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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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">Module 的语法<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="module-的语法"><a href="#module-的语法" class="header-anchor">#</a> Module 的语法</h1> <h2 id="概述"><a href="#概述" class="header-anchor">#</a> 概述</h2> <p>历史上，JavaScript 一直没有模块（module）体系，无法将一个大程序拆分成互相依赖的小文件，再用简单的方法拼装起来。其他语言都有这项功能，比如 Ruby 的<code>require</code>、Python 的<code>import</code>，甚至就连 CSS 都有<code>@import</code>，但是 JavaScript 任何这方面的支持都没有，这对开发大型的、复杂的项目形成了巨大障碍。
    26. 

  26. 

  27. 在 ES6 之前，社区制定了一些模块加载方案，最主要的有 CommonJS 和 AMD 两种。前者用于服务器，后者用于浏览器。ES6 在语言标准的层面上，实现了模块功能，而且实现得相当简单，完全可以取代 CommonJS 和 AMD 规范，成为浏览器和服务器通用的模块解决方案。</p> <p>ES6 模块的设计思想是尽量的静态化，使得编译时就能确定模块的依赖关系，以及输入和输出的变量。CommonJS 和 AMD 模块，都只能在运行时确定这些东西。比如，CommonJS 模块就是对象，输入时必须查找对象属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// CommonJS模块</span>
    28. 

  28. <span class="token keyword">let</span> <span class="token punctuation">{</span> stat<span class="token punctuation">,</span> exists<span class="token punctuation">,</span> readFile <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'fs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    29. 

  29. 

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

  31. <span class="token keyword">let</span> _fs <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'fs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    32. 

  32. <span class="token keyword">let</span> stat <span class="token operator">=</span> _fs<span class="token punctuation">.</span>stat<span class="token punctuation">;</span>
    33. 

  33. <span class="token keyword">let</span> exists <span class="token operator">=</span> _fs<span class="token punctuation">.</span>exists<span class="token punctuation">;</span>
    34. 

  34. <span class="token keyword">let</span> readfile <span class="token operator">=</span> _fs<span class="token punctuation">.</span>readfile<span class="token punctuation">;</span>
    35. 

  35. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码的实质是整体加载<code>fs</code>模块（即加载<code>fs</code>的所有方法），生成一个对象（<code>_fs</code>），然后再从这个对象上面读取 3 个方法。<strong>这种加载称为“运行时加载”</strong>，因为只有运行时才能得到这个对象，导致完全没办法在编译时做“静态优化”。</p> <p>ES6 模块不是对象，而是通过<code>export</code>命令显式指定输出的代码，再通过<code>import</code>命令输入。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES6模块</span>
    36. 

  36. <span class="token keyword">import</span> <span class="token punctuation">{</span> stat<span class="token punctuation">,</span> exists<span class="token punctuation">,</span> readFile <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'fs'</span><span class="token punctuation">;</span>
    37. 

