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和 for-of 循环</a></li><li><a href="/blog/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/blog/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/blog/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/blog/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/blog/pages/83f8c3a0cd87dd83/" 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-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06225672>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>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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">let 和 const 命令<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="let-和-const-命令"><a href="#let-和-const-命令" class="header-anchor">#</a> let 和 const 命令</h1> <h2 id="let-命令"><a href="#let-命令" class="header-anchor">#</a> let 命令</h2> <h3 id="基本用法"><a href="#基本用法" class="header-anchor">#</a> 基本用法</h3> <p>ES6 新增了<code>let</code>命令，用来声明变量。它的用法类似于<code>var</code>，但是所声明的变量，<strong>只在<code>let</code>命令所在的代码块内有效(块级作用域)</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    26. 

  26.   <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    27. 

  27.   <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    28. 

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

  29. 

  30. a <span class="token comment">// ReferenceError: a is not defined.</span>
    31. 

  31. b <span class="token comment">// 1</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><span class="line-number">7</span><br></div></div> <p>上面代码在代码块之中，分别用<code>let</code>和<code>var</code>声明了两个变量。然后在代码块之外调用这两个变量，结果<code>let</code>声明的变量报错，<code>var</code>声明的变量返回了正确的值。这表明，<code>let</code>声明的变量只在它所在的代码块有效。</p> <p><code>for</code>循环的计数器，就很合适使用<code>let</code>命令。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    33. 

  33.   <span class="token comment">// ...</span>
    34. 

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

  35. 

  36. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    37. 

  37. <span class="token comment">// ReferenceError: i is not defined</span>
    38. 

  38. </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，计数器<code>i</code>只在<code>for</code>循环体内有效，在循环体外引用就会报错。</p> <blockquote><p>如果for循环内用<code>var</code>声明<code>i</code>，则会打印10</p></blockquote> <p>下面的代码如果使用<code>var</code>，最后输出的是<code>10</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    39. 

  39. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    40. 

  40.   a<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 循环时只是将函数赋值给a[i],并未执行函数。函数内的i是全局的i</span>
    41. 

  41.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    42. 

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

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

  44. a<span class="token punctuation">[</span><span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 10</span>
    45. 

  45. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>i</code>是<code>var</code>命令声明的，在全局范围内都有效，所以全局只有一个变量<code>i</code>。每一次循环，变量<code>i</code>的值都会发生改变，而循环内被赋给数组<code>a</code>的函数内部的<code>console.log(i)</code>，<strong>里面的<code>i</code>指向的就是全局的<code>i</code></strong>。也就是说，所有数组<code>a</code>的成员里面的<code>i</code>，指向的都是同一个<code>i</code>，导致运行时输出的是最后一轮的<code>i</code>的值，也就是 10。</p> <p>如果使用<code>let</code>，声明的变量仅在块级作用域内有效，最后输出的是 6。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    46. 

  46. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    47. 

  47.   a<span class="token punctuation">[</span>i<span class="token punctuation">]</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>
    48. 

  48.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    49. 

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

  50. <span class="token punctuation">}</span>
    51. 

  51. a<span class="token punctuation">[</span><span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 6</span>
    52. 

  52. </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，变量<code>i</code>是<code>let</code>声明的，<strong>当前的<code>i</code>只在本轮循环有效，所以每一次循环的<code>i</code>其实都是一个新的变量</strong>，所以最后输出的是<code>6</code>。你可能会问，如果每一轮循环的变量<code>i</code>都是重新声明的，那它怎么知道上一轮循环的值，从而计算出本轮循环的值？这是因为 JavaScript 引擎内部会记住上一轮循环的值，初始化本轮的变量<code>i</code>时，就在上一轮循环的基础上进行计算。</p> <p>另外，<code>for</code>循环还有一个特别之处，就是设置循环变量的那部分是一个父作用域，而循环体内部是一个单独的子作用域。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">3</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    53. 

  53.   <span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span>
    54. 

  54.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    55. 

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

  56. <span class="token comment">// abc</span>
    57. 

  57. <span class="token comment">// abc</span>
    58. 

  58. <span class="token comment">// abc</span>
    59. 

