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href="/blog/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/blog/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/blog/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">对象的扩展<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="对象的扩展"><a href="#对象的扩展" class="header-anchor">#</a> 对象的扩展</h1> <p>对象（object）是 JavaScript 最重要的数据结构。ES6 对它进行了重大升级，本章介绍数据结构本身的改变，下一章介绍<code>Object</code>对象的新增方法。
    26. 

  26. </p> <h2 id="属性的简洁表示法"><a href="#属性的简洁表示法" class="header-anchor">#</a> 属性的简洁表示法</h2> <p>ES6 允许在大括号里面，<strong>直接写入变量和函数，作为对象的属性和方法</strong>。这样的书写更加简洁。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    27. 

  27. <span class="token keyword">const</span> baz <span class="token operator">=</span> <span class="token punctuation">{</span>foo<span class="token punctuation">}</span><span class="token punctuation">;</span>
    28. 

  28. baz <span class="token comment">// {foo: &quot;bar&quot;}</span>
    29. 

  29. 

  30. <span class="token comment">// 等同于</span>
    31. 

  31. <span class="token keyword">const</span> baz <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">foo</span><span class="token operator">:</span> foo<span class="token punctuation">}</span><span class="token punctuation">;</span>
    32. 

  32. </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>上面代码中，变量<code>foo</code>直接写在大括号里面。这时，属性名就是变量名, 属性值就是变量值。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">function</span> <span class="token function">f</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>
    33. 

  33.   <span class="token keyword">return</span> <span class="token punctuation">{</span>x<span class="token punctuation">,</span> y<span class="token punctuation">}</span><span class="token punctuation">;</span>
    34. 

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

  35. 

  36. <span class="token comment">// 等同于</span>
    37. 

  37. 

  38. <span class="token keyword">function</span> <span class="token function">f</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>
    39. 

  39.   <span class="token keyword">return</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> x<span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> y<span class="token punctuation">}</span><span class="token punctuation">;</span>
    40. 

  40. <span class="token punctuation">}</span>
    41. 

  41. 

  42. <span class="token function">f</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 comment">// Object {x: 1, y: 2}</span>
    43. 

  43. </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>除了属性简写，方法也可以简写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> o <span class="token operator">=</span> <span class="token punctuation">{</span>
    44. 

  44.   <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    45. 

  45.     <span class="token keyword">return</span> <span class="token string">&quot;Hello!&quot;</span><span class="token punctuation">;</span>
    46. 

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

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

  48. 

  49. <span class="token comment">// 等同于</span>
    50. 

  50. 

  51. <span class="token keyword">const</span> o <span class="token operator">=</span> <span class="token punctuation">{</span>
    52. 

  52.   <span class="token function-variable function">method</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>
    53. 

  53.     <span class="token keyword">return</span> <span class="token string">&quot;Hello!&quot;</span><span class="token punctuation">;</span>
    54. 

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

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

  56. </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>下面是一个实际的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> birth <span class="token operator">=</span> <span class="token string">'2000/01/01'</span><span class="token punctuation">;</span>
    57. 

  57. 

  58. <span class="token keyword">const</span> Person <span class="token operator">=</span> <span class="token punctuation">{</span>
    59. 

  59. 

  60.   <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'张三'</span><span class="token punctuation">,</span>
    61. 

  61. 

  62.   <span class="token comment">//等同于birth: birth</span>
    63. 

  63.   birth<span class="token punctuation">,</span>
    64. 

  64. 

  65.   <span class="token comment">// 等同于hello: function ()...</span>
    66. 

  66.   <span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'我的名字是'</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> <span class="token punctuation">}</span>
    67. 

  67. 

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

  69. </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>这种写法用于函数的返回值，将会非常方便。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">getPoint</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    70. 

  70.   <span class="token keyword">const</span> x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    71. 

  71.   <span class="token keyword">const</span> y <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    72. 

  72.   <span class="token keyword">return</span> <span class="token punctuation">{</span>x<span class="token punctuation">,</span> y<span class="token punctuation">}</span><span class="token punctuation">;</span>
    73. 

  73. <span class="token punctuation">}</span>
    74. 

  74. 

  75. <span class="token function">getPoint</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    76. 

  76. <span class="token comment">// {x:1, y:10}</span>
    77. 

  77. </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>CommonJS 模块输出一组变量，就非常合适使用简洁写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> ms <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    78. 

  78. 

  79. <span class="token keyword">function</span> <span class="token function">getItem</span> <span class="token punctuation">(</span><span class="token parameter">key</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    80. 

  80.   <span class="token keyword">return</span> key <span class="token keyword">in</span> ms <span class="token operator">?</span> ms<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">:</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    81. 

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

  82. 

  83. <span class="token keyword">function</span> <span class="token function">setItem</span> <span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    84. 

  84.   ms<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> value<span class="token punctuation">;</span>
    85. 

  85. <span class="token punctuation">}</span>
    86. 

  86. 

  87. <span class="token keyword">function</span> <span class="token function">clear</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    88. 

  88.   ms <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    89. 

  89. <span class="token punctuation">}</span>
    90. 

  90. 

  91. module<span class="token punctuation">.</span>exports <span class="token operator">=</span> <span class="token punctuation">{</span> getItem<span class="token punctuation">,</span> setItem<span class="token punctuation">,</span> clear <span class="token punctuation">}</span><span class="token punctuation">;</span>
    92. 

  92. <span class="token comment">// 等同于</span>
    93. 

  93. module<span class="token punctuation">.</span>exports <span class="token operator">=</span> <span class="token punctuation">{</span>
    94. 

  94.   <span class="token literal-property property">getItem</span><span class="token operator">:</span> getItem<span class="token punctuation">,</span>
    95. 

  95.   <span class="token literal-property property">setItem</span><span class="token operator">:</span> setItem<span class="token punctuation">,</span>
    96. 

  96.   <span class="token literal-property property">clear</span><span class="token operator">:</span> clear
    97. 

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

  98. </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>属性的赋值器（setter）和取值器（getter），事实上也是采用这种写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> cart <span class="token operator">=</span> <span class="token punctuation">{</span>
    99. 

  99.   <span class="token literal-property property">_wheels</span><span class="token operator">:</span> <span class="token number">4</span><span class="token punctuation">,</span>
    100. 

  100. 

  101.   <span class="token keyword">get</span> <span class="token function">wheels</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    102. 

  102.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_wheels<span class="token punctuation">;</span>
    103. 

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

  104. 

  105.   <span class="token keyword">set</span> <span class="token function">wheels</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    106. 

  106.     <span class="token keyword">if</span> <span class="token punctuation">(</span>value <span class="token operator">&lt;</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_wheels<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    107. 

  107.       <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">'数值太小了！'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    108. 

  108.     <span class="token punctuation">}</span>
    109. 

