    88. 

  88. <span class="token punctuation">}</span>
    89. 

  89. 

  90. Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    91. 

  91.   <span class="token function">toString</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>
    92. 

  92.   <span class="token function">toValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>
    93. 

  93. <span class="token punctuation">}</span><span class="token punctuation">)</span><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><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p><code>prototype</code>对象的<code>constructor</code>属性，直接指向“类”的本身，这与 ES5 的行为是一致的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>constructor <span class="token operator">===</span> Point <span class="token comment">// true</span>
    95. 

  95. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>另外，<strong>类的内部所有定义的方法，都是不可枚举的</strong>（non-enumerable）。</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>
    96. 

  96.   <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>
    97. 

  97.     <span class="token comment">// ...</span>
    98. 

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

  99. 

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

  101.     <span class="token comment">// ...</span>
    102. 

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

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

  104. 

  105. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span>
    106. 

  106. <span class="token comment">// []</span>
    107. 

  107. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span><span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span>
    108. 

  108. <span class="token comment">// [&quot;constructor&quot;,&quot;toString&quot;]</span>
    109. 

  109. </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>toString</code>方法是<code>Point</code>类内部定义的方法，它是不可枚举的。这一点与 ES5 的行为不一致。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">Point</span> <span class="token operator">=</span> <span class="token keyword">function</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>
    110. 

  110.   <span class="token comment">// ...</span>
    111. 

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

  112. 

  113. <span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">toString</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>
    114. 

  114.   <span class="token comment">// ...</span>
    115. 

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

  116. 

  117. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span>
    118. 

  118. <span class="token comment">// [&quot;toString&quot;]</span>
    119. 

  119. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span><span class="token class-name">Point</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span>
    120. 

  120. <span class="token comment">// [&quot;constructor&quot;,&quot;toString&quot;]</span>
    121. 

  121. </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>上面代码采用 ES5 的写法，<code>toString</code>方法就是可枚举的。</p> <h3 id="constructor-方法"><a href="#constructor-方法" class="header-anchor">#</a> constructor 方法</h3> <p><code>constructor</code>方法是类的默认方法，<strong>通过<code>new</code>命令生成对象实例时，自动调用该方法</strong>。一个类必须有<code>constructor</code>方法，如果没有显式定义，一个空的<code>constructor</code>方法会被默认添加。</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>
    122. 

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

  123. 

  124. <span class="token comment">// 等同于</span>
    125. 

  125. <span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    126. 

  126.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    127. 

  127. <span class="token punctuation">}</span>
    128. 

  128. </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，定义了一个空的类<code>Point</code>，JavaScript 引擎会自动为它添加一个空的<code>constructor</code>方法。</p> <p><code>constructor</code>方法默认返回实例对象（即<code>this</code>），完全可以指定返回另外一个对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    129. 

  129.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    130. 

  130.     <span class="token keyword">return</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    131. 

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

  132. <span class="token punctuation">}</span>
    133. 

  133. 

  134. <span class="token keyword">new</span> <span class="token class-name">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">instanceof</span> <span class="token class-name">Foo</span>
    135. 

  135. <span class="token comment">// false</span>
    136. 

  136. </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>constructor</code>函数返回一个全新的对象，结果导致实例对象不是<code>Foo</code>类的实例。</p> <p><strong>类必须使用<code>new</code>调用，否则会报错</strong>。这是它跟普通构造函数的一个主要区别，后者不用<code>new</code>也可以执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    137. 

  137.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    138. 

  138.     <span class="token keyword">return</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    139. 

  139.   <span class="token punctuation">}</span>
    140. 

  140. <span class="token punctuation">}</span>
    141. 

  141. 

  142. <span class="token function">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    143. 

  143. <span class="token comment">// TypeError: Class constructor Foo cannot be invoked without 'new'</span>
    144. 

  144. </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><h3 id="类的实例"><a href="#类的实例" class="header-anchor">#</a> 类的实例</h3> <p>生成类的实例的写法，与 ES5 完全一样，也是使用<code>new</code>命令。前面说过，如果忘记加上<code>new</code>，像函数那样调用<code>Class</code>，将会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    145. 

  145.   <span class="token comment">// ...</span>
    146. 

  146. <span class="token punctuation">}</span>
    147. 

  147. 

  148. <span class="token comment">// 报错</span>
    149. 

  149. <span class="token keyword">var</span> point <span class="token operator">=</span> <span class="token function">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>
    150. 

  150. 

  151. <span class="token comment">// 正确</span>
    152. 

  152. <span class="token keyword">var</span> point <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>
    153. 

  153. </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>与 ES5 一样，实例的属性除非显式定义在其本身（即定义在<code>this</code>对象上），否则都是定义在原型上（即定义在<code>class</code>上）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">//定义类</span>
    154. 

  154. <span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    155. 

  155. 

  156.   <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> <span class="token comment">// constructor属性定义在原型上</span>
    157. 

  157.     <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> x<span class="token punctuation">;</span> <span class="token comment">// x属性定义在实例上</span>
    158. 

  158.     <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">=</span> y<span class="token punctuation">;</span> <span class="token comment">// y属性定义在实例上</span>
    159. 

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

  160. 

  161.   <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">// toString属性定义在原型上</span>
    162. 

  162.     <span class="token keyword">return</span> <span class="token string">'('</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">+</span> <span class="token string">', '</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">+</span> <span class="token string">')'</span><span class="token punctuation">;</span>
    163. 

  163.   <span class="token punctuation">}</span>
    164. 

  164. 

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

  166. 

  167. <span class="token keyword">var</span> point <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>
    168. 

  168. 

  169. point<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// (2, 3)</span>
    170. 

  170. 

  171. point<span class="token punctuation">.</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span><span class="token string">'x'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    172. 

  172. point<span class="token punctuation">.</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span><span class="token string">'y'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    173. 

  173. point<span class="token punctuation">.</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span><span class="token string">'toString'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    174. 

  174. point<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span><span class="token string">'toString'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    175. 