  59. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码正确运行，输出了 3 次<code>abc</code>。这表明函数内部的变量<code>i</code>与循环变量<code>i</code>不在同一个作用域，有各自单独的作用域。</p> <h3 id="不存在变量提升"><a href="#不存在变量提升" class="header-anchor">#</a> 不存在变量提升</h3> <p><code>var</code>命令会发生“变量提升”现象，即变量可以在声明之前使用，值为<code>undefined</code>。这种现象多多少少是有些奇怪的，按照一般的逻辑，变量应该在声明语句之后才可以使用。</p> <p>为了纠正这种现象，<code>let</code>命令改变了语法行为，它所声明的变量一定要在声明后使用，否则报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// var 的情况</span>
    60. 

  60. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出undefined</span>
    61. 

  61. <span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    62. 

  62. 

  63. <span class="token comment">// let 的情况</span>
    64. 

  64. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>bar<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错ReferenceError</span>
    65. 

  65. <span class="token keyword">let</span> bar <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    66. 

  66. </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，变量<code>foo</code>用<code>var</code>命令声明，会发生变量提升，即脚本开始运行时，变量<code>foo</code>已经存在了，但是没有值，所以会输出<code>undefined</code>。变量<code>bar</code>用<code>let</code>命令声明，不会发生变量提升。这表示在声明它之前，变量<code>bar</code>是不存在的，这时如果用到它，就会抛出一个错误。</p> <h3 id="暂时性死区"><a href="#暂时性死区" class="header-anchor">#</a> 暂时性死区</h3> <p><strong>只要块级作用域内存在<code>let</code>命令，它所声明的变量就“绑定”（binding）这个区域，不再受外部的影响。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> tmp <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    67. 

  67. 

  68. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    69. 

  69.   tmp <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    70. 

  70.   <span class="token keyword">let</span> tmp<span class="token punctuation">;</span>
    71. 

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

  72. </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，存在全局变量<code>tmp</code>，但是块级作用域内<code>let</code>又声明了一个局部变量<code>tmp</code>，导致后者绑定这个块级作用域，所以在<code>let</code>声明变量前，对<code>tmp</code>赋值会报错。</p> <p>ES6 明确规定，<strong>如果区块中存在<code>let</code>和<code>const</code>命令，这个区块对这些命令声明的变量，从一开始就形成了封闭作用域。凡是在声明之前就使用这些变量，就会报错。</strong></p> <p>总之，在代码块内，**使用<code>let</code>命令声明变量之前，该变量都是不可用的。这在语法上，称为“暂时性死区”（**temporal dead zone，简称 TDZ）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    73. 

  73.   <span class="token comment">// TDZ开始</span>
    74. 

  74.   tmp <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    75. 

  75.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>tmp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    76. 

  76. 

  77.   <span class="token keyword">let</span> tmp<span class="token punctuation">;</span> <span class="token comment">// TDZ结束</span>
    78. 

  78.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>tmp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
    79. 

  79. 

  80.   tmp <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    81. 

  81.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>tmp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 123</span>
    82. 

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

  83. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，在<code>let</code>命令声明变量<code>tmp</code>之前，都属于变量<code>tmp</code>的“死区”。</p> <p>“暂时性死区”也意味着<code>typeof</code>不再是一个百分之百安全的操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">typeof</span> x<span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    84. 

  84. <span class="token keyword">let</span> x<span class="token punctuation">;</span>
    85. 

  85. </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，变量<code>x</code>使用<code>let</code>命令声明，所以在声明之前，都属于<code>x</code>的“死区”，只要用到该变量就会报错。因此，<code>typeof</code>运行时就会抛出一个<code>ReferenceError</code>。</p> <p>作为比较，如果一个变量根本没有被声明，使用<code>typeof</code>反而不会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">typeof</span> undeclared_variable <span class="token comment">// &quot;undefined&quot;</span>
    86. 

  86. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>undeclared_variable</code>是一个不存在的变量名，结果返回“undefined”。所以，在没有<code>let</code>之前，<code>typeof</code>运算符是百分之百安全的，永远不会报错。现在这一点不成立了。这样的设计是为了让大家养成良好的编程习惯，变量一定要在声明之后使用，否则就报错。</p> <p>有些“死区”比较隐蔽，不太容易发现。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token parameter">x <span class="token operator">=</span> y<span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token number">2</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    87. 

  87.   <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>
    88. 

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

  89. 

  90. <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错</span>
    91. 