  109.     <span class="token keyword">this</span><span class="token punctuation">.</span>_wheels <span class="token operator">=</span> value<span class="token punctuation">;</span>
    110. 

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

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

  112. </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>简洁写法在打印对象时也很有用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> user <span class="token operator">=</span> <span class="token punctuation">{</span>
    113. 

  113.   <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'test'</span>
    114. 

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

  115. 

  116. <span class="token keyword">let</span> foo <span class="token operator">=</span> <span class="token punctuation">{</span>
    117. 

  117.   <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'baz'</span>
    118. 

  118. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    119. 

  119. 

  120. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>user<span class="token punctuation">,</span> foo<span class="token punctuation">)</span>
    121. 

  121. <span class="token comment">// {name: &quot;test&quot;} {bar: &quot;baz&quot;}</span>
    122. 

  122. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">{</span>user<span class="token punctuation">,</span> foo<span class="token punctuation">}</span><span class="token punctuation">)</span>
    123. 

  123. <span class="token comment">// {user: {name: &quot;test&quot;}, foo: {bar: &quot;baz&quot;}}</span>
    124. 

  124. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，<code>console.log</code>直接输出<code>user</code>和<code>foo</code>两个对象时，就是两组键值对，可能会混淆。把它们放在大括号里面输出，就变成了对象的简洁表示法，每组键值对前面会打印对象名，这样就比较清晰了。</p> <p>注意，<strong>简写的对象方法不能用作构造函数</strong>，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    125. 

  125.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    126. 

  126.     <span class="token keyword">this</span><span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    127. 

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

  128. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    129. 

  129. 

  130. <span class="token keyword">new</span> <span class="token class-name">obj<span class="token punctuation">.</span>f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
    131. 

  131. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>f</code>是一个简写的对象方法，所以<code>obj.f</code>不能当作构造函数使用。</p> <h2 id="属性名表达式"><a href="#属性名表达式" class="header-anchor">#</a> 属性名表达式</h2> <p>JavaScript 定义对象的属性，有两种方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 方法一</span>
    132. 

  132. obj<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    133. 

  133. 

  134. <span class="token comment">// 方法二</span>
    135. 

  135. obj<span class="token punctuation">[</span><span class="token string">'a'</span> <span class="token operator">+</span> <span class="token string">'bc'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</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></div></div><p>上面代码的方法一是直接用标识符作为属性名，方法二是用表达式作为属性名，这时要将表达式放在方括号之内。</p> <p>但是，如果使用字面量方式定义对象（使用大括号），在 ES5 中只能使用方法一（标识符）定义属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    137. 

  137.   <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    138. 

  138.   <span class="token literal-property property">abc</span><span class="token operator">:</span> <span class="token number">123</span>
    139. 

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

  140. </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><strong>ES6 允许字面量定义对象时，用方法二（表达式）作为对象的属性名</strong>，即把表达式放在方括号内。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> propKey <span class="token operator">=</span> <span class="token string">'foo'</span><span class="token punctuation">;</span>
    141. 

  141. 

  142. <span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    143. 

  143.   <span class="token punctuation">[</span>propKey<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    144. 

  144.   <span class="token punctuation">[</span><span class="token string">'a'</span> <span class="token operator">+</span> <span class="token string">'bc'</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token number">123</span>
    145. 

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

  146. </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>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">let</span> lastWord <span class="token operator">=</span> <span class="token string">'last word'</span><span class="token punctuation">;</span>
    147. 

  147. 

  148. <span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span>
    149. 

  149.   <span class="token string-property property">'first word'</span><span class="token operator">:</span> <span class="token string">'hello'</span><span class="token punctuation">,</span>
    150. 

  150.   <span class="token punctuation">[</span>lastWord<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token string">'world'</span>
    151. 

  151. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    152. 

  152. 

  153. a<span class="token punctuation">[</span><span class="token string">'first word'</span><span class="token punctuation">]</span> <span class="token comment">// &quot;hello&quot;</span>
    154. 

  154. a<span class="token punctuation">[</span>lastWord<span class="token punctuation">]</span> <span class="token comment">// &quot;world&quot;</span>
    155. 

  155. a<span class="token punctuation">[</span><span class="token string">'last word'</span><span class="token punctuation">]</span> <span class="token comment">// &quot;world&quot;</span>
    156. 

  156. </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>表达式还可以用于定义方法名。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    157. 

  157.   <span class="token punctuation">[</span><span class="token string">'h'</span> <span class="token operator">+</span> <span class="token string">'ello'</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    158. 

  158.     <span class="token keyword">return</span> <span class="token string">'hi'</span><span class="token punctuation">;</span>
    159. 

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

  160. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    161. 

  161. 

  162. obj<span class="token punctuation">.</span><span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// hi</span>
    163. 

  163. </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>注意，属性名表达式与简洁表示法，不能同时使用，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    164. 

  164. <span class="token keyword">const</span> foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    165. 

  165. <span class="token keyword">const</span> bar <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span>
    166. 

  166. <span class="token keyword">const</span> baz <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    167. 

  167. 

  168. <span class="token comment">// 正确</span>
    169. 

  169. <span class="token keyword">const</span> foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    170. 

  170. <span class="token keyword">const</span> baz <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token string">'abc'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    171. 

  171. </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>注意，属性名表达式如果是一个对象，默认情况下会自动将对象转为字符串<code>[object Object]</code>，这一点要特别小心。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> keyA <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    172. 

  172. <span class="token keyword">const</span> keyB <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    173. 

  173. 

  174. <span class="token keyword">const</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
    175. 

  175.   <span class="token punctuation">[</span>keyA<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token string">'valueA'</span><span class="token punctuation">,</span>
    176. 

  176.   <span class="token punctuation">[</span>keyB<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token string">'valueB'</span>
    177. 

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

  178. 

  179. myObject <span class="token comment">// Object {[object Object]: &quot;valueB&quot;}</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><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>上面代码中，<code>[keyA]</code>和<code>[keyB]</code>得到的都是<code>[object Object]</code>，所以<code>[keyB]</code>会把<code>[keyA]</code>覆盖掉，而<code>myObject</code>最后只有一个<code>[object Object]</code>属性。</p> <h2 id="方法的-name-属性"><a href="#方法的-name-属性" class="header-anchor">#</a> 方法的 name 属性</h2> <p>函数的<code>name</code>属性，返回函数名。对象方法也是函数，因此也有<code>name</code>属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> person <span class="token operator">=</span> <span class="token punctuation">{</span>
    181. 

  181.   <span class="token function">sayName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    182. 

  182.     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>
    183. 

  183.   <span class="token punctuation">}</span><span class="token punctuation">,</span>
    184. 

  184. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    185. 

  185. 

  186. person<span class="token punctuation">.</span>sayName<span class="token punctuation">.</span>name   <span class="token comment">// &quot;sayName&quot;</span>
    187. 