  175. </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></div></div><p>上面代码中，<code>x</code>和<code>y</code>都是实例对象<code>point</code>自身的属性（因为定义在<code>this</code>变量上），所以<code>hasOwnProperty</code>方法返回<code>true</code>，而<code>toString</code>是原型对象的属性（因为定义在<code>Point</code>类上），所以<code>hasOwnProperty</code>方法返回<code>false</code>。这些都与 ES5 的行为保持一致。</p> <p>与 ES5 一样，类的所有实例共享一个原型对象。</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>
    176. 

  176. <span class="token keyword">var</span> p2 <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">3</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    177. 

  177. 

  178. p1<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> p2<span class="token punctuation">.</span>__proto__
    179. 

  179. <span class="token comment">//true</span>
    180. 

  180. </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>p1</code>和<code>p2</code>都是<code>Point</code>的实例，它们的原型都是<code>Point.prototype</code>，所以<code>__proto__</code>属性是相等的。</p> <p>这也意味着，可以通过实例的<code>__proto__</code>属性为“类”添加方法。</p> <blockquote><p><code>__proto__</code> 并不是语言本身的特性，这是各大厂商具体实现时添加的私有属性，虽然目前很多现代浏览器的 JS 引擎中都提供了这个私有属性，但依旧不建议在生产中使用该属性，避免对环境产生依赖。生产环境中，我们可以使用 <code>Object.getPrototypeOf</code> 方法来获取实例对象的原型，然后再来为原型添加方法/属性。</p></blockquote> <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>
    181. 

  181. <span class="token keyword">var</span> p2 <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">3</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    182. 

  182. 

  183. p1<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> <span class="token keyword">return</span> <span class="token string">'Oops'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    184. 

  184. 

  185. 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;Oops&quot;</span>
    186. 

  186. p2<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;Oops&quot;</span>
    187. 

  187. 

  188. <span class="token keyword">var</span> p3 <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">4</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    189. 

  189. p3<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;Oops&quot;</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></div></div><p>上面代码在<code>p1</code>的原型上添加了一个<code>printName</code>方法，由于<code>p1</code>的原型就是<code>p2</code>的原型，因此<code>p2</code>也可以调用这个方法。而且，此后新建的实例<code>p3</code>也可以调用这个方法。这意味着，使用实例的<code>__proto__</code>属性改写原型，必须相当谨慎，不推荐使用，因为这会改变“类”的原始定义，影响到所有实例。</p> <h3 id="取值函数-getter-和存值函数-setter"><a href="#取值函数-getter-和存值函数-setter" class="header-anchor">#</a> 取值函数（getter）和存值函数（setter）</h3> <p>与 ES5 一样，在“类”的内部可以使用<code>get</code>和<code>set</code>关键字，对某个属性设置存值函数和取值函数，拦截该属性的存取行为。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">MyClass</span> <span class="token punctuation">{</span>
    191. 

  191.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    192. 

  192.     <span class="token comment">// ...</span>
    193. 

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

  194.   <span class="token keyword">get</span> <span class="token function">prop</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    195. 

  195.     <span class="token keyword">return</span> <span class="token string">'getter'</span><span class="token punctuation">;</span>
    196. 

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

  197.   <span class="token keyword">set</span> <span class="token function">prop</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    198. 

  198.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'setter: '</span><span class="token operator">+</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    199. 

  199.   <span class="token punctuation">}</span>
    200. 

  200. <span class="token punctuation">}</span>
    201. 

  201. 

  202. <span class="token keyword">let</span> inst <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    203. 

  203. 

  204. inst<span class="token punctuation">.</span>prop <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    205. 

  205. <span class="token comment">// setter: 123</span>
    206. 

  206. 

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

  208. <span class="token comment">// 'getter'</span>
    209. 

  209. </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></div></div><p>上面代码中，<code>prop</code>属性有对应的存值函数和取值函数，因此赋值和读取行为都被自定义了。</p> <p><strong>存值函数和取值函数是设置在属性的 Descriptor 对象上的。</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">CustomHTMLElement</span> <span class="token punctuation">{</span>
    210. 

  210.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">element</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    211. 

  211.     <span class="token keyword">this</span><span class="token punctuation">.</span>element <span class="token operator">=</span> element<span class="token punctuation">;</span>
    212. 

  212.   <span class="token punctuation">}</span>
    213. 

  213. 

  214.   <span class="token keyword">get</span> <span class="token function">html</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    215. 

  215.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>element<span class="token punctuation">.</span>innerHTML<span class="token punctuation">;</span>
    216. 

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

  217. 

  218.   <span class="token keyword">set</span> <span class="token function">html</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    219. 

  219.     <span class="token keyword">this</span><span class="token punctuation">.</span>element<span class="token punctuation">.</span>innerHTML <span class="token operator">=</span> value<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">var</span> descriptor <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>
    224. 

  224.   <span class="token class-name">CustomHTMLElement</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token string">&quot;html&quot;</span>
    225. 

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

  226. 

  227. <span class="token string">&quot;get&quot;</span> <span class="token keyword">in</span> descriptor  <span class="token comment">// true</span>
    228. 

  228. <span class="token string">&quot;set&quot;</span> <span class="token keyword">in</span> descriptor  <span class="token comment">// true</span>
    229. 

  229. </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></div></div><p>上面代码中，存值函数和取值函数是定义在<code>html</code>属性的描述对象上面，这与 ES5 完全一致。</p> <h3 id="属性表达式"><a href="#属性表达式" class="header-anchor">#</a> 属性表达式</h3> <p>类的属性名，可以采用表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> methodName <span class="token operator">=</span> <span class="token string">'getArea'</span><span class="token punctuation">;</span>
    230. 

  230. 

  231. <span class="token keyword">class</span> <span class="token class-name">Square</span> <span class="token punctuation">{</span>
    232. 

  232.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">length</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    233. 

  233.     <span class="token comment">// ...</span>
    234. 

  234.   <span class="token punctuation">}</span>
    235. 

  235. 

  236.   <span class="token punctuation">[</span>methodName<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    237. 

  237.     <span class="token comment">// ...</span>
    238. 

  238.   <span class="token punctuation">}</span>
    239. 

  239. <span class="token punctuation">}</span>
    240. 