  91. </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，调用<code>bar</code>函数之所以报错（某些实现可能不报错），是因为参数<code>x</code>默认值等于另一个参数<code>y</code>，而此时<code>y</code>还没有声明，属于“死区”。如果<code>y</code>的默认值是<code>x</code>，就不会报错，因为此时<code>x</code>已经声明了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token parameter">x <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">,</span> y <span class="token operator">=</span> x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    92. 

  92.   <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>
    93. 

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

  94. <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [2, 2]</span>
    95. 

  95. </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>另外，下面的代码也会报错，与<code>var</code>的行为不同。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 不报错</span>
    96. 

  96. <span class="token keyword">var</span> x <span class="token operator">=</span> x<span class="token punctuation">;</span>
    97. 

  97. 

  98. <span class="token comment">// 报错</span>
    99. 

  99. <span class="token keyword">let</span> x <span class="token operator">=</span> x<span class="token punctuation">;</span>
    100. 

  100. <span class="token comment">// ReferenceError: x is not defined</span>
    101. 

  101. </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>上面代码报错，也是因为暂时性死区。使用<code>let</code>声明变量时，只要变量在还没有声明完成前使用，就会报错。上面这行就属于这个情况，在变量<code>x</code>的声明语句还没有执行完成前，就去取<code>x</code>的值，导致报错”x 未定义“。</p> <p>ES6 规定暂时性死区和<code>let</code>、<code>const</code>语句不出现变量提升，主要是为了减少运行时错误，防止在变量声明前就使用这个变量，从而导致意料之外的行为。这样的错误在 ES5 是很常见的，现在有了这种规定，避免此类错误就很容易了。</p> <p>总之，暂时性死区的本质就是，只要一进入当前作用域，所要使用的变量就已经存在了，但是不可获取，只有等到声明变量的那一行代码出现，才可以获取和使用该变量。</p> <h3 id="不允许重复声明"><a href="#不允许重复声明" class="header-anchor">#</a> 不允许重复声明</h3> <p><strong><code>let</code>不允许在相同作用域内，重复声明同一个变量。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    102. 

  102. <span class="token keyword">function</span> <span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    103. 

  103.   <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    104. 

  104.   <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    105. 

  105. <span class="token punctuation">}</span>
    106. 

  106. 

  107. <span class="token comment">// 报错</span>
    108. 

  108. <span class="token keyword">function</span> <span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    109. 

  109.   <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    110. 

  110.   <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span><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></div></div><p>因此，不能在函数内部重新声明参数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">func</span><span class="token punctuation">(</span><span class="token parameter">arg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    113. 

  113.   <span class="token keyword">let</span> arg<span class="token punctuation">;</span>
    114. 

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

  115. <span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
    116. 

  116. 

  117. <span class="token keyword">function</span> <span class="token function">func</span><span class="token punctuation">(</span><span class="token parameter">arg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    118. 

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

  119.     <span class="token keyword">let</span> arg<span class="token punctuation">;</span>
    120. 

  120.   <span class="token punctuation">}</span>
    121. 

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

  122. <span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 不报错</span>
    123. 

  123. </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><h2 id="块级作用域"><a href="#块级作用域" class="header-anchor">#</a> 块级作用域</h2> <h3 id="为什么需要块级作用域"><a href="#为什么需要块级作用域" class="header-anchor">#</a> 为什么需要块级作用域？</h3> <p>ES5 只有全局作用域和函数作用域，没有块级作用域，这带来很多不合理的场景。</p> <p>第一种场景，内层变量可能会覆盖外层变量。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> tmp <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    124. 

  124. 

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

  126.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>tmp<span class="token punctuation">)</span><span class="token punctuation">;</span>
    127. 

  127.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    128. 

  128.     <span class="token keyword">var</span> tmp <span class="token operator">=</span> <span class="token string">'hello world'</span><span class="token punctuation">;</span>
    129. 

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

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

  131. 

  132. <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
    133. 

  133. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码的原意是，<code>if</code>代码块的外部使用外层的<code>tmp</code>变量，内部使用内层的<code>tmp</code>变量。但是，函数<code>f</code>执行后，输出结果为<code>undefined</code>，原因在于变量提升，导致内层的<code>tmp</code>变量覆盖了外层的<code>tmp</code>变量。</p> <p>第二种场景，用来计数的循环变量泄露为全局变量。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    134. 

  134. 

  135. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> s<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    136. 

  136.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>s<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    137. 

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

  138. 