  37. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码的实质是从<code>fs</code>模块加载 3 个方法，其他方法不加载。<strong>这种加载称为“编译时加载”或者静态加载</strong>，**即 ES6 可以在编译时就完成模块加载，**效率要比 CommonJS 模块的加载方式高。当然，这也导致了没法引用 ES6 模块本身，因为它不是对象。</p> <p>由于 ES6 模块是编译时加载，使得静态分析成为可能。有了它，就能进一步拓宽 JavaScript 的语法，比如引入宏（macro）和类型检验（type system）这些只能靠静态分析实现的功能。</p> <p>除了静态加载带来的各种好处，ES6 模块还有以下好处。</p> <ul><li>不再需要<code>UMD</code>模块格式了，将来服务器和浏览器都会支持 ES6 模块格式。目前，通过各种工具库，其实已经做到了这一点。</li> <li>将来浏览器的新 API 就能用模块格式提供，不再必须做成全局变量或者<code>navigator</code>对象的属性。</li> <li>不再需要对象作为命名空间（比如<code>Math</code>对象），未来这些功能可以通过模块提供。</li></ul> <p>本章介绍 ES6 模块的语法，下一章介绍如何在浏览器和 Node 之中，加载 ES6 模块。</p> <h2 id="严格模式"><a href="#严格模式" class="header-anchor">#</a> 严格模式</h2> <p><strong>ES6 的模块自动采用严格模式</strong>，不管你有没有在模块头部加上<code>&quot;use strict&quot;;</code>。</p> <p>严格模式主要有以下限制。</p> <ul><li>变量必须声明后再使用</li> <li>函数的参数不能有同名属性，否则报错</li> <li>不能使用<code>with</code>语句</li> <li>不能对只读属性赋值，否则报错</li> <li>不能使用前缀 0 表示八进制数，否则报错</li> <li>不能删除不可删除的属性，否则报错</li> <li>不能删除变量<code>delete prop</code>，会报错，只能删除属性<code>delete global[prop]</code></li> <li><code>eval</code>不会在它的外层作用域引入变量</li> <li><code>eval</code>和<code>arguments</code>不能被重新赋值</li> <li><code>arguments</code>不会自动反映函数参数的变化</li> <li>不能使用<code>arguments.callee</code></li> <li>不能使用<code>arguments.caller</code></li> <li>禁止<code>this</code>指向全局对象</li> <li>不能使用<code>fn.caller</code>和<code>fn.arguments</code>获取函数调用的堆栈</li> <li>增加了保留字（比如<code>protected</code>、<code>static</code>和<code>interface</code>）</li></ul> <p>上面这些限制，模块都必须遵守。由于严格模式是 ES5 引入的，不属于 ES6，所以请参阅相关 ES5 书籍，本书不再详细介绍了。</p> <p>其中，尤其需要注意<code>this</code>的限制。ES6 模块之中，顶层的<code>this</code>指向<code>undefined</code>，即不应该在顶层代码使用<code>this</code>。</p> <h2 id="export-命令"><a href="#export-命令" class="header-anchor">#</a> export 命令</h2> <p>模块功能主要由两个命令构成：<code>export</code>和<code>import</code>。<code>export</code>命令用于规定模块的对外接口，<code>import</code>命令用于输入其他模块提供的功能。</p> <p><strong>一个模块就是一个独立的文件。该文件内部的所有变量，外部无法获取。如果你希望外部能够读取模块内部的某个变量，就必须使用<code>export</code>关键字输出该变量</strong>。下面是一个 JS 文件，里面使用<code>export</code>命令输出变量。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// profile.js</span>
    38. 

  38. <span class="token keyword">export</span> <span class="token keyword">var</span> firstName <span class="token operator">=</span> <span class="token string">'Michael'</span><span class="token punctuation">;</span>
    39. 

  39. <span class="token keyword">export</span> <span class="token keyword">var</span> lastName <span class="token operator">=</span> <span class="token string">'Jackson'</span><span class="token punctuation">;</span>
    40. 

  40. <span class="token keyword">export</span> <span class="token keyword">var</span> year <span class="token operator">=</span> <span class="token number">1958</span><span class="token punctuation">;</span>
    41. 

  41. </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>上面代码是<code>profile.js</code>文件，保存了用户信息。ES6 将其视为一个模块，里面用<code>export</code>命令对外部输出了三个变量。</p> <p><code>export</code>的写法，除了像上面这样，还有另外一种。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// profile.js</span>
    42. 

  42. <span class="token keyword">var</span> firstName <span class="token operator">=</span> <span class="token string">'Michael'</span><span class="token punctuation">;</span>
    43. 

  43. <span class="token keyword">var</span> lastName <span class="token operator">=</span> <span class="token string">'Jackson'</span><span class="token punctuation">;</span>
    44. 

  44. <span class="token keyword">var</span> year <span class="token operator">=</span> <span class="token number">1958</span><span class="token punctuation">;</span>
    45. 

  45. 

  46. <span class="token keyword">export</span> <span class="token punctuation">{</span> firstName<span class="token punctuation">,</span> lastName<span class="token punctuation">,</span> year <span class="token punctuation">}</span><span class="token punctuation">;</span>
    47. 

  47. </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>上面代码在<code>export</code>命令后面，使用大括号指定所要输出的一组变量。它与前一种写法（直接放置在<code>var</code>语句前）是等价的，但是应该优先考虑使用这种写法。因为这样就可以在脚本尾部，一眼看清楚输出了哪些变量。</p> <p><code>export</code>命令除了输出变量，还可以输出函数或类（class）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">multiply</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>
    48. 