  240. </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，<code>Square</code>类的方法名<code>getArea</code>，是从表达式得到的。</p> <h3 id="class-表达式"><a href="#class-表达式" class="header-anchor">#</a> Class 表达式</h3> <p>与函数一样，类也可以使用表达式的形式定义。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> MyClass <span class="token operator">=</span> <span class="token keyword">class</span> <span class="token class-name">Me</span> <span class="token punctuation">{</span>
    241. 

  241.   <span class="token function">getClassName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    242. 

  242.     <span class="token keyword">return</span> Me<span class="token punctuation">.</span>name<span class="token punctuation">;</span>
    243. 

  243.   <span class="token punctuation">}</span>
    244. 

  244. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    245. 

  245. </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>上面代码使用表达式定义了一个类。需要注意的是，这个类的名字是<code>Me</code>，但是<code>Me</code>只在 Class 的内部可用，指代当前类。在 Class 外部，这个类只能用<code>MyClass</code>引用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> inst <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    246. 

  246. inst<span class="token punctuation">.</span><span class="token function">getClassName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Me</span>
    247. 

  247. Me<span class="token punctuation">.</span>name <span class="token comment">// ReferenceError: Me is not defined</span>
    248. 

  248. </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>上面代码表示，<code>Me</code>只在 Class 内部有定义。</p> <p>如果类的内部没用到的话，可以省略<code>Me</code>，也就是可以写成下面的形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> MyClass <span class="token operator">=</span> <span class="token keyword">class</span> <span class="token punctuation">{</span> <span class="token comment">/* ... */</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    249. 

  249. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>采用 Class 表达式，可以写出立即执行的 Class。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">class</span> <span class="token punctuation">{</span>
    250. 

  250.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    251. 

  251.     <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
    252. 

  252.   <span class="token punctuation">}</span>
    253. 

  253. 

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

  255.     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>name<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><span class="token string">'张三'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    258. 

  258. 

  259. person<span class="token punctuation">.</span><span class="token function">sayName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;张三&quot;</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><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，<code>person</code>是一个立即执行的类的实例。</p> <h3 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h3> <p><strong>（1）严格模式</strong></p> <p>类和模块的内部，默认就是严格模式，所以不需要使用<code>use strict</code>指定运行模式。只要你的代码写在类或模块之中，就只有严格模式可用。考虑到未来所有的代码，其实都是运行在模块之中，所以 ES6 实际上把整个语言升级到了严格模式。</p> <p><strong>（2）不存在提升</strong></p> <p>类不存在变量提升（hoist），这一点与 ES5 完全不同。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">new</span> <span class="token class-name">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    261. 

  261. <span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    262. 

  262. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>Foo</code>类使用在前，定义在后，这样会报错，因为 ES6 不会把类的声明提升到代码头部。这种规定的原因与下文要提到的继承有关，必须保证子类在父类之后定义。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    263. 

  263.   <span class="token keyword">let</span> Foo <span class="token operator">=</span> <span class="token keyword">class</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    264. 

  264.   <span class="token keyword">class</span> <span class="token class-name">Bar</span> <span class="token keyword">extends</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    265. 

  265.   <span class="token punctuation">}</span>
    266. 

  266. <span class="token punctuation">}</span>
    267. 

  267. </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>上面的代码不会报错，因为<code>Bar</code>继承<code>Foo</code>的时候，<code>Foo</code>已经有定义了。但是，如果存在<code>class</code>的提升，上面代码就会报错，因为<code>class</code>会被提升到代码头部，而<code>let</code>命令是不提升的，所以导致<code>Bar</code>继承<code>Foo</code>的时候，<code>Foo</code>还没有定义。</p> <p><strong>（3）name 属性</strong></p> <p>由于本质上，ES6 的类只是 ES5 的构造函数的一层包装，所以函数的许多特性都被<code>Class</code>继承，包括<code>name</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">Point</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    268. 

  268. Point<span class="token punctuation">.</span>name <span class="token comment">// &quot;Point&quot;</span>
    269. 

  269. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>name</code>属性总是返回紧跟在<code>class</code>关键字后面的类名。</p> <p><strong>（4）Generator 方法</strong></p> <p>如果某个方法之前加上星号（<code>*</code>），就表示该方法是一个 Generator 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">Foo</span> <span class="token punctuation">{</span>
    270. 

  270.   <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>
    271. 

  271.     <span class="token keyword">this</span><span class="token punctuation">.</span>args <span class="token operator">=</span> args<span class="token punctuation">;</span>
    272. 

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

  273.   <span class="token operator">*</span> <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    274. 

  274.     <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> arg <span class="token keyword">of</span> <span class="token keyword">this</span><span class="token punctuation">.</span>args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    275. 

  275.       <span class="token keyword">yield</span> arg<span class="token punctuation">;</span>
    276. 

  276.     <span class="token punctuation">}</span>
    277. 

  277.   <span class="token punctuation">}</span>
    278. 

  278. <span class="token punctuation">}</span>
    279. 

  279. 

  280. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> <span class="token keyword">new</span> <span class="token class-name">Foo</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">,</span> <span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    281. 

  281.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    282. 

  282. <span class="token punctuation">}</span>
    283. 

  283. <span class="token comment">// hello</span>
    284. 

  284. <span class="token comment">// world</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><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></div></div><p>上面代码中，<code>Foo</code>类的<code>Symbol.iterator</code>方法前有一个星号，表示该方法是一个 Generator 函数。<code>Symbol.iterator</code>方法返回一个<code>Foo</code>类的默认遍历器，<code>for...of</code>循环会自动调用这个遍历器。</p> <p><strong>（5）this 的指向</strong></p> <p>类的方法内部如果含有<code>this</code>，它默认指向类的实例。但是，必须非常小心，一旦单独使用该方法，很可能报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Logger</span> <span class="token punctuation">{</span>
    286. 

  286.   <span class="token function">printName</span><span class="token punctuation">(</span>name <span class="token operator">=</span> <span class="token string">'there'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    287. 

  287.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Hello </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>name<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    288. 

  288.   <span class="token punctuation">}</span>
    289. 

  289. 

  290.   <span class="token function">print</span><span class="token punctuation">(</span><span class="token parameter">text</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    291. 