  139. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 5</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><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，变量<code>i</code>只用来控制循环，但是循环结束后，它并没有消失，泄露成了全局变量。</p> <h3 id="es6-的块级作用域"><a href="#es6-的块级作用域" class="header-anchor">#</a> ES6 的块级作用域</h3> <p><code>let</code>实际上为 JavaScript 新增了块级作用域。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">f1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    141. 

  141.   <span class="token keyword">let</span> n <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">;</span>
    142. 

  142.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    143. 

  143.     <span class="token keyword">let</span> n <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    144. 

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

  145.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 5</span>
    146. 

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

  147. </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>上面的函数有两个代码块，都声明了变量<code>n</code>，运行后输出 5。这表示外层代码块不受内层代码块的影响。如果两次都使用<code>var</code>定义变量<code>n</code>，最后输出的值才是 10。</p> <p>ES6 允许块级作用域的任意嵌套。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span><span class="token punctuation">{</span><span class="token punctuation">{</span><span class="token punctuation">{</span>
    148. 

  148.   <span class="token punctuation">{</span><span class="token keyword">let</span> insane <span class="token operator">=</span> <span class="token string">'Hello World'</span><span class="token punctuation">}</span>
    149. 

  149.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>insane<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错</span>
    150. 

  150. <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>
    151. 

  151. </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>上面代码使用了一个五层的块级作用域，每一层都是一个单独的作用域。第四层作用域无法读取第五层作用域的内部变量。</p> <p>内层作用域可以定义外层作用域的同名变量。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span><span class="token punctuation">{</span><span class="token punctuation">{</span><span class="token punctuation">{</span>
    152. 

  152.   <span class="token keyword">let</span> insane <span class="token operator">=</span> <span class="token string">'Hello World'</span><span class="token punctuation">;</span>
    153. 

  153.   <span class="token punctuation">{</span><span class="token keyword">let</span> insane <span class="token operator">=</span> <span class="token string">'Hello World'</span><span class="token punctuation">}</span>
    154. 

  154. <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>
    155. 

  155. </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>块级作用域的出现，实际上使得获得广泛应用的匿名立即执行函数表达式（匿名 IIFE）不再必要了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// IIFE 写法</span>
    156. 

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

  157.   <span class="token keyword">var</span> tmp <span class="token operator">=</span> <span class="token operator">...</span><span class="token punctuation">;</span>
    158. 

  158.   <span class="token operator">...</span>
    159. 

  159. <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>
    160. 

  160. 

  161. <span class="token comment">// 块级作用域写法</span>
    162. 

  162. <span class="token punctuation">{</span>
    163. 

  163.   <span class="token keyword">let</span> tmp <span class="token operator">=</span> <span class="token operator">...</span><span class="token punctuation">;</span>
    164. 

  164.   <span class="token operator">...</span>
    165. 

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

  166. </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><h3 id="块级作用域与函数声明"><a href="#块级作用域与函数声明" class="header-anchor">#</a> 块级作用域与函数声明</h3> <p>函数能不能在块级作用域之中声明？这是一个相当令人混淆的问题。</p> <p>ES5 规定，函数只能在顶层作用域和函数作用域之中声明，不能在块级作用域声明。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 情况一</span>
    167. 

  167. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    168. 

  168.   <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    169. 

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

  170. 

  171. <span class="token comment">// 情况二</span>
    172. 

  172. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    173. 

  173.   <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    174. 

  174. <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    175. 

  175.   <span class="token comment">// ...</span>
    176. 

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

  177. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面两种函数声明，根据 ES5 的规定都是非法的。</p> <p>但是，浏览器没有遵守这个规定，为了兼容以前的旧代码，还是支持在块级作用域之中声明函数，因此上面两种情况实际都能运行，不会报错。</p> <p>ES6 引入了块级作用域，明确允许在块级作用域之中声明函数。ES6 规定，块级作用域之中，函数声明语句的行为类似于<code>let</code>，在块级作用域之外不可引用。</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 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">'I am outside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    178. 

  178. 

  179. <span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    180. 

  180.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    181. 

  181.     <span class="token comment">// 重复声明一次函数f</span>
    182. 

  182.     <span class="token keyword">function</span> <span class="token function">f</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">'I am inside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    183. 

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

  184. 

  185.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    186. 