  48.   <span class="token keyword">return</span> x <span class="token operator">*</span> y<span class="token punctuation">;</span>
    49. 

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

  50. </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>上面代码对外输出一个函数<code>multiply</code>。</p> <p>通常情况下，<code>export</code>输出的变量就是本来的名字，但是<strong>可以使用<code>as</code>关键字重命名</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">v1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
    51. 

  51. <span class="token keyword">function</span> <span class="token function">v2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
    52. 

  52. 

  53. <span class="token keyword">export</span> <span class="token punctuation">{</span>
    54. 

  54.   v1 <span class="token keyword">as</span> streamV1<span class="token punctuation">,</span>
    55. 

  55.   v2 <span class="token keyword">as</span> streamV2<span class="token punctuation">,</span>
    56. 

  56.   v2 <span class="token keyword">as</span> streamLatestVersion
    57. 

  57. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    58. 

  58. </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>上面代码使用<code>as</code>关键字，重命名了函数<code>v1</code>和<code>v2</code>的对外接口。重命名后，<code>v2</code>可以用不同的名字输出两次。</p> <p>需要特别注意的是，<code>export</code>命令规定的是对外的接口，必须与模块内部的变量建立一一对应关系。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    59. 

  59. <span class="token keyword">export</span> <span class="token number">1</span><span class="token punctuation">;</span>
    60. 

  60. 

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

  62. <span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    63. 

  63. <span class="token keyword">export</span> m<span class="token punctuation">;</span>
    64. 

  64. </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>上面两种写法都会报错，因为没有提供对外的接口。第一种写法直接输出 1，第二种写法通过变量<code>m</code>，还是直接输出 1。<code>1</code>只是一个值，不是接口。正确的写法是下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 写法一</span>
    65. 

  65. <span class="token keyword">export</span> <span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    66. 

  66. 

  67. <span class="token comment">// 写法二</span>
    68. 

  68. <span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    69. 

  69. <span class="token keyword">export</span> <span class="token punctuation">{</span>m<span class="token punctuation">}</span><span class="token punctuation">;</span>
    70. 

  70. 

  71. <span class="token comment">// 写法三</span>
    72. 

  72. <span class="token keyword">var</span> n <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    73. 

  73. <span class="token keyword">export</span> <span class="token punctuation">{</span>n <span class="token keyword">as</span> m<span class="token punctuation">}</span><span class="token punctuation">;</span>
    74. 

  74. </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>上面三种写法都是正确的，规定了对外的接口<code>m</code>。其他脚本可以通过这个接口，取到值<code>1</code>。它们的实质是，在接口名与模块内部变量之间，建立了一一对应的关系。</p> <p>同样的，<code>function</code>和<code>class</code>的输出，也必须遵守这样的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    75. 

  75. <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>
    76. 

  76. <span class="token keyword">export</span> f<span class="token punctuation">;</span>
    77. 

  77. 

  78. <span class="token comment">// 正确</span>
    79. 

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

  80. 

  81. <span class="token comment">// 正确</span>
    82. 

  82. <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>
    83. 

  83. <span class="token keyword">export</span> <span class="token punctuation">{</span>f<span class="token punctuation">}</span><span class="token punctuation">;</span>
    84. 

  84. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><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>另外，<code>export</code>语句输出的接口，与其对应的<strong>值是动态绑定关系</strong>，即通过该接口，可以取到模块内部实时的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    85. 

  85. <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> foo <span class="token operator">=</span> <span class="token string">'baz'</span><span class="token punctuation">,</span> <span class="token number">500</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    86. 

  86. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码输出变量<code>foo</code>，值为<code>bar</code>，500 毫秒之后变成<code>baz</code>。</p> <p>这一点与 CommonJS 规范完全不同。CommonJS 模块输出的是值的缓存，不存在动态更新，详见下文《Module 的加载实现》一节。</p> <p>最后，<code>export</code>命令可以出现在模块的任何位置，只要处于模块顶层就可以。如果处于块级作用域内，就会报错，下一节的<code>import</code>命令也是如此。这是因为处于条件代码块之中，就没法做静态优化了，违背了 ES6 模块的设计初衷。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    87. 

  87.   <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token string">'bar'</span> <span class="token comment">// SyntaxError</span>
    88. 

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

  89. <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    90. 