  291.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>text<span class="token punctuation">)</span><span class="token punctuation">;</span>
    292. 

  292.   <span class="token punctuation">}</span>
    293. 

  293. <span class="token punctuation">}</span>
    294. 

  294. 

  295. <span class="token keyword">const</span> logger <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Logger</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    296. 

  296. <span class="token keyword">const</span> <span class="token punctuation">{</span> printName <span class="token punctuation">}</span> <span class="token operator">=</span> logger<span class="token punctuation">;</span>
    297. 

  297. <span class="token function">printName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// TypeError: Cannot read property 'print' of undefined</span>
    298. 

  298. </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>上面代码中，<code>printName</code>方法中的<code>this</code>，默认指向<code>Logger</code>类的实例。但是，如果将这个方法提取出来单独使用，<code>this</code>会指向该方法运行时所在的环境（由于 class 内部是严格模式，所以 this 实际指向的是<code>undefined</code>），从而导致找不到<code>print</code>方法而报错。</p> <p>一个比较简单的解决方法是，在构造方法中绑定<code>this</code>，这样就不会找不到<code>print</code>方法了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Logger</span> <span class="token punctuation">{</span>
    299. 

  299.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    300. 

  300.     <span class="token keyword">this</span><span class="token punctuation">.</span>printName <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">printName</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    301. 

  301.   <span class="token punctuation">}</span>
    302. 

  302. 

  303.   <span class="token comment">// ...</span>
    304. 

  304. <span class="token punctuation">}</span>
    305. 

  305. </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>另一种解决方法是使用箭头函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Obj</span> <span class="token punctuation">{</span>
    306. 

  306.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    307. 

  307.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function-variable function">getThis</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
    308. 

  308.   <span class="token punctuation">}</span>
    309. 

  309. <span class="token punctuation">}</span>
    310. 

  310. 

  311. <span class="token keyword">const</span> myObj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Obj</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    312. 

  312. myObj<span class="token punctuation">.</span><span class="token function">getThis</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">===</span> myObj <span class="token comment">// true</span>
    313. 

  313. </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>箭头函数内部的<code>this</code>总是指向定义时所在的对象。上面代码中，箭头函数位于构造函数内部，它的定义生效的时候，是在构造函数执行的时候。这时，箭头函数所在的运行环境，肯定是实例对象，所以<code>this</code>会总是指向实例对象。</p> <p>还有一种解决方法是使用<code>Proxy</code>，获取方法的时候，自动绑定<code>this</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">selfish</span> <span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    314. 

  314.   <span class="token keyword">const</span> cache <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">WeakMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    315. 

  315.   <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    316. 

  316.     <span class="token function">get</span> <span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    317. 

  317.       <span class="token keyword">const</span> value <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    318. 

  318.       <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> value <span class="token operator">!==</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    319. 

  319.         <span class="token keyword">return</span> value<span class="token punctuation">;</span>
    320. 

  320.       <span class="token punctuation">}</span>
    321. 

  321.       <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>cache<span class="token punctuation">.</span><span class="token function">has</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    322. 

  322.         cache<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token function">value</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span>target<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    323. 

  323.       <span class="token punctuation">}</span>
    324. 

  324.       <span class="token keyword">return</span> cache<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    325. 

  325.     <span class="token punctuation">}</span>
    326. 

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

  327.   <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    328. 

  328.   <span class="token keyword">return</span> proxy<span class="token punctuation">;</span>
    329. 

  329. <span class="token punctuation">}</span>
    330. 

  330. 

  331. <span class="token keyword">const</span> logger <span class="token operator">=</span> <span class="token function">selfish</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Logger</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    332. 

  332. </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></div></div><h2 id="静态方法"><a href="#静态方法" class="header-anchor">#</a> 静态方法</h2> <p>类相当于实例的原型，所有在类中定义的方法，都会被实例继承。如果在一个方法前，<strong>加上<code>static</code>关键字，就表示该方法不会被实例继承，而是直接通过类来调用，这就称为“静态方法”</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">Foo</span> <span class="token punctuation">{</span>
    333. 

  333.   <span class="token keyword">static</span> <span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    334. 

  334.     <span class="token keyword">return</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    335. 

  335.   <span class="token punctuation">}</span>
    336. 

  336. <span class="token punctuation">}</span>
    337. 

  337. 

  338. Foo<span class="token punctuation">.</span><span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 'hello'</span>
    339. 

  339. 

  340. <span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    341. 

  341. foo<span class="token punctuation">.</span><span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    342. 

  342. <span class="token comment">// TypeError: foo.classMethod is not a function</span>
    343. 

  343. </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，<code>Foo</code>类的<code>classMethod</code>方法前有<code>static</code>关键字，表明该方法是一个静态方法，可以直接在<code>Foo</code>类上调用（<code>Foo.classMethod()</code>），而不是在<code>Foo</code>类的实例上调用。如果在实例上调用静态方法，会抛出一个错误，表示不存在该方法。</p> <p>注意，<strong>如果静态方法包含<code>this</code>关键字，这个<code>this</code>指的是类，而不是实例。</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">Foo</span> <span class="token punctuation">{</span>
    344. 

  344.   <span class="token keyword">static</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    345. 

  345.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    346. 

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

  347.   <span class="token keyword">static</span> <span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    348. 

  348.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    349. 

  349.   <span class="token punctuation">}</span>
    350. 

  350.   <span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    351. 

  351.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    352. 

  352.   <span class="token punctuation">}</span>
    353. 

  353. <span class="token punctuation">}</span>
    354. 

  354. 

  355. Foo<span class="token punctuation">.</span><span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// hello</span>
    356. 

  356. </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>上面代码中，静态方法<code>bar</code>调用了<code>this.baz</code>，这里的<code>this</code>指的是<code>Foo</code>类，而不是<code>Foo</code>的实例，等同于调用<code>Foo.baz</code>。另外，从这个例子还可以看出，静态方法可以与非静态方法重名。</p> <p>父类的静态方法，可以被子类继承。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    357. 

  357.   <span class="token keyword">static</span> <span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    358. 

  358.     <span class="token keyword">return</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    359. 