  186. <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>
    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><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码在 ES5 中运行，会得到“I am inside!”，因为在<code>if</code>内声明的函数<code>f</code>会被提升到函数头部，实际运行的代码如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5 环境</span>
    188. 

  188. <span class="token keyword">function</span> <span class="token function">f</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">'I am outside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    189. 

  189. 

  190. <span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    191. 

  191.   <span class="token keyword">function</span> <span class="token function">f</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">'I am inside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    192. 

  192.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    193. 

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

  194.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    195. 

  195. <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>
    196. 

  196. </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>ES6 就完全不一样了，理论上会得到“I am outside!”。因为块级作用域内声明的函数类似于<code>let</code>，对作用域之外没有影响。但是，如果你真的在 ES6 浏览器中运行一下上面的代码，是会报错的，这是为什么呢？</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 浏览器的 ES6 环境</span>
    197. 

  197. <span class="token keyword">function</span> <span class="token function">f</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">'I am outside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    198. 

  198. 

  199. <span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    200. 

  200.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    201. 

  201.     <span class="token comment">// 重复声明一次函数f</span>
    202. 

  202.     <span class="token keyword">function</span> <span class="token function">f</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">'I am inside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    203. 

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

  204. 

  205.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    206. 

  206. <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>
    207. 

  207. <span class="token comment">// Uncaught TypeError: f is not a function</span>
    208. 

  208. </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>上面的代码在 ES6 浏览器中，都会报错。</p> <p>原来，如果改变了块级作用域内声明的函数的处理规则，显然会对老代码产生很大影响。为了减轻因此产生的不兼容问题，ES6 在<a href="http://www.ecma-international.org/ecma-262/6.0/index.html#sec-block-level-function-declarations-web-legacy-compatibility-semantics" target="_blank" rel="noopener noreferrer">附录 B<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>里面规定，浏览器的实现可以不遵守上面的规定，有自己的<a href="http://stackoverflow.com/questions/31419897/what-are-the-precise-semantics-of-block-level-functions-in-es6" target="_blank" rel="noopener noreferrer">行为方式<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p> <ul><li>允许在块级作用域内声明函数。</li> <li>函数声明类似于<code>var</code>，即会提升到全局作用域或函数作用域的头部。</li> <li>同时，函数声明还会提升到所在的块级作用域的头部。</li></ul> <p>注意，上面三条规则只对 ES6 的浏览器实现有效，其他环境的实现不用遵守，还是将块级作用域的函数声明当作<code>let</code>处理。</p> <p>根据这三条规则，浏览器的 ES6 环境中，块级作用域内声明的函数，行为类似于<code>var</code>声明的变量。上面的例子实际运行的代码如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 浏览器的 ES6 环境</span>
    209. 

  209. <span class="token keyword">function</span> <span class="token function">f</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">'I am outside!'</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 keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    211. 

  211.   <span class="token keyword">var</span> f <span class="token operator">=</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span>
    212. 

  212.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    213. 

  213.     <span class="token keyword">function</span> <span class="token function">f</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">'I am inside!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    214. 

  214.   <span class="token punctuation">}</span>
    215. 

  215. 

  216.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    217. 

  217. <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>
    218. 

  218. <span class="token comment">// Uncaught TypeError: f is not a function</span>
    219. 

  219. </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>考虑到环境导致的行为差异太大，应该避免在块级作用域内声明函数。如果确实需要，也应该写成函数表达式，而不是函数声明语句。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 块级作用域内部的函数声明语句，建议不要使用</span>
    220. 

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

  221.   <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token string">'secret'</span><span class="token punctuation">;</span>
    222. 

  222.   <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    223. 

  223.     <span class="token keyword">return</span> a<span class="token punctuation">;</span>
    224. 

  224.   <span class="token punctuation">}</span>
    225. 

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

  226. 

  227. <span class="token comment">// 块级作用域内部，优先使用函数表达式</span>
    228. 

  228. <span class="token punctuation">{</span>
    229. 

  229.   <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token string">'secret'</span><span class="token punctuation">;</span>
    230. 

  230.   <span class="token keyword">let</span> <span class="token function-variable function">f</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>
    231. 

  231.     <span class="token keyword">return</span> a<span class="token punctuation">;</span>
    232. 

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

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

  234. </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>另外，还有一个需要注意的地方。ES6 的块级作用域必须有大括号，如果没有大括号，JavaScript 引擎就认为不存在块级作用域。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 第一种写法，报错</span>
    235. 