  90. </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>export</code>语句放在函数之中，结果报错。</p> <h2 id="import-命令"><a href="#import-命令" class="header-anchor">#</a> import 命令</h2> <p>使用<code>export</code>命令定义了模块的对外接口以后，其他 JS 文件就可以通过<code>import</code>命令加载这个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    91. 

  91. <span class="token keyword">import</span> <span class="token punctuation">{</span> firstName<span class="token punctuation">,</span> lastName<span class="token punctuation">,</span> year <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./profile.js'</span><span class="token punctuation">;</span>
    92. 

  92. 

  93. <span class="token keyword">function</span> <span class="token function">setName</span><span class="token punctuation">(</span><span class="token parameter">element</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    94. 

  94.   element<span class="token punctuation">.</span>textContent <span class="token operator">=</span> firstName <span class="token operator">+</span> <span class="token string">' '</span> <span class="token operator">+</span> lastName<span class="token punctuation">;</span>
    95. 

  95. <span class="token punctuation">}</span>
    96. 

  96. </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>上面代码的<code>import</code>命令，用于加载<code>profile.js</code>文件，并从中输入变量。<code>import</code>命令接受一对大括号，里面指定要从其他模块导入的变量名。大括号里面的<strong>变量名，必须与被导入模块（<code>profile.js</code>）对外接口的名称相同。</strong></p> <p>如果想为输入的变量重新取一个名字**，<code>import</code>命令要使用<code>as</code>关键字，将输入的变量重命名。**</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> lastName <span class="token keyword">as</span> surname <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./profile.js'</span><span class="token punctuation">;</span>
    97. 

  97. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><strong><code>import</code>命令输入的变量都是只读的</strong>，因为它的本质是输入接口。也就是说，不允许在加载模块的脚本里面，改写接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span>a<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./xxx.js'</span>
    98. 

  98. 

  99. a <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Syntax Error : 'a' is read-only;</span>
    100. 

  100. </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，脚本加载了变量<code>a</code>，对其重新赋值就会报错，因为<code>a</code>是一个只读的接口。但是**，如果<code>a</code>是一个对象，改写<code>a</code>的属性是允许的**。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span>a<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./xxx.js'</span>
    101. 

  101. 

  102. a<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span> <span class="token comment">// 合法操作</span>
    103. 

  103. </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>a</code>的属性可以成功改写，并且其他模块也可以读到改写后的值。不过，这种写法很难查错，<strong>建议凡是输入的变量，都当作完全只读，不要轻易改变它的属性</strong>。</p> <p><code>import</code>后面的<code>from</code>指定模块文件的位置，可以是相对路径，也可以是绝对路径，<strong><code>.js</code>后缀可以省略</strong>。<strong>如果只是模块名，不带有路径，那么必须有配置文件</strong>，告诉 JavaScript 引擎该模块的位置。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span>myMethod<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'util'</span><span class="token punctuation">;</span>
    104. 

  104. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>util</code>是模块文件名，由于不带有路径，必须通过配置，告诉引擎怎么取到这个模块。</p> <p>注意，<strong><code>import</code>命令具有提升效果，会提升到整个模块的头部</strong>，首先执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    105. 

  105. 

  106. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    107. 

  107. </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>上面的代码不会报错，因为<code>import</code>的执行早于<code>foo</code>的调用。这种行为的本质是，<code>import</code>命令是编译阶段执行的，在代码运行之前。</p> <p>由于<code>import</code>是静态执行，所以<strong>不能使用表达式和变量</strong>，这些只有在运行时才能得到结果的语法结构。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    108. 

  108. <span class="token keyword">import</span> <span class="token punctuation">{</span> <span class="token string">'f'</span> <span class="token operator">+</span> <span class="token string">'oo'</span> <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    109. 

  109. 

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

  111. <span class="token keyword">let</span> module <span class="token operator">=</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    112. 

  112. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> from module<span class="token punctuation">;</span>
    113. 

  113. 

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

  115. <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    116. 

  116.   <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'module1'</span><span class="token punctuation">;</span>
    117. 

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

  118.   <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'module2'</span><span class="token punctuation">;</span>
    119. 

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

  120. </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>上面三种写法都会报错，因为它们用到了表达式、变量和<code>if</code>结构。在静态分析阶段，这些语法都是没法得到值的。</p> <p>最后，<code>import</code>语句会执行所加载的模块，因此可以有下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    121. 