  359.   <span class="token punctuation">}</span>
    360. 

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

  361. 

  362. <span class="token keyword">class</span> <span class="token class-name">Bar</span> <span class="token keyword">extends</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    363. 

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

  364. 

  365. Bar<span class="token punctuation">.</span><span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 'hello'</span>
    366. 

  366. </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，父类<code>Foo</code>有一个静态方法，子类<code>Bar</code>可以调用这个方法。</p> <p>静态方法也是可以从<code>super</code>对象上调用的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    367. 

  367.   <span class="token keyword">static</span> <span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    368. 

  368.     <span class="token keyword">return</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    369. 

  369.   <span class="token punctuation">}</span>
    370. 

  370. <span class="token punctuation">}</span>
    371. 

  371. 

  372. <span class="token keyword">class</span> <span class="token class-name">Bar</span> <span class="token keyword">extends</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    373. 

  373.   <span class="token keyword">static</span> <span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    374. 

  374.     <span class="token keyword">return</span> <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">', too'</span><span class="token punctuation">;</span>
    375. 

  375.   <span class="token punctuation">}</span>
    376. 

  376. <span class="token punctuation">}</span>
    377. 

  377. 

  378. Bar<span class="token punctuation">.</span><span class="token function">classMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;hello, too&quot;</span>
    379. 

  379. </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><h2 id="实例属性的新写法"><a href="#实例属性的新写法" class="header-anchor">#</a> 实例属性的新写法</h2> <p>实例属性除了定义在<code>constructor()</code>方法里面的<code>this</code>上面，也可以定义在类的最顶层。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">IncreasingCounter</span> <span class="token punctuation">{</span>
    380. 

  380.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    381. 

  381.     <span class="token keyword">this</span><span class="token punctuation">.</span>_count <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    382. 

  382.   <span class="token punctuation">}</span>
    383. 

  383.   <span class="token keyword">get</span> <span class="token function">value</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    384. 

  384.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Getting the current value!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    385. 

  385.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_count<span class="token punctuation">;</span>
    386. 

  386.   <span class="token punctuation">}</span>
    387. 

  387.   <span class="token function">increment</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    388. 

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

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

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

  391. </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，实例属性<code>this._count</code>定义在<code>constructor()</code>方法里面。另一种写法是，这个属性也可以定义在类的最顶层，其他都不变。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">IncreasingCounter</span> <span class="token punctuation">{</span>
    392. 

  392.   _count <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    393. 

  393.   <span class="token keyword">get</span> <span class="token function">value</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    394. 

  394.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Getting the current value!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    395. 

  395.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_count<span class="token punctuation">;</span>
    396. 

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

  397.   <span class="token function">increment</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    398. 

  398.     <span class="token keyword">this</span><span class="token punctuation">.</span>_count<span class="token operator">++</span><span class="token punctuation">;</span>
    399. 

  399.   <span class="token punctuation">}</span>
    400. 

  400. <span class="token punctuation">}</span>
    401. 

  401. </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，实例属性<code>_count</code>与取值函数<code>value()</code>和<code>increment()</code>方法，处于同一个层级。这时，不需要在实例属性前面加上<code>this</code>。</p> <p>这种新写法的好处是，所有实例对象自身的属性都定义在类的头部，看上去比较整齐，一眼就能看出这个类有哪些实例属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">foo</span> <span class="token punctuation">{</span>
    402. 

  402.   bar <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    403. 

  403.   baz <span class="token operator">=</span> <span class="token string">'world'</span><span class="token punctuation">;</span>
    404. 

  404. 

  405.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    406. 

  406.     <span class="token comment">// ...</span>
    407. 

  407.   <span class="token punctuation">}</span>
    408. 

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

  409. </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>上面的代码，一眼就能看出，<code>foo</code>类有两个实例属性，一目了然。另外，写起来也比较简洁。</p> <h2 id="静态属性"><a href="#静态属性" class="header-anchor">#</a> 静态属性</h2> <p>静态属性指的是 Class 本身的属性，即<code>Class.propName</code>，而不是定义在实例对象（<code>this</code>）上的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    410. 

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

  411. 

  412. Foo<span class="token punctuation">.</span>prop <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    413. 

  413. Foo<span class="token punctuation">.</span>prop <span class="token comment">// 1</span>
    414. 

  414. </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>上面的写法为<code>Foo</code>类定义了一个静态属性<code>prop</code>。</p> <p>目前，只有这种写法可行，因为 ES6 明确规定，Class 内部只有静态方法，没有静态属性。现在有一个<a href="https://github.com/tc39/proposal-class-fields" target="_blank" rel="noopener noreferrer">提案<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>提供了类的静态属性，写法是在实例属性的前面，加上<code>static</code>关键字。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">MyClass</span> <span class="token punctuation">{</span>
    415. 

  415.   <span class="token keyword">static</span> myStaticProp <span class="token operator">=</span> <span class="token number">42</span><span class="token punctuation">;</span>
    416. 

  416. 

  417.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    418. 

  418.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>MyClass<span class="token punctuation">.</span>myStaticProp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 42</span>
    419. 

  419.   <span class="token punctuation">}</span>
    420. 

  420. <span class="token punctuation">}</span>
    421. 

  421. </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>这个新写法大大方便了静态属性的表达。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 老写法</span>
    422. 

  422. <span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    423. 

  423.   <span class="token comment">// ...</span>
    424. 

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

  425. Foo<span class="token punctuation">.</span>prop <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    426. 

  426. 

  427. <span class="token comment">// 新写法</span>
    428. 

  428. <span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    429. 

  429.   <span class="token keyword">static</span> prop <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    430. 

  430. <span class="token punctuation">}</span>
    431. 

  431. </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，老写法的静态属性定义在类的外部。整个类生成以后，再生成静态属性。这样让人很容易忽略这个静态属性，也不符合相关代码应该放在一起的代码组织原则。另外，新写法是显式声明（declarative），而不是赋值处理，语义更好。</p> <h2 id="私有方法和私有属性"><a href="#私有方法和私有属性" class="header-anchor">#</a> 私有方法和私有属性</h2> <h3 id="现有的解决方案"><a href="#现有的解决方案" class="header-anchor">#</a> 现有的解决方案</h3> <p>私有方法和私有属性，是只能在类的内部访问的方法和属性，外部不能访问。这是常见需求，有利于代码的封装，但 ES6 不提供，只能通过变通方法模拟实现。</p> <p>一种做法是在命名上加以区别。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Widget</span> <span class="token punctuation">{</span>
    432. 