  235. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    236. 

  236. 

  237. <span class="token comment">// 第二种写法，不报错</span>
    238. 

  238. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    239. 

  239.   <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    240. 

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

  241. </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，第一种写法没有大括号，所以不存在块级作用域，而<code>let</code>只能出现在当前作用域的顶层，所以报错。第二种写法有大括号，所以块级作用域成立。</p> <p>函数声明也是如此，严格模式下，函数只能声明在当前作用域的顶层。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 不报错</span>
    242. 

  242. <span class="token string">'use strict'</span><span class="token punctuation">;</span>
    243. 

  243. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    244. 

  244.   <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    245. 

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

  246. 

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

  248. <span class="token string">'use strict'</span><span class="token punctuation">;</span>
    249. 

  249. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span>
    250. 

  250.   <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    251. 

  251. </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><h2 id="const-命令"><a href="#const-命令" class="header-anchor">#</a> const 命令</h2> <h3 id="基本用法-2"><a href="#基本用法-2" class="header-anchor">#</a> 基本用法</h3> <p><code>const</code>声明一个<strong>只读的常量</strong>。一旦声明，<strong>常量的值就不能改变</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token constant">PI</span> <span class="token operator">=</span> <span class="token number">3.1415</span><span class="token punctuation">;</span>
    252. 

  252. <span class="token constant">PI</span> <span class="token comment">// 3.1415</span>
    253. 

  253. 

  254. <span class="token constant">PI</span> <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    255. 

  255. <span class="token comment">// TypeError: Assignment to constant variable.</span>
    256. 

  256. </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>上面代码表明改变常量的值会报错。</p> <p><code>const</code>声明的变量不得改变值，这意味着，<code>const</code>一旦声明变量，就必须立即初始化，不能留到以后赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> foo<span class="token punctuation">;</span>
    257. 

  257. <span class="token comment">// SyntaxError: Missing initializer in const declaration</span>
    258. 

  258. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示，对于<code>const</code>来说，只声明不赋值，就会报错。</p> <p><code>const</code>的作用域与<code>let</code>命令相同：只在声明所在的块级作用域内有效。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    259. 

  259.   <span class="token keyword">const</span> <span class="token constant">MAX</span> <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">;</span>
    260. 

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

  261. 

  262. <span class="token constant">MAX</span> <span class="token comment">// Uncaught ReferenceError: MAX is not defined</span>
    263. 

  263. </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><code>const</code>命令声明的常量也是不提升，同样存在暂时性死区，只能在声明的位置后面使用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    264. 

  264.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token constant">MAX</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
    265. 

  265.   <span class="token keyword">const</span> <span class="token constant">MAX</span> <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">;</span>
    266. 

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

  267. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码在常量<code>MAX</code>声明之前就调用，结果报错。</p> <p><code>const</code>声明的常量，也与<code>let</code>一样不可重复声明。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> message <span class="token operator">=</span> <span class="token string">&quot;Hello!&quot;</span><span class="token punctuation">;</span>
    268. 

  268. <span class="token keyword">let</span> age <span class="token operator">=</span> <span class="token number">25</span><span class="token punctuation">;</span>
    269. 

  269. 

  270. <span class="token comment">// 以下两行都会报错</span>
    271. 

  271. <span class="token keyword">const</span> message <span class="token operator">=</span> <span class="token string">&quot;Goodbye!&quot;</span><span class="token punctuation">;</span>
    272. 

  272. <span class="token keyword">const</span> age <span class="token operator">=</span> <span class="token number">30</span><span class="token punctuation">;</span>
    273. 

  273. </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><h3 id="本质"><a href="#本质" class="header-anchor">#</a> 本质</h3> <p><code>const</code>实际上保证的，并不是变量的值不得改动，而是变量指向的那个内存地址所保存的数据不得改动。对于简单类型的数据（数值、字符串、布尔值），值就保存在变量指向的那个内存地址，因此等同于常量。但对于复合类型的数据（主要是对象和数组），变量指向的内存地址，保存的只是一个指向实际数据的指针，<code>const</code>只能保证这个指针是固定的（即总是指向另一个固定的地址），至于它指向的数据结构是不是可变的，就完全不能控制了。因此，将一个对象声明为常量必须非常小心。</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 punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    274. 

  274. 