  187. </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，方法的<code>name</code>属性返回函数名（即方法名）。</p> <p>如果对象的方法使用了取值函数（<code>getter</code>）和存值函数（<code>setter</code>），则<code>name</code>属性不是在该方法上面，而是该方法的属性的描述对象的<code>get</code>和<code>set</code>属性上面，返回值是方法名前加上<code>get</code>和<code>set</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    188. 

  188.   <span class="token keyword">get</span> <span class="token function">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 punctuation">,</span>
    189. 

  189.   <span class="token keyword">set</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    190. 

  190. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    191. 

  191. 

  192. obj<span class="token punctuation">.</span>foo<span class="token punctuation">.</span>name
    193. 

  193. <span class="token comment">// TypeError: Cannot read property 'name' of undefined</span>
    194. 

  194. 

  195. <span class="token keyword">const</span> descriptor <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    196. 

  196. 

  197. descriptor<span class="token punctuation">.</span>get<span class="token punctuation">.</span>name <span class="token comment">// &quot;get foo&quot;</span>
    198. 

  198. descriptor<span class="token punctuation">.</span>set<span class="token punctuation">.</span>name <span class="token comment">// &quot;set foo&quot;</span>
    199. 

  199. </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>有两种特殊情况：<code>bind</code>方法创造的函数，<code>name</code>属性返回<code>bound</code>加上原函数的名字；<code>Function</code>构造函数创造的函数，<code>name</code>属性返回<code>anonymous</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span>name <span class="token comment">// &quot;anonymous&quot;</span>
    200. 

  200. 

  201. <span class="token keyword">var</span> <span class="token function-variable function">doSomething</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>
    202. 

  202.   <span class="token comment">// ...</span>
    203. 

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

  204. <span class="token function">doSomething</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>name <span class="token comment">// &quot;bound doSomething&quot;</span>
    205. 

  205. </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>如果对象的方法是一个 Symbol 值，那么<code>name</code>属性返回的是这个 Symbol 值的描述。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> key1 <span class="token operator">=</span> <span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token string">'description'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    206. 

  206. <span class="token keyword">const</span> key2 <span class="token operator">=</span> <span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    207. 

  207. <span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    208. 

  208.   <span class="token punctuation">[</span>key1<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><span class="token punctuation">,</span>
    209. 

  209.   <span class="token punctuation">[</span>key2<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><span class="token punctuation">,</span>
    210. 

  210. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    211. 

  211. obj<span class="token punctuation">[</span>key1<span class="token punctuation">]</span><span class="token punctuation">.</span>name <span class="token comment">// &quot;[description]&quot;</span>
    212. 

  212. obj<span class="token punctuation">[</span>key2<span class="token punctuation">]</span><span class="token punctuation">.</span>name <span class="token comment">// &quot;&quot;</span>
    213. 

  213. </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>key1</code>对应的 Symbol 值有描述，<code>key2</code>没有。</p> <h2 id="属性的可枚举性和遍历"><a href="#属性的可枚举性和遍历" class="header-anchor">#</a> 属性的可枚举性和遍历</h2> <h3 id="可枚举性"><a href="#可枚举性" class="header-anchor">#</a> 可枚举性</h3> <p>对象的每个属性都有一个描述对象（Descriptor），用来控制该属性的行为。<code>Object.getOwnPropertyDescriptor</code>方法可以获取该属性的描述对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">123</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    214. 

  214. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span>
    215. 

  215. <span class="token comment">//  {</span>
    216. 

  216. <span class="token comment">//    value: 123,</span>
    217. 

  217. <span class="token comment">//    writable: true,</span>
    218. 

  218. <span class="token comment">//    enumerable: true,</span>
    219. 

  219. <span class="token comment">//    configurable: true</span>
    220. 

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

  221. </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>描述对象的<code>enumerable</code>属性，称为“可枚举性”，如果该属性为<code>false</code>，就表示某些操作会忽略当前属性。</p> <p>目前，有四个操作会忽略<code>enumerable</code>为<code>false</code>的属性。</p> <ul><li><code>for...in</code>循环：只遍历对象自身的和继承的可枚举的属性。</li> <li><code>Object.keys()</code>：返回对象自身的所有可枚举的属性的键名。</li> <li><code>JSON.stringify()</code>：只串行化对象自身的可枚举的属性。</li> <li><code>Object.assign()</code>： 忽略<code>enumerable</code>为<code>false</code>的属性，只拷贝对象自身的可枚举的属性。</li></ul> <p>这四个操作之中，前三个是 ES5 就有的，最后一个<code>Object.assign()</code>是 ES6 新增的。其中，只有<code>for...in</code>会返回继承的属性，其他三个方法都会忽略继承的属性，只处理对象自身的属性。实际上，引入“可枚举”（<code>enumerable</code>）这个概念的最初目的，就是让某些属性可以规避掉<code>for...in</code>操作，不然所有内部属性和方法都会被遍历到。比如，对象原型的<code>toString</code>方法，以及数组的<code>length</code>属性，就通过“可枚举性”，从而避免被<code>for...in</code>遍历到。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token string">'toString'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>enumerable
    222. 

  222. <span class="token comment">// false</span>
    223. 

  223. 

  224. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">'length'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>enumerable
    225. 

  225. <span class="token comment">// false</span>
    226. 

  226. </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>toString</code>和<code>length</code>属性的<code>enumerable</code>都是<code>false</code>，因此<code>for...in</code>不会遍历到这两个继承自原型的属性。</p> <p>另外，ES6 规定，所有 Class 的原型的方法都是不可枚举的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span><span class="token keyword">class</span> <span class="token punctuation">{</span><span class="token function">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 punctuation">}</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>enumerable
    227. 

  227. <span class="token comment">// false</span>
    228. 

  228. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>总的来说，操作中引入继承的属性会让问题复杂化，大多数时候，我们只关心对象自身的属性。所以，尽量不要用<code>for...in</code>循环，而用<code>Object.keys()</code>代替。</p> <h3 id="属性的遍历"><a href="#属性的遍历" class="header-anchor">#</a> 属性的遍历</h3> <p>ES6 一共有 5 种方法可以遍历对象的属性。</p> <p><strong>（1）for...in</strong></p> <p><code>for...in</code>循环遍历对象自身的和继承的可枚举属性（不含 Symbol 属性）。</p> <p><strong>（2）Object.keys(obj)</strong></p> <p><code>Object.keys</code>返回一个数组，包括对象自身的（不含继承的）所有可枚举属性（不含 Symbol 属性）的键名。</p> <p><strong>（3）Object.getOwnPropertyNames(obj)</strong></p> <p><code>Object.getOwnPropertyNames</code>返回一个数组，包含对象自身的所有属性（不含 Symbol 属性，但是包括不可枚举属性）的键名。</p> <p><strong>（4）Object.getOwnPropertySymbols(obj)</strong></p> <p><code>Object.getOwnPropertySymbols</code>返回一个数组，包含对象自身的所有 Symbol 属性的键名。</p> <p><strong>（5）Reflect.ownKeys(obj)</strong></p> <p><code>Reflect.ownKeys</code>返回一个数组，包含对象自身的所有键名，不管键名是 Symbol 或字符串，也不管是否可枚举。</p> <p>以上的 5 种方法遍历对象的键名，都遵守同样的属性遍历的次序规则。</p> <ul><li>首先遍历所有数值键，按照数值升序排列。</li> <li>其次遍历所有字符串键，按照加入时间升序排列。</li> <li>最后遍历所有 Symbol 键，按照加入时间升序排列。</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token punctuation">[</span><span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token operator">:</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token operator">:</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token literal-property property">a</span><span class="token operator">:</span><span class="token number">0</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    229. 