  432. 

  433.   <span class="token comment">// 公有方法</span>
    434. 

  434.   <span class="token function">foo</span> <span class="token punctuation">(</span><span class="token parameter">baz</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    435. 

  435.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">_bar</span><span class="token punctuation">(</span>baz<span class="token punctuation">)</span><span class="token punctuation">;</span>
    436. 

  436.   <span class="token punctuation">}</span>
    437. 

  437. 

  438.   <span class="token comment">// 私有方法</span>
    439. 

  439.   <span class="token function">_bar</span><span class="token punctuation">(</span><span class="token parameter">baz</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    440. 

  440.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>snaf <span class="token operator">=</span> baz<span class="token punctuation">;</span>
    441. 

  441.   <span class="token punctuation">}</span>
    442. 

  442. 

  443.   <span class="token comment">// ...</span>
    444. 

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

  445. </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>_bar</code>方法前面的下划线，表示这是一个只限于内部使用的私有方法。但是，这种命名是不保险的，在类的外部，还是可以调用到这个方法。</p> <p>另一种方法就是索性将私有方法移出模块，因为模块内部的所有方法都是对外可见的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Widget</span> <span class="token punctuation">{</span>
    446. 

  446.   <span class="token function">foo</span> <span class="token punctuation">(</span><span class="token parameter">baz</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    447. 

  447.     <span class="token function">bar</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> baz<span class="token punctuation">)</span><span class="token punctuation">;</span>
    448. 

  448.   <span class="token punctuation">}</span>
    449. 

  449. 

  450.   <span class="token comment">// ...</span>
    451. 

  451. <span class="token punctuation">}</span>
    452. 

  452. 

  453. <span class="token keyword">function</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token parameter">baz</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    454. 

  454.   <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>snaf <span class="token operator">=</span> baz<span class="token punctuation">;</span>
    455. 

  455. <span class="token punctuation">}</span>
    456. 

  456. </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，<code>foo</code>是公开方法，内部调用了<code>bar.call(this, baz)</code>。这使得<code>bar</code>实际上成为了当前模块的私有方法。</p> <p>还有一种方法是利用<code>Symbol</code>值的唯一性，将私有方法的名字命名为一个<code>Symbol</code>值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> bar <span class="token operator">=</span> <span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token string">'bar'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    457. 

  457. <span class="token keyword">const</span> snaf <span class="token operator">=</span> <span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token string">'snaf'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    458. 

  458. 

  459. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">class</span> <span class="token class-name">myClass</span><span class="token punctuation">{</span>
    460. 

  460. 

  461.   <span class="token comment">// 公有方法</span>
    462. 

  462.   <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">baz</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    463. 

  463.     <span class="token keyword">this</span><span class="token punctuation">[</span>bar<span class="token punctuation">]</span><span class="token punctuation">(</span>baz<span class="token punctuation">)</span><span class="token punctuation">;</span>
    464. 

  464.   <span class="token punctuation">}</span>
    465. 

  465. 

  466.   <span class="token comment">// 私有方法</span>
    467. 

  467.   <span class="token punctuation">[</span>bar<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token parameter">baz</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    468. 

  468.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">[</span>snaf<span class="token punctuation">]</span> <span class="token operator">=</span> baz<span class="token punctuation">;</span>
    469. 

  469.   <span class="token punctuation">}</span>
    470. 

  470. 

  471.   <span class="token comment">// ...</span>
    472. 

  472. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    473. 

  473. </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>bar</code>和<code>snaf</code>都是<code>Symbol</code>值，一般情况下无法获取到它们，因此达到了私有方法和私有属性的效果。但是也不是绝对不行，<code>Reflect.ownKeys()</code>依然可以拿到它们。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> inst <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">myClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    474. 

  474. 

  475. Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>myClass<span class="token punctuation">.</span>prototype<span class="token punctuation">)</span>
    476. 

  476. <span class="token comment">// [ 'constructor', 'foo', Symbol(bar) ]</span>
    477. 

  477. </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，Symbol 值的属性名依然可以从类的外部拿到。</p> <h3 id="私有属性的提案"><a href="#私有属性的提案" class="header-anchor">#</a> 私有属性的提案</h3> <p>目前，有一个<a href="https://github.com/tc39/proposal-private-methods" target="_blank" rel="noopener noreferrer">提案<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>，为<code>class</code>加了私有属性。方法是在属性名之前，使用<code>#</code>表示。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">IncreasingCounter</span> <span class="token punctuation">{</span>
    478. 

  478.   #count <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    479. 

  479.   <span class="token keyword">get</span> <span class="token function">value</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    480. 

  480.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Getting the current value!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    481. 

  481.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>#count<span class="token punctuation">;</span>
    482. 

  482.   <span class="token punctuation">}</span>
    483. 

  483.   <span class="token function">increment</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    484. 

  484.     <span class="token keyword">this</span><span class="token punctuation">.</span>#count<span class="token operator">++</span><span class="token punctuation">;</span>
    485. 

  485.   <span class="token punctuation">}</span>
    486. 

  486. <span class="token punctuation">}</span>
    487. 

  487. </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>#count</code>就是私有属性，只能在类的内部使用（<code>this.#count</code>）。如果在类的外部使用，就会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> counter <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">IncreasingCounter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    488. 

  488. counter<span class="token punctuation">.</span>#count <span class="token comment">// 报错</span>
    489. 

  489. counter<span class="token punctuation">.</span>#count <span class="token operator">=</span> <span class="token number">42</span> <span class="token comment">// 报错</span>
    490. 

  490. </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>上面代码在类的外部，读取私有属性，就会报错。</p> <p>下面是另一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    491. 

  491.   #x<span class="token punctuation">;</span>
    492. 

  492. 

  493.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x <span class="token operator">=</span> <span class="token number">0</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    494. 