  121. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码仅仅执行<code>lodash</code>模块，但是不输入任何值。</p> <p>如果多次重复执行同一句<code>import</code>语句，那么只会执行一次，而不会执行多次。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    122. 

  122. <span class="token keyword">import</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    123. 

  123. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码加载了两次<code>lodash</code>，但是只会执行一次。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    124. 

  124. <span class="token keyword">import</span> <span class="token punctuation">{</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    125. 

  125. 

  126. <span class="token comment">// 等同于</span>
    127. 

  127. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    128. 

  128. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，虽然<code>foo</code>和<code>bar</code>在两个语句中加载，但是它们对应的是同一个<code>my_module</code>实例。也就是说，<code>import</code>语句是 Singleton 模式。</p> <p>目前阶段，通过 Babel 转码，CommonJS 模块的<code>require</code>命令和 ES6 模块的<code>import</code>命令，可以写在同一个模块里面，但是最好不要这样做。因为<code>import</code>在静态解析阶段执行，所以它是一个模块之中最早执行的。下面的代码可能不会得到预期结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'core-js/modules/es6.symbol'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    129. 

  129. <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'core-js/modules/es6.promise'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    130. 

  130. <span class="token keyword">import</span> React <span class="token keyword">from</span> <span class="token string">'React'</span><span class="token punctuation">;</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></div></div><h2 id="模块的整体加载-星号"><a href="#模块的整体加载-星号" class="header-anchor">#</a> 模块的整体加载-(星号*)</h2> <p>除了指定加载某个输出值，还可以使用整体加载，即用星号（<code>*</code>）指定一个对象，所有输出值都加载在这个对象上面。</p> <p>下面是一个<code>circle.js</code>文件，它输出两个方法<code>area</code>和<code>circumference</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// circle.js</span>
    132. 

  132. 

  133. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">area</span><span class="token punctuation">(</span><span class="token parameter">radius</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    134. 

  134.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> radius <span class="token operator">*</span> radius<span class="token punctuation">;</span>
    135. 

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

  136. 

  137. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">circumference</span><span class="token punctuation">(</span><span class="token parameter">radius</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    138. 

  138.   <span class="token keyword">return</span> <span class="token number">2</span> <span class="token operator">*</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> radius<span class="token punctuation">;</span>
    139. 

  139. <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><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>现在，加载这个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    141. 

  141. 

  142. <span class="token keyword">import</span> <span class="token punctuation">{</span> area<span class="token punctuation">,</span> circumference <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./circle'</span><span class="token punctuation">;</span>
    143. 

  143. 

  144. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆面积：'</span> <span class="token operator">+</span> <span class="token function">area</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    145. 

  145. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆周长：'</span> <span class="token operator">+</span> <span class="token function">circumference</span><span class="token punctuation">(</span><span class="token number">14</span><span class="token punctuation">)</span><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>上面写法是逐一指定要加载的方法，整体加载的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> circle <span class="token keyword">from</span> <span class="token string">'./circle'</span><span class="token punctuation">;</span>
    147. 

  147. 

  148. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆面积：'</span> <span class="token operator">+</span> circle<span class="token punctuation">.</span><span class="token function">area</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    149. 

  149. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆周长：'</span> <span class="token operator">+</span> circle<span class="token punctuation">.</span><span class="token function">circumference</span><span class="token punctuation">(</span><span class="token number">14</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    150. 

  150. </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>注意，<strong>模块整体加载所在的那个对象（上例是<code>circle</code>），应该是可以静态分析的，所以不允许运行时改变</strong>。下面的写法都是不允许的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> circle <span class="token keyword">from</span> <span class="token string">'./circle'</span><span class="token punctuation">;</span>
    151. 

  151. 

  152. <span class="token comment">// 下面两行都是不允许的</span>
    153. 

  153. circle<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    154. 