  229. <span class="token comment">// ['2', '10', 'b', 'a', Symbol()]</span>
    230. 

  230. </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>Reflect.ownKeys</code>方法返回一个数组，包含了参数对象的所有属性。这个数组的属性次序是这样的，首先是数值属性<code>2</code>和<code>10</code>，其次是字符串属性<code>b</code>和<code>a</code>，最后是 Symbol 属性。</p> <h2 id="super-关键字"><a href="#super-关键字" class="header-anchor">#</a> super 关键字</h2> <p>我们知道，<code>this</code>关键字总是指向函数所在的当前对象，ES6 又新增了另一个类似的关键字<code>super</code>，<strong>指向当前对象的原型对象</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> proto <span class="token operator">=</span> <span class="token punctuation">{</span>
    231. 

  231.   <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'hello'</span>
    232. 

  232. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    233. 

  233. 

  234. <span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    235. 

  235.   <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'world'</span><span class="token punctuation">,</span>
    236. 

  236.   <span class="token function">find</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    237. 

  237.     <span class="token keyword">return</span> <span class="token keyword">super</span><span class="token punctuation">.</span>foo<span class="token punctuation">;</span>
    238. 

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

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

  240. 

  241. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    242. 

  242. obj<span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;hello&quot;</span>
    243. 

  243. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><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，对象<code>obj.find()</code>方法之中，通过<code>super.foo</code>引用了原型对象<code>proto</code>的<code>foo</code>属性。</p> <p>注意，<code>super</code>关键字表示原型对象时，只能用在对象的方法之中，用在其他地方都会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    244. 

  244. <span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    245. 

  245.   <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token keyword">super</span><span class="token punctuation">.</span>foo
    246. 

  246. <span class="token punctuation">}</span>
    247. 

  247. 

  248. <span class="token comment">// 报错</span>
    249. 

  249. <span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    250. 

  250.   <span class="token function-variable function">foo</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">super</span><span class="token punctuation">.</span>foo
    251. 

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

  252. 

  253. <span class="token comment">// 报错</span>
    254. 

  254. <span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    255. 

  255.   <span class="token function-variable function">foo</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>
    256. 

  256.     <span class="token keyword">return</span> <span class="token keyword">super</span><span class="token punctuation">.</span>foo
    257. 

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

  258. <span class="token punctuation">}</span>
    259. 

  259. </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>上面三种<code>super</code>的用法都会报错，因为对于 JavaScript 引擎来说，这里的<code>super</code>都没有用在对象的方法之中。第一种写法是<code>super</code>用在属性里面，第二种和第三种写法是<code>super</code>用在一个函数里面，然后赋值给<code>foo</code>属性。目前，只有对象方法的简写法可以让 JavaScript 引擎确认，定义的是对象的方法。</p> <p>JavaScript 引擎内部，<code>super.foo</code>等同于<code>Object.getPrototypeOf(this).foo</code>（属性）或<code>Object.getPrototypeOf(this).foo.call(this)</code>（方法）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> proto <span class="token operator">=</span> <span class="token punctuation">{</span>
    260. 

  260.   <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token string">'hello'</span><span class="token punctuation">,</span>
    261. 

  261.   <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    262. 

  262.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    263. 

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

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

  265. 

  266. <span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    267. 

  267.   <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token string">'world'</span><span class="token punctuation">,</span>
    268. 

  268.   <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    269. 

  269.     <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    270. 

  270.   <span class="token punctuation">}</span>
    271. 

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

  272. 

  273. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    274. 

  274. 

  275. obj<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;world&quot;</span>
    276. 

  276. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，<code>super.foo</code>指向原型对象<code>proto</code>的<code>foo</code>方法，但是绑定的<code>this</code>却还是当前对象<code>obj</code>，因此输出的就是<code>world</code>。</p> <h2 id="对象的扩展运算符"><a href="#对象的扩展运算符" class="header-anchor">#</a> 对象的扩展运算符</h2> <p>《数组的扩展》一章中，已经介绍过扩展运算符（<code>...</code>）。ES2018 将这个运算符<a href="https://github.com/sebmarkbage/ecmascript-rest-spread" target="_blank" rel="noopener noreferrer">引入<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>了对象。</p> <h3 id="解构赋值"><a href="#解构赋值" class="header-anchor">#</a> 解构赋值</h3> <p>对象的解构赋值用于从一个对象取值，相当于将目标对象自身的所有可遍历的（enumerable）、但尚未被读取的属性，分配到指定的对象上面。所有的键和它们的值，都会拷贝到新对象上面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> <span class="token operator">...</span>z <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">4</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    277. 

  277. x <span class="token comment">// 1</span>
    278. 

  278. y <span class="token comment">// 2</span>
    279. 

  279. z <span class="token comment">// { a: 3, b: 4 }</span>
    280. 

  280. </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，变量<code>z</code>是解构赋值所在的对象。它获取等号右边的所有尚未读取的键（<code>a</code>和<code>b</code>），将它们连同值一起拷贝过来。</p> <p>由于解构赋值要求等号右边是一个对象，所以如果等号右边是<code>undefined</code>或<code>null</code>，就会报错，因为它们无法转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token operator">...</span>z <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// 运行时错误</span>
    281. 

  281. <span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token operator">...</span>z <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span> <span class="token comment">// 运行时错误</span>
    282. 

  282. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>解构赋值必须是最后一个参数，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token operator">...</span>x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z <span class="token punctuation">}</span> <span class="token operator">=</span> someObject<span class="token punctuation">;</span> <span class="token comment">// 句法错误</span>
    283. 

  283. <span class="token keyword">let</span> <span class="token punctuation">{</span> x<span class="token punctuation">,</span> <span class="token operator">...</span>y<span class="token punctuation">,</span> <span class="token operator">...</span>z <span class="token punctuation">}</span> <span class="token operator">=</span> someObject<span class="token punctuation">;</span> <span class="token comment">// 句法错误</span>
    284. 