  494.     <span class="token keyword">this</span><span class="token punctuation">.</span>#x <span class="token operator">=</span> <span class="token operator">+</span>x<span class="token punctuation">;</span>
    495. 

  495.   <span class="token punctuation">}</span>
    496. 

  496. 

  497.   <span class="token keyword">get</span> <span class="token function">x</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    498. 

  498.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>#x<span class="token punctuation">;</span>
    499. 

  499.   <span class="token punctuation">}</span>
    500. 

  500. 

  501.   <span class="token keyword">set</span> <span class="token function">x</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    502. 

  502.     <span class="token keyword">this</span><span class="token punctuation">.</span>#x <span class="token operator">=</span> <span class="token operator">+</span>value<span class="token punctuation">;</span>
    503. 

  503.   <span class="token punctuation">}</span>
    504. 

  504. <span class="token punctuation">}</span>
    505. 

  505. </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></div></div><p>上面代码中，<code>#x</code>就是私有属性，在<code>Point</code>类之外是读取不到这个属性的。由于井号<code>#</code>是属性名的一部分，使用时必须带有<code>#</code>一起使用，所以<code>#x</code>和<code>x</code>是两个不同的属性。</p> <p>之所以要引入一个新的前缀<code>#</code>表示私有属性，而没有采用<code>private</code>关键字，是因为 JavaScript 是一门动态语言，没有类型声明，使用独立的符号似乎是唯一的比较方便可靠的方法，能够准确地区分一种属性是否为私有属性。另外，Ruby 语言使用<code>@</code>表示私有属性，ES6 没有用这个符号而使用<code>#</code>，是因为<code>@</code>已经被留给了 Decorator。</p> <p>这种写法不仅可以写私有属性，还可以用来写私有方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    506. 

  506.   #a<span class="token punctuation">;</span>
    507. 

  507.   #b<span class="token punctuation">;</span>
    508. 

  508.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    509. 

  509.     <span class="token keyword">this</span><span class="token punctuation">.</span>#a <span class="token operator">=</span> a<span class="token punctuation">;</span>
    510. 

  510.     <span class="token keyword">this</span><span class="token punctuation">.</span>#b <span class="token operator">=</span> b<span class="token punctuation">;</span>
    511. 

  511.   <span class="token punctuation">}</span>
    512. 

  512.   <span class="token function">#sum</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    513. 

  513.     <span class="token keyword">return</span> #a <span class="token operator">+</span> #b<span class="token punctuation">;</span>
    514. 

  514.   <span class="token punctuation">}</span>
    515. 

  515.   <span class="token function">printSum</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    516. 

  516.     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><span class="token function">#sum</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    517. 

  517.   <span class="token punctuation">}</span>
    518. 

  518. <span class="token punctuation">}</span>
    519. 

  519. </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>#sum()</code>就是一个私有方法。</p> <p>另外，私有属性也可以设置 getter 和 setter 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">Counter</span> <span class="token punctuation">{</span>
    520. 

  520.   #xValue <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    521. 

  521. 

  522.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    523. 

  523.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    524. 

  524.     <span class="token comment">// ...</span>
    525. 

  525.   <span class="token punctuation">}</span>
    526. 

  526. 

  527.   <span class="token keyword">get</span> <span class="token function">#x</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> #xValue<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    528. 

  528.   <span class="token keyword">set</span> <span class="token function">#x</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    529. 

  529.     <span class="token keyword">this</span><span class="token punctuation">.</span>#xValue <span class="token operator">=</span> value<span class="token punctuation">;</span>
    530. 

  530.   <span class="token punctuation">}</span>
    531. 

  531. <span class="token punctuation">}</span>
    532. 

  532. </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>上面代码中，<code>#x</code>是一个私有属性，它的读写都通过<code>get #x()</code>和<code>set #x()</code>来完成。</p> <p>私有属性不限于从<code>this</code>引用，只要是在类的内部，实例也可以引用私有属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    533. 

  533.   #privateValue <span class="token operator">=</span> <span class="token number">42</span><span class="token punctuation">;</span>
    534. 

  534.   <span class="token keyword">static</span> <span class="token function">getPrivateValue</span><span class="token punctuation">(</span><span class="token parameter">foo</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    535. 

  535.     <span class="token keyword">return</span> foo<span class="token punctuation">.</span>#privateValue<span class="token punctuation">;</span>
    536. 

  536.   <span class="token punctuation">}</span>
    537. 

  537. <span class="token punctuation">}</span>
    538. 

  538. 

  539. Foo<span class="token punctuation">.</span><span class="token function">getPrivateValue</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Foo</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">// 42</span>
    540. 

  540. </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>上面代码允许从实例<code>foo</code>上面引用私有属性。</p> <p>私有属性和私有方法前面，也可以加上<code>static</code>关键字，表示这是一个静态的私有属性或私有方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">FakeMath</span> <span class="token punctuation">{</span>
    541. 

  541.   <span class="token keyword">static</span> <span class="token constant">PI</span> <span class="token operator">=</span> <span class="token number">22</span> <span class="token operator">/</span> <span class="token number">7</span><span class="token punctuation">;</span>
    542. 

  542.   <span class="token keyword">static</span> #totallyRandomNumber <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span>
    543. 

  543. 

  544.   <span class="token keyword">static</span> <span class="token function">#computeRandomNumber</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    545. 

  545.     <span class="token keyword">return</span> FakeMath<span class="token punctuation">.</span>#totallyRandomNumber<span class="token punctuation">;</span>
    546. 

  546.   <span class="token punctuation">}</span>
    547. 

  547. 

  548.   <span class="token keyword">static</span> <span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    549. 

  549.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'I heard you like random numbers…'</span><span class="token punctuation">)</span>
    550. 

  550.     <span class="token keyword">return</span> FakeMath<span class="token punctuation">.</span><span class="token function">#computeRandomNumber</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    551. 

  551.   <span class="token punctuation">}</span>
    552. 

  552. <span class="token punctuation">}</span>
    553. 

  553. 

  554. FakeMath<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token comment">// 3.142857142857143</span>
    555. 

  555. FakeMath<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    556. 

  556. <span class="token comment">// I heard you like random numbers…</span>
    557. 