  275. <span class="token comment">// 为 foo 添加一个属性，可以成功</span>
    276. 

  276. foo<span class="token punctuation">.</span>prop <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    277. 

  277. foo<span class="token punctuation">.</span>prop <span class="token comment">// 123</span>
    278. 

  278. 

  279. <span class="token comment">// 将 foo 指向另一个对象，就会报错</span>
    280. 

  280. foo <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// TypeError: &quot;foo&quot; is read-only</span>
    281. 

  281. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，常量<code>foo</code>储存的是一个地址，这个地址指向一个对象。不可变的只是这个地址，即不能把<code>foo</code>指向另一个地址，但对象本身是可变的，所以依然可以为其添加新属性。</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> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    282. 

  282. a<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token string">'Hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 可执行</span>
    283. 

  283. a<span class="token punctuation">.</span>length <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>    <span class="token comment">// 可执行</span>
    284. 

  284. a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'Dave'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>    <span class="token comment">// 报错</span>
    285. 

  285. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，常量<code>a</code>是一个数组，这个数组本身是可写的，但是如果将另一个数组赋值给<code>a</code>，就会报错。</p> <p>如果真的想将对象冻结，应该使用<code>Object.freeze</code>方法。</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> Object<span class="token punctuation">.</span><span class="token function">freeze</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>
    286. 

  286. 

  287. <span class="token comment">// 常规模式时，下面一行不起作用；</span>
    288. 

  288. <span class="token comment">// 严格模式时，该行会报错</span>
    289. 

  289. foo<span class="token punctuation">.</span>prop <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    290. 

  290. </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，常量<code>foo</code>指向一个冻结的对象，所以添加新属性不起作用，严格模式时还会报错。</p> <p>除了将对象本身冻结，对象的属性也应该冻结。下面是一个将对象彻底冻结的函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">constantize</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    291. 

  291.   Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    292. 

  292.   Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span> <span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> i</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    293. 

  293.     <span class="token keyword">if</span> <span class="token punctuation">(</span> <span class="token keyword">typeof</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token string">'object'</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span>
    294. 

  294.       <span class="token function">constantize</span><span class="token punctuation">(</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
    295. 

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

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

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

  298. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="es6-声明变量的六种方法"><a href="#es6-声明变量的六种方法" class="header-anchor">#</a> ES6 声明变量的六种方法</h3> <p>ES5 只有两种声明变量的方法：<code>var</code>命令和<code>function</code>命令。ES6 除了添加<code>let</code>和<code>const</code>命令，后面章节还会提到，另外两种声明变量的方法：<code>import</code>命令和<code>class</code>命令。所以，ES6 一共有 6 种声明变量的方法。</p> <h2 id="顶层对象的属性"><a href="#顶层对象的属性" class="header-anchor">#</a> 顶层对象的属性</h2> <p>顶层对象，在浏览器环境指的是<code>window</code>对象，在 Node 指的是<code>global</code>对象。ES5 之中，顶层对象的属性与全局变量是等价的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>window<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    299. 

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

  300. 

  301. a <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    302. 

  302. window<span class="token punctuation">.</span>a <span class="token comment">// 2</span>
    303. 

  303. </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，顶层对象的属性赋值与全局变量的赋值，是同一件事。</p> <p>顶层对象的属性与全局变量挂钩，被认为是 JavaScript 语言最大的设计败笔之一。这样的设计带来了几个很大的问题，首先是没法在编译时就报出变量未声明的错误，只有运行时才能知道（因为全局变量可能是顶层对象的属性创造的，而属性的创造是动态的）；其次，程序员很容易不知不觉地就创建了全局变量（比如打字出错）；最后，顶层对象的属性是到处可以读写的，这非常不利于模块化编程。另一方面，<code>window</code>对象有实体含义，指的是浏览器的窗口对象，顶层对象是一个有实体含义的对象，也是不合适的。</p> <p>ES6 为了改变这一点，一方面规定，为了保持兼容性，<code>var</code>命令和<code>function</code>命令声明的全局变量，依旧是顶层对象的属性；另一方面规定，<code>let</code>命令、<code>const</code>命令、<code>class</code>命令声明的全局变量，不属于顶层对象的属性。也就是说，<strong>从 ES6 开始，全局变量将逐步与顶层对象的属性脱钩。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    304. 

  304. <span class="token comment">// 如果在 Node 的 REPL 环境，可以写成 global.a</span>
    305. 