  284. </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，解构赋值不是最后一个参数，所以会报错。</p> <p>注意，解构赋值的拷贝是浅拷贝，即如果一个键的值是复合类型的值（数组、对象、函数）、那么解构赋值拷贝的是这个值的引用，而不是这个值的副本。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">1</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    285. 

  285. <span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token operator">...</span>x <span class="token punctuation">}</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span>
    286. 

  286. obj<span class="token punctuation">.</span>a<span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    287. 

  287. x<span class="token punctuation">.</span>a<span class="token punctuation">.</span>b <span class="token comment">// 2</span>
    288. 

  288. </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>x</code>是解构赋值所在的对象，拷贝了对象<code>obj</code>的<code>a</code>属性。<code>a</code>属性引用了一个对象，修改这个对象的值，会影响到解构赋值对它的引用。</p> <p>另外，扩展运算符的解构赋值，不能复制继承自原型对象的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> o1 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    289. 

  289. <span class="token keyword">let</span> o2 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    290. 

  290. o2<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> o1<span class="token punctuation">;</span>
    291. 

  291. <span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token operator">...</span>o3 <span class="token punctuation">}</span> <span class="token operator">=</span> o2<span class="token punctuation">;</span>
    292. 

  292. o3 <span class="token comment">// { b: 2 }</span>
    293. 

  293. o3<span class="token punctuation">.</span>a <span class="token comment">// undefined</span>
    294. 

  294. </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>上面代码中，对象<code>o3</code>复制了<code>o2</code>，但是只复制了<code>o2</code>自身的属性，没有复制它的原型对象<code>o1</code>的属性。</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">const</span> o <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    295. 

  295. o<span class="token punctuation">.</span>z <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    296. 

  296. 

  297. <span class="token keyword">let</span> <span class="token punctuation">{</span> x<span class="token punctuation">,</span> <span class="token operator">...</span>newObj <span class="token punctuation">}</span> <span class="token operator">=</span> o<span class="token punctuation">;</span>
    298. 

  298. <span class="token keyword">let</span> <span class="token punctuation">{</span> y<span class="token punctuation">,</span> z <span class="token punctuation">}</span> <span class="token operator">=</span> newObj<span class="token punctuation">;</span>
    299. 

  299. x <span class="token comment">// 1</span>
    300. 

  300. y <span class="token comment">// undefined</span>
    301. 

  301. z <span class="token comment">// 3</span>
    302. 

  302. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><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，变量<code>x</code>是单纯的解构赋值，所以可以读取对象<code>o</code>继承的属性；变量<code>y</code>和<code>z</code>是扩展运算符的解构赋值，只能读取对象<code>o</code>自身的属性，所以变量<code>z</code>可以赋值成功，变量<code>y</code>取不到值。ES6 规定，变量声明语句之中，如果使用解构赋值，扩展运算符后面必须是一个变量名，而不能是一个解构赋值表达式，所以上面代码引入了中间变量<code>newObj</code>，如果写成下面这样会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> x<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">{</span> y<span class="token punctuation">,</span> z <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token operator">=</span> o<span class="token punctuation">;</span>
    303. 

  303. <span class="token comment">// SyntaxError: ... must be followed by an identifier in declaration contexts</span>
    304. 

  304. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>解构赋值的一个用处，是扩展某个函数的参数，引入其他操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">baseFunction</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span> a<span class="token punctuation">,</span> b <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    305. 

  305.   <span class="token comment">// ...</span>
    306. 

  306. <span class="token punctuation">}</span>
    307. 

  307. <span class="token keyword">function</span> <span class="token function">wrapperFunction</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span> x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> <span class="token operator">...</span>restConfig <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    308. 

  308.   <span class="token comment">// 使用 x 和 y 参数进行操作</span>
    309. 

  309.   <span class="token comment">// 其余参数传给原始函数</span>
    310. 

  310.   <span class="token keyword">return</span> <span class="token function">baseFunction</span><span class="token punctuation">(</span>restConfig<span class="token punctuation">)</span><span class="token punctuation">;</span>
    311. 

  311. <span class="token punctuation">}</span>
    312. 

  312. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><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，原始函数<code>baseFunction</code>接受<code>a</code>和<code>b</code>作为参数，函数<code>wrapperFunction</code>在<code>baseFunction</code>的基础上进行了扩展，能够接受多余的参数，并且保留原始函数的行为。</p> <h3 id="扩展运算符"><a href="#扩展运算符" class="header-anchor">#</a> 扩展运算符</h3> <p>对象的扩展运算符（<code>...</code>）用于取出参数对象的所有可遍历属性，拷贝到当前对象之中。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> z <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">4</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    313. 

  313. <span class="token keyword">let</span> n <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>z <span class="token punctuation">}</span><span class="token punctuation">;</span>
    314. 

  314. n <span class="token comment">// { a: 3, b: 4 }</span>
    315. 

  315. </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>由于数组是特殊的对象，所以对象的扩展运算符也可以用于数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> foo <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    316. 

  316. foo
    317. 

  317. <span class="token comment">// {0: &quot;a&quot;, 1: &quot;b&quot;, 2: &quot;c&quot;}</span>
    318. 

  318. </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>如果扩展运算符后面是一个空对象，则没有任何效果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span><span class="token operator">...</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span>
    319. 

  319. <span class="token comment">// { a: 1 }</span>
    320. 

  320. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果扩展运算符后面不是对象，则会自动将其转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 等同于 {...Object(1)}</span>
    321. 

  321. <span class="token punctuation">{</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token number">.1</span><span class="token punctuation">}</span> <span class="token comment">// {}</span>
    322. 

  322. </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，扩展运算符后面是整数<code>1</code>，会自动转为数值的包装对象<code>Number{1}</code>。由于该对象没有自身属性，所以返回一个空对象。</p> <p>下面的例子都是类似的道理。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 等同于 {...Object(true)}</span>
    323. 

  323. <span class="token punctuation">{</span><span class="token operator">...</span><span class="token boolean">true</span><span class="token punctuation">}</span> <span class="token comment">// {}</span>
    324. 

  324. 

  325. <span class="token comment">// 等同于 {...Object(undefined)}</span>
    326. 

  326. <span class="token punctuation">{</span><span class="token operator">...</span><span class="token keyword">undefined</span><span class="token punctuation">}</span> <span class="token comment">// {}</span>
    327. 

  327. 

  328. <span class="token comment">// 等同于 {...Object(null)}</span>
    329. 

  329. <span class="token punctuation">{</span><span class="token operator">...</span><span class="token keyword">null</span><span class="token punctuation">}</span> <span class="token comment">// {}</span>
    330. 

  330. </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>但是，如果扩展运算符后面是字符串，它会自动转成一个类似数组的对象，因此返回的不是空对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span><span class="token operator">...</span><span class="token string">'hello'</span><span class="token punctuation">}</span>
    331. 