  557. <span class="token comment">// 4</span>
    558. 

  558. FakeMath<span class="token punctuation">.</span>#totallyRandomNumber <span class="token comment">// 报错</span>
    559. 

  559. FakeMath<span class="token punctuation">.</span><span class="token function">#computeRandomNumber</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
    560. 

  560. </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></div></div><p>上面代码中，<code>#totallyRandomNumber</code>是私有属性，<code>#computeRandomNumber()</code>是私有方法，只能在<code>FakeMath</code>这个类的内部调用，外部调用就会报错。</p> <h2 id="new-target-属性"><a href="#new-target-属性" class="header-anchor">#</a> new.target 属性</h2> <p><code>new</code>是从构造函数生成实例对象的命令。ES6 为<code>new</code>命令引入了一个<code>new.target</code>属性，该属性一般用在构造函数之中，返回<code>new</code>命令作用于的那个构造函数。如果构造函数不是通过<code>new</code>命令或<code>Reflect.construct()</code>调用的，<code>new.target</code>会返回<code>undefined</code>，因此这个属性可以用来确定构造函数是怎么调用的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Person</span><span class="token punctuation">(</span><span class="token parameter">name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    561. 

  561.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span><span class="token punctuation">.</span>target <span class="token operator">!==</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    562. 

  562.     <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
    563. 

  563.   <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    564. 

  564.     <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'必须使用 new 命令生成实例'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    565. 

  565.   <span class="token punctuation">}</span>
    566. 

  566. <span class="token punctuation">}</span>
    567. 

  567. 

  568. <span class="token comment">// 另一种写法</span>
    569. 

  569. <span class="token keyword">function</span> <span class="token function">Person</span><span class="token punctuation">(</span><span class="token parameter">name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    570. 

  570.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span><span class="token punctuation">.</span>target <span class="token operator">===</span> Person<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    571. 

  571.     <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
    572. 

  572.   <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    573. 

  573.     <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'必须使用 new 命令生成实例'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    574. 

  574.   <span class="token punctuation">}</span>
    575. 

  575. <span class="token punctuation">}</span>
    576. 

  576. 

  577. <span class="token keyword">var</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">'张三'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 正确</span>
    578. 

  578. <span class="token keyword">var</span> notAPerson <span class="token operator">=</span> <span class="token function">Person</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>person<span class="token punctuation">,</span> <span class="token string">'张三'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 报错</span>
    579. 

  579. </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></div></div><p>上面代码确保构造函数只能通过<code>new</code>命令调用。</p> <p>Class 内部调用<code>new.target</code>，返回当前 Class。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    580. 

  580.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">length<span class="token punctuation">,</span> width</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    581. 

  581.     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 operator">===</span> Rectangle<span class="token punctuation">)</span><span class="token punctuation">;</span>
    582. 

  582.     <span class="token keyword">this</span><span class="token punctuation">.</span>length <span class="token operator">=</span> length<span class="token punctuation">;</span>
    583. 

  583.     <span class="token keyword">this</span><span class="token punctuation">.</span>width <span class="token operator">=</span> width<span class="token punctuation">;</span>
    584. 

  584.   <span class="token punctuation">}</span>
    585. 

  585. <span class="token punctuation">}</span>
    586. 

  586. 

  587. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Rectangle</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 true</span>
    588. 

  588. </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>需要注意的是，子类继承父类时，<code>new.target</code>会返回子类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    589. 

  589.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">length<span class="token punctuation">,</span> width</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    590. 

  590.     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 operator">===</span> Rectangle<span class="token punctuation">)</span><span class="token punctuation">;</span>
    591. 

  591.     <span class="token comment">// ...</span>
    592. 

  592.   <span class="token punctuation">}</span>
    593. 

  593. <span class="token punctuation">}</span>
    594. 

  594. 

  595. <span class="token keyword">class</span> <span class="token class-name">Square</span> <span class="token keyword">extends</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    596. 

  596.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">length</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    597. 

  597.     <span class="token keyword">super</span><span class="token punctuation">(</span>length<span class="token punctuation">,</span> width<span class="token punctuation">)</span><span class="token punctuation">;</span>
    598. 

  598.   <span class="token punctuation">}</span>
    599. 

  599. <span class="token punctuation">}</span>
    600. 

  600. 

  601. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Square</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 false</span>
    602. 

  602. </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>new.target</code>会返回子类。</p> <p>利用这个特点，可以写出不能独立使用、必须继承后才能使用的类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Shape</span> <span class="token punctuation">{</span>
    603. 

  603.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    604. 

  604.     <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span><span class="token punctuation">.</span>target <span class="token operator">===</span> Shape<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    605. 

  605.       <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'本类不能实例化'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    606. 

  606.     <span class="token punctuation">}</span>
    607. 

  607.   <span class="token punctuation">}</span>
    608. 

  608. <span class="token punctuation">}</span>
    609. 

  609. 

  610. <span class="token keyword">class</span> <span class="token class-name">Rectangle</span> <span class="token keyword">extends</span> <span class="token class-name">Shape</span> <span class="token punctuation">{</span>
    611. 

  611.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">length<span class="token punctuation">,</span> width</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    612. 

  612.     <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    613. 

  613.     <span class="token comment">// ...</span>
    614. 

  614.   <span class="token punctuation">}</span>
    615. 

  615. <span class="token punctuation">}</span>
    616. 

  616. 

  617. <span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Shape</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 报错</span>
    618. 

  618. <span class="token keyword">var</span> y <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Rectangle</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 正确</span>
    619. 

  619. </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>Shape</code>类不能被实例化，只能用于继承。</p> <p>注意，在函数外部，使用<code>new.target</code>会报错。</p></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/21.Class 的基本语法.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/3777253e65bac487/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">async 函数</div></a> <a href="/blog/pages/83f8c3a0cd87dd83/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Class 的继承</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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  621.         <a href="/blog/pages/3777253e65bac487/" class="prev">async 函数</a></span> <span class="next"><a href="/blog/pages/83f8c3a0cd87dd83/">Class 的继承</a>→
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