  305. <span class="token comment">// 或者采用通用方法，写成 this.a</span>
    306. 

  306. window<span class="token punctuation">.</span>a <span class="token comment">// 1</span>
    307. 

  307. 

  308. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    309. 

  309. window<span class="token punctuation">.</span>b <span class="token comment">// undefined</span>
    310. 

  310. </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，全局变量<code>a</code>由<code>var</code>命令声明，所以它是顶层对象的属性；全局变量<code>b</code>由<code>let</code>命令声明，所以它不是顶层对象的属性，返回<code>undefined</code>。</p> <h2 id="globalthis-对象"><a href="#globalthis-对象" class="header-anchor">#</a> globalThis 对象</h2> <p>JavaScript 语言存在一个顶层对象，它提供全局环境（即全局作用域），所有代码都是在这个环境中运行。但是，顶层对象在各种实现里面是不统一的。</p> <ul><li>浏览器里面，顶层对象是<code>window</code>，但 Node 和 Web Worker 没有<code>window</code>。</li> <li>浏览器和 Web Worker 里面，<code>self</code>也指向顶层对象，但是 Node 没有<code>self</code>。</li> <li>Node 里面，顶层对象是<code>global</code>，但其他环境都不支持。</li></ul> <p>同一段代码为了能够在各种环境，都能取到顶层对象，现在一般是使用<code>this</code>变量，但是有局限性。</p> <ul><li>全局环境中，<code>this</code>会返回顶层对象。但是，Node 模块和 ES6 模块中，<code>this</code>返回的是当前模块。</li> <li>函数里面的<code>this</code>，如果函数不是作为对象的方法运行，而是单纯作为函数运行，<code>this</code>会指向顶层对象。但是，严格模式下，这时<code>this</code>会返回<code>undefined</code>。</li> <li>不管是严格模式，还是普通模式，<code>new Function('return this')()</code>，总是会返回全局对象。但是，如果浏览器用了 CSP（Content Security Policy，内容安全策略），那么<code>eval</code>、<code>new Function</code>这些方法都可能无法使用。</li></ul> <p>综上所述，很难找到一种方法，可以在所有情况下，都取到顶层对象。下面是两种勉强可以使用的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 方法一</span>
    311. 

  311. <span class="token punctuation">(</span><span class="token keyword">typeof</span> window <span class="token operator">!==</span> <span class="token string">'undefined'</span>
    312. 

  312.    <span class="token operator">?</span> window
    313. 

  313.    <span class="token operator">:</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> process <span class="token operator">===</span> <span class="token string">'object'</span> <span class="token operator">&amp;&amp;</span>
    314. 

  314.       <span class="token keyword">typeof</span> require <span class="token operator">===</span> <span class="token string">'function'</span> <span class="token operator">&amp;&amp;</span>
    315. 

  315.       <span class="token keyword">typeof</span> global <span class="token operator">===</span> <span class="token string">'object'</span><span class="token punctuation">)</span>
    316. 

  316.      <span class="token operator">?</span> global
    317. 

  317.      <span class="token operator">:</span> <span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    318. 

  318. 

  319. <span class="token comment">// 方法二</span>
    320. 

  320. <span class="token keyword">var</span> <span class="token function-variable function">getGlobal</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>
    321. 

  321.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> self <span class="token operator">!==</span> <span class="token string">'undefined'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> self<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    322. 

  322.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> window <span class="token operator">!==</span> <span class="token string">'undefined'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> window<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    323. 

  323.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> global <span class="token operator">!==</span> <span class="token string">'undefined'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> global<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    324. 

  324.   <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">'unable to locate global object'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    325. 

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

  326. </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><a href="https://github.com/tc39/proposal-global" 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> 在语言标准的层面，引入<code>globalThis</code>作为顶层对象。也就是说，任何环境下，<code>globalThis</code>都是存在的，都可以从它拿到顶层对象，指向全局环境下的<code>this</code>。</p> <p>垫片库<a href="https://github.com/ungap/global-this" target="_blank" rel="noopener noreferrer"><code>global-this</code><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>模拟了这个提案，可以在所有环境拿到<code>globalThis</code>。</p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/02.let 和 const 命令.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/f344d070a1031ef7/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">ECMAScript 6 简介</div></a> <a href="/blog/pages/b1ab10a62f7564da/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">变量的解构赋值</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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