  331. <span class="token comment">// {0: &quot;h&quot;, 1: &quot;e&quot;, 2: &quot;l&quot;, 3: &quot;l&quot;, 4: &quot;o&quot;}</span>
    332. 

  332. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>对象的扩展运算符等同于使用<code>Object.assign()</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> aClone <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>a <span class="token punctuation">}</span><span class="token punctuation">;</span>
    333. 

  333. <span class="token comment">// 等同于</span>
    334. 

  334. <span class="token keyword">let</span> aClone <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span><span class="token punctuation">;</span>
    335. 

  335. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面的例子只是拷贝了对象实例的属性，如果想完整克隆一个对象，还拷贝对象原型的属性，可以采用下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 写法一</span>
    336. 

  336. <span class="token keyword">const</span> clone1 <span class="token operator">=</span> <span class="token punctuation">{</span>
    337. 

  337.   <span class="token literal-property property">__proto__</span><span class="token operator">:</span> Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">,</span>
    338. 

  338.   <span class="token operator">...</span>obj
    339. 

  339. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    340. 

  340. 

  341. <span class="token comment">// 写法二</span>
    342. 

  342. <span class="token keyword">const</span> clone2 <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>
    343. 

  343.   Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    344. 

  344.   obj
    345. 

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

  346. 

  347. <span class="token comment">// 写法三</span>
    348. 

  348. <span class="token keyword">const</span> clone3 <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>
    349. 

  349.   Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">,</span>
    350. 

  350.   Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
    351. 

  351. <span class="token punctuation">)</span>
    352. 

  352. </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，写法一的<code>__proto__</code>属性在非浏览器的环境不一定部署，因此推荐使用写法二和写法三。</p> <p>扩展运算符可以用于合并两个对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> ab <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>a<span class="token punctuation">,</span> <span class="token operator">...</span>b <span class="token punctuation">}</span><span class="token punctuation">;</span>
    353. 

  353. <span class="token comment">// 等同于</span>
    354. 

  354. <span class="token keyword">let</span> ab <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> a<span class="token punctuation">,</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>
    355. 

  355. </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>如果用户自定义的属性，放在扩展运算符后面，则扩展运算符内部的同名属性会被覆盖掉。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> aWithOverrides <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>a<span class="token punctuation">,</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    356. 

  356. <span class="token comment">// 等同于</span>
    357. 

  357. <span class="token keyword">let</span> aWithOverrides <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>a<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    358. 

  358. <span class="token comment">// 等同于</span>
    359. 

  359. <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">,</span> aWithOverrides <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>a<span class="token punctuation">,</span> x<span class="token punctuation">,</span> y <span class="token punctuation">}</span><span class="token punctuation">;</span>
    360. 

  360. <span class="token comment">// 等同于</span>
    361. 

  361. <span class="token keyword">let</span> aWithOverrides <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> a<span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    362. 

  362. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>a</code>对象的<code>x</code>属性和<code>y</code>属性，拷贝到新对象后会被覆盖掉。</p> <p>这用来修改现有对象部分的属性就很方便了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> newVersion <span class="token operator">=</span> <span class="token punctuation">{</span>
    363. 

  363.   <span class="token operator">...</span>previousVersion<span class="token punctuation">,</span>
    364. 

  364.   <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'New Name'</span> <span class="token comment">// Override the name property</span>
    365. 

  365. <span class="token punctuation">}</span><span class="token punctuation">;</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></div></div><p>上面代码中，<code>newVersion</code>对象自定义了<code>name</code>属性，其他属性全部复制自<code>previousVersion</code>对象。</p> <p>如果把自定义属性放在扩展运算符前面，就变成了设置新对象的默认属性值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> aWithDefaults <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">...</span>a <span class="token punctuation">}</span><span class="token punctuation">;</span>
    367. 

  367. <span class="token comment">// 等同于</span>
    368. 

  368. <span class="token keyword">let</span> aWithDefaults <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</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> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span><span class="token punctuation">;</span>
    369. 

  369. <span class="token comment">// 等同于</span>
    370. 

  370. <span class="token keyword">let</span> aWithDefaults <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span><span class="token punctuation">;</span>
    371. 

  371. </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>与数组的扩展运算符一样，对象的扩展运算符后面可以跟表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    372. 

  372.   <span class="token operator">...</span><span class="token punctuation">(</span>x <span class="token operator">&gt;</span> <span class="token number">1</span> <span class="token operator">?</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span> <span class="token operator">:</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    373. 

  373.   <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
    374. 

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

  375. </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>扩展运算符的参数对象之中，如果有取值函数<code>get</code>，这个函数是会执行的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 并不会抛出错误，因为 x 属性只是被定义，但没执行</span>
    376. 

  376. <span class="token keyword">let</span> aWithXGetter <span class="token operator">=</span> <span class="token punctuation">{</span>
    377. 

  377.   <span class="token operator">...</span>a<span class="token punctuation">,</span>
    378. 

  378.   <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>
    379. 

  379.     <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">'not throw yet'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    380. 

  380.   <span class="token punctuation">}</span>
    381. 

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

  382. 

  383. <span class="token comment">// 会抛出错误，因为 x 属性被执行了</span>
    384. 

  384. <span class="token keyword">let</span> runtimeError <span class="token operator">=</span> <span class="token punctuation">{</span>
    385. 

  385.   <span class="token operator">...</span>a<span class="token punctuation">,</span>
    386. 

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

  387.     <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>
    388. 

  388.       <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">'throw now'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    389. 

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

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

  391. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    392. 

  392. </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><h2 id="链判断运算符"><a href="#链判断运算符" class="header-anchor">#</a> 链判断运算符</h2> <p>编程实务中，如果读取对象内部的某个属性，往往需要判断一下该对象是否存在。比如，要读取<code>message.body.user.firstName</code>，安全的写法是写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> firstName <span class="token operator">=</span> <span class="token punctuation">(</span>message
    393. 

  393.   <span class="token operator">&amp;&amp;</span> message<span class="token punctuation">.</span>body
    394. 

  394.   <span class="token operator">&amp;&amp;</span> message<span class="token punctuation">.</span>body<span class="token punctuation">.</span>user
    395. 

  395.   <span class="token operator">&amp;&amp;</span> message<span class="token punctuation">.</span>body<span class="token punctuation">.</span>user<span class="token punctuation">.</span>firstName<span class="token punctuation">)</span> <span class="token operator">||</span> <span class="token string">'default'</span><span class="token punctuation">;</span>
    396. 

  396. </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>或者使用三元运算符<code>?:</code>，判断一个对象是否存在。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> fooInput <span class="token operator">=</span> myForm<span class="token punctuation">.</span><span class="token function">querySelector</span><span class="token punctuation">(</span><span class="token string">'input[name=foo]'</span><span class="token punctuation">)</span>
    397. 

  397. <span class="token keyword">const</span> fooValue <span class="token operator">=</span> fooInput <span class="token operator">?</span> fooInput<span class="token punctuation">.</span>value <span class="token operator">:</span> <span class="token keyword">undefined</span>
    398. 

  398. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这样的层层判断非常麻烦，因此 <a href="https://github.com/tc39/proposal-optional-chaining" target="_blank" rel="noopener noreferrer">ES2020<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> 引入了“链判断运算符”（optional chaining operator）<code>?.</code>，简化上面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> firstName <span class="token operator">=</span> message<span class="token operator">?.</span>body<span class="token operator">?.</span>user<span class="token operator">?.</span>firstName <span class="token operator">||</span> <span class="token string">'default'</span><span class="token punctuation">;</span>
    399. 

  399. <span class="token keyword">const</span> fooValue <span class="token operator">=</span> myForm<span class="token punctuation">.</span><span class="token function">querySelector</span><span class="token punctuation">(</span><span class="token string">'input[name=foo]'</span><span class="token punctuation">)</span><span class="token operator">?.</span>value
    400. 

  400. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码使用了<code>?.</code>运算符，直接在链式调用的时候判断，左侧的对象是否为<code>null</code>或<code>undefined</code>。如果是的，就不再往下运算，而是返回<code>undefined</code>。</p> <p>链判断运算符有三种用法。</p> <ul><li><code>obj?.prop</code> // 对象属性</li> <li><code>obj?.[expr]</code> // 同上</li> <li><code>func?.(...args)</code> // 函数或对象方法的调用</li></ul> <p>下面是判断对象方法是否存在，如果存在就立即执行的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>iterator<span class="token punctuation">.</span>return<span class="token operator">?.</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    401. 

  401. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>iterator.return</code>如果有定义，就会调用该方法，否则直接返回<code>undefined</code>。</p> <p>对于那些可能没有实现的方法，这个运算符尤其有用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>myForm<span class="token punctuation">.</span>checkValidity<span class="token operator">?.</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    402. 

  402.   <span class="token comment">// 表单校验失败</span>
    403. 

  403.   <span class="token keyword">return</span><span class="token punctuation">;</span>
    404. 

  404. <span class="token punctuation">}</span>
    405. 

  405. </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，老式浏览器的表单可能没有<code>checkValidity</code>这个方法，这时<code>?.</code>运算符就会返回<code>undefined</code>，判断语句就变成了<code>undefined === false</code>，所以就会跳过下面的代码。</p> <p>下面是这个运算符常见的使用形式，以及不使用该运算符时的等价形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>a<span class="token operator">?.</span>b
    406. 

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

  407. a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> a<span class="token punctuation">.</span>b
    408. 

  408. 

  409. a<span class="token operator">?.</span><span class="token punctuation">[</span>x<span class="token punctuation">]</span>
    410. 

  410. <span class="token comment">// 等同于</span>
    411. 

  411. a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> a<span class="token punctuation">[</span>x<span class="token punctuation">]</span>
    412. 

  412. 

  413. a<span class="token operator">?.</span><span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    414. 

  414. <span class="token comment">// 等同于</span>
    415. 

  415. a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> a<span class="token punctuation">.</span><span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    416. 

  416. 

  417. a<span class="token operator">?.</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    418. 

  418. <span class="token comment">// 等同于</span>
    419. 

  419. a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> <span class="token function">a</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    420. 

  420. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><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，特别注意后两种形式，如果<code>a?.b()</code>里面的<code>a.b</code>不是函数，不可调用，那么<code>a?.b()</code>是会报错的。<code>a?.()</code>也是如此，如果<code>a</code>不是<code>null</code>或<code>undefined</code>，但也不是函数，那么<code>a?.()</code>会报错。</p> <p>使用这个运算符，有几个注意点。</p> <p>（1）短路机制</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>a<span class="token operator">?.</span><span class="token punctuation">[</span><span class="token operator">++</span>x<span class="token punctuation">]</span>
    421. 

  421. <span class="token comment">// 等同于</span>
    422. 

  422. a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> a<span class="token punctuation">[</span><span class="token operator">++</span>x<span class="token punctuation">]</span>
    423. 

  423. </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>上面代码中，如果<code>a</code>是<code>undefined</code>或<code>null</code>，那么<code>x</code>不会进行递增运算。也就是说，链判断运算符一旦为真，右侧的表达式就不再求值。</p> <p>（2）delete 运算符</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">delete</span> a<span class="token operator">?.</span>b
    424. 

  424. <span class="token comment">// 等同于</span>
    425. 

  425. a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> <span class="token keyword">delete</span> a<span class="token punctuation">.</span>b
    426. 

  426. </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>上面代码中，如果<code>a</code>是<code>undefined</code>或<code>null</code>，会直接返回<code>undefined</code>，而不会进行<code>delete</code>运算。</p> <p>（3）括号的影响</p> <p>如果属性链有圆括号，链判断运算符对圆括号外部没有影响，只对圆括号内部有影响。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span>a<span class="token operator">?.</span>b<span class="token punctuation">)</span><span class="token punctuation">.</span>c
    427. 

  427. <span class="token comment">// 等价于</span>
    428. 

  428. <span class="token punctuation">(</span>a <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">undefined</span> <span class="token operator">:</span> a<span class="token punctuation">.</span>b<span class="token punctuation">)</span><span class="token punctuation">.</span>c
    429. 

  429. </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>上面代码中，<code>?.</code>对圆括号外部没有影响，不管<code>a</code>对象是否存在，圆括号后面的<code>.c</code>总是会执行。</p> <p>一般来说，使用<code>?.</code>运算符的场合，不应该使用圆括号。</p> <p>（4）报错场合</p> <p>以下写法是禁止的，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 构造函数</span>
    430. 

  430. <span class="token keyword">new</span> <span class="token class-name">a</span><span class="token operator">?.</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    431. 

  431. <span class="token keyword">new</span> <span class="token class-name">a</span><span class="token operator">?.</span><span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    432. 

  432. 

  433. <span class="token comment">// 链判断运算符的右侧有模板字符串</span>
    434. 

  434. a<span class="token operator">?.</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">{b}</span><span class="token template-punctuation string">`</span></span>
    435. 

  435. a<span class="token operator">?.</span>b<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">{c}</span><span class="token template-punctuation string">`</span></span>
    436. 

  436. 

  437. <span class="token comment">// 链判断运算符的左侧是 super</span>
    438. 

  438. <span class="token keyword">super</span><span class="token operator">?.</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    439. 

  439. <span class="token keyword">super</span><span class="token operator">?.</span>foo
    440. 

  440. 

  441. <span class="token comment">// 链运算符用于赋值运算符左侧</span>
    442. 

  442. a<span class="token operator">?.</span>b <span class="token operator">=</span> c
    443. 

  443. </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>（5）右侧不得为十进制数值</p> <p>为了保证兼容以前的代码，允许<code>foo?.3:0</code>被解析成<code>foo ? .3 : 0</code>，因此规定如果<code>?.</code>后面紧跟一个十进制数字，那么<code>?.</code>不再被看成是一个完整的运算符，而会按照三元运算符进行处理，也就是说，那个小数点会归属于后面的十进制数字，形成一个小数。</p> <h2 id="null-判断运算符"><a href="#null-判断运算符" class="header-anchor">#</a> Null 判断运算符</h2> <p>读取对象属性的时候，如果某个属性的值是<code>null</code>或<code>undefined</code>，有时候需要为它们指定默认值。常见做法是通过<code>||</code>运算符指定默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> headerText <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token punctuation">.</span>headerText <span class="token operator">||</span> <span class="token string">'Hello, world!'</span><span class="token punctuation">;</span>
    444. 

  444. <span class="token keyword">const</span> animationDuration <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token punctuation">.</span>animationDuration <span class="token operator">||</span> <span class="token number">300</span><span class="token punctuation">;</span>
    445. 

  445. <span class="token keyword">const</span> showSplashScreen <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token punctuation">.</span>showSplashScreen <span class="token operator">||</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    446. 

  446. </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>上面的三行代码都通过<code>||</code>运算符指定默认值，但是这样写是错的。开发者的原意是，只要属性的值为<code>null</code>或<code>undefined</code>，默认值就会生效，但是属性的值如果为空字符串或<code>false</code>或<code>0</code>，默认值也会生效。</p> <p>为了避免这种情况，<a href="https://github.com/tc39/proposal-nullish-coalescing" target="_blank" rel="noopener noreferrer">ES2020<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> 引入了一个新的 Null 判断运算符<code>??</code>。它的行为类似<code>||</code>，但是只有运算符左侧的值为<code>null</code>或<code>undefined</code>时，才会返回右侧的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> headerText <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token punctuation">.</span>headerText <span class="token operator">??</span> <span class="token string">'Hello, world!'</span><span class="token punctuation">;</span>
    447. 

  447. <span class="token keyword">const</span> animationDuration <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token punctuation">.</span>animationDuration <span class="token operator">??</span> <span class="token number">300</span><span class="token punctuation">;</span>
    448. 

  448. <span class="token keyword">const</span> showSplashScreen <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token punctuation">.</span>showSplashScreen <span class="token operator">??</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    449. 

  449. </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>上面代码中，默认值只有在属性值为<code>null</code>或<code>undefined</code>时，才会生效。</p> <p>这个运算符的一个目的，就是跟链判断运算符<code>?.</code>配合使用，为<code>null</code>或<code>undefined</code>的值设置默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> animationDuration <span class="token operator">=</span> response<span class="token punctuation">.</span>settings<span class="token operator">?.</span>animationDuration <span class="token operator">??</span> <span class="token number">300</span><span class="token punctuation">;</span>
    450. 

  450. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>response.settings</code>如果是<code>null</code>或<code>undefined</code>，就会返回默认值300。</p> <p>这个运算符很适合判断函数参数是否赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Component</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    451. 

  451.   <span class="token keyword">const</span> enable <span class="token operator">=</span> props<span class="token punctuation">.</span>enabled <span class="token operator">??</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    452. 

  452.   <span class="token comment">// …</span>
    453. 

  453. <span class="token punctuation">}</span>
    454. 

  454. </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>上面代码判断<code>props</code>参数的<code>enabled</code>属性是否赋值，等同于下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Component</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    455. 

  455.   <span class="token keyword">const</span> <span class="token punctuation">{</span>
    456. 

  456.     <span class="token literal-property property">enabled</span><span class="token operator">:</span> enable <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    457. 

  457.   <span class="token punctuation">}</span> <span class="token operator">=</span> props<span class="token punctuation">;</span>
    458. 

  458.   <span class="token comment">// …</span>
    459. 

  459. <span class="token punctuation">}</span>
    460. 

  460. </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><code>??</code>有一个运算优先级问题，它与<code>&amp;&amp;</code>和<code>||</code>的优先级孰高孰低。现在的规则是，如果多个逻辑运算符一起使用，必须用括号表明优先级，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    461. 

  461. lhs <span class="token operator">&amp;&amp;</span> middle <span class="token operator">??</span> rhs
    462. 

  462. lhs <span class="token operator">??</span> middle <span class="token operator">&amp;&amp;</span> rhs
    463. 

  463. lhs <span class="token operator">||</span> middle <span class="token operator">??</span> rhs
    464. 

  464. lhs <span class="token operator">??</span> middle <span class="token operator">||</span> rhs
    465. 

  465. </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>上面四个表达式都会报错，必须加入表明优先级的括号。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span>lhs <span class="token operator">&amp;&amp;</span> middle<span class="token punctuation">)</span> <span class="token operator">??</span> rhs<span class="token punctuation">;</span>
    466. 

  466. lhs <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span>middle <span class="token operator">??</span> rhs<span class="token punctuation">)</span><span class="token punctuation">;</span>
    467. 

  467. 

  468. <span class="token punctuation">(</span>lhs <span class="token operator">??</span> middle<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> rhs<span class="token punctuation">;</span>
    469. 

  469. lhs <span class="token operator">??</span> <span class="token punctuation">(</span>middle <span class="token operator">&amp;&amp;</span> rhs<span class="token punctuation">)</span><span class="token punctuation">;</span>
    470. 

  470. 

  471. <span class="token punctuation">(</span>lhs <span class="token operator">||</span> middle<span class="token punctuation">)</span> <span class="token operator">??</span> rhs<span class="token punctuation">;</span>
    472. 

  472. lhs <span class="token operator">||</span> <span class="token punctuation">(</span>middle <span class="token operator">??</span> rhs<span class="token punctuation">)</span><span class="token punctuation">;</span>
    473. 

  473. 

  474. <span class="token punctuation">(</span>lhs <span class="token operator">??</span> middle<span class="token punctuation">)</span> <span class="token operator">||</span> rhs<span class="token punctuation">;</span>
    475. 

  475. lhs <span class="token operator">??</span> <span class="token punctuation">(</span>middle <span class="token operator">||</span> rhs<span class="token punctuation">)</span><span class="token punctuation">;</span>
    476. 

  476. </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></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/10.对象的扩展.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/e34009d60d8bc4b2/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">数组的扩展</div></a> <a href="/blog/pages/e85e68947502cf90/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">对象的新增方法</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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  478.         <a href="/blog/pages/e34009d60d8bc4b2/" class="prev">数组的扩展</a></span> <span class="next"><a href="/blog/pages/e85e68947502cf90/">对象的新增方法</a>→
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