  154. circle<span class="token punctuation">.</span><span class="token function-variable function">area</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 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><span class="line-number">5</span><br></div></div><h2 id="export-default-命令"><a href="#export-default-命令" class="header-anchor">#</a> export default 命令</h2> <p>从前面的例子可以看出，使用<code>import</code>命令的时候，用户需要知道所要加载的变量名或函数名，否则无法加载。但是，用户肯定希望快速上手，未必愿意阅读文档，去了解模块有哪些属性和方法。</p> <p>为了给用户提供方便，让他们不用阅读文档就能加载模块，就要用到**<code>export default</code>命令，为模块指定默认输出。**</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// export-default.js</span>
    156. 

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

  157.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    158. 

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

  159. </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>上面代码是一个模块文件<code>export-default.js</code>，它的默认输出是一个函数。</p> <p>其他模块加载该模块时，<strong><code>import</code>命令可以为该匿名函数指定任意名字。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// import-default.js</span>
    160. 

  160. <span class="token keyword">import</span> customName <span class="token keyword">from</span> <span class="token string">'./export-default'</span><span class="token punctuation">;</span>
    161. 

  161. <span class="token function">customName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'foo'</span>
    162. 

  162. </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>上面代码的<code>import</code>命令，可以用任意名称指向<code>export-default.js</code>输出的方法，这时就不需要知道原模块输出的函数名。需要注意的是，<strong>这时<code>import</code>命令后面，不使用大括号</strong>。</p> <p><code>export default</code>命令用在非匿名函数前，也是可以的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// export-default.js</span>
    163. 

  163. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    164. 

  164.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    165. 

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

  166. 

  167. <span class="token comment">// 或者写成</span>
    168. 

  168. 

  169. <span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    170. 

  170.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    171. 

  171. <span class="token punctuation">}</span>
    172. 

  172. 

  173. <span class="token keyword">export</span> <span class="token keyword">default</span> foo<span class="token punctuation">;</span>
    174. 

  174. </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，<strong><code>foo</code>函数的函数名<code>foo</code>，在模块外部是无效的。加载的时候，视同匿名函数加载</strong>。</p> <p>下面比较一下默认输出和正常输出。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 第一组</span>
    175. 

  175. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token function">crc32</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 输出</span>
    176. 

  176.   <span class="token comment">// ...</span>
    177. 

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

  178. 

  179. <span class="token keyword">import</span> crc32 <span class="token keyword">from</span> <span class="token string">'crc32'</span><span class="token punctuation">;</span> <span class="token comment">// 输入</span>
    180. 

  180. 

  181. <span class="token comment">// 第二组</span>
    182. 

  182. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">crc32</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 输出</span>
    183. 

  183.   <span class="token comment">// ...</span>
    184. 

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

  185. 

  186. <span class="token keyword">import</span> <span class="token punctuation">{</span>crc32<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'crc32'</span><span class="token punctuation">;</span> <span class="token comment">// 输入</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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码的两组写法，第一组是使用<code>export default</code>时，对应的<code>import</code>语句不需要使用大括号；第二组是不使用<code>export default</code>时，对应的<code>import</code>语句需要使用大括号。</p> <p><code>export default</code>命令用于指定模块的默认输出。显然，<strong>一个模块只能有一个默认输出</strong>，因此<code>export default</code>命令只能使用一次。所以，import命令后面才不用加大括号，因为只可能唯一对应<code>export default</code>命令。</p> <p>**本质上，<code>export default</code>就是输出一个叫做<code>default</code>的变量或方法，**然后系统允许你为它取任意名字。所以，下面的写法是有效的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// modules.js</span>
    188. 

  188. <span class="token keyword">function</span> <span class="token function">add</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>
    189. 

  189.   <span class="token keyword">return</span> x <span class="token operator">*</span> y<span class="token punctuation">;</span>
    190. 

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

  191. <span class="token keyword">export</span> <span class="token punctuation">{</span>add <span class="token keyword">as</span> <span class="token keyword">default</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    192. 

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

  193. <span class="token comment">// export default add;</span>
    194. 

  194. 

  195. <span class="token comment">// app.js</span>
    196. 

  196. <span class="token keyword">import</span> <span class="token punctuation">{</span> <span class="token keyword">default</span> <span class="token keyword">as</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'modules'</span><span class="token punctuation">;</span>
    197. 

  197. <span class="token comment">// 等同于</span>
    198. 

  198. <span class="token comment">// import foo from 'modules';</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>正是因为<code>export default</code>命令其实只是输出一个叫做<code>default</code>的变量，所以它后面不能跟变量声明语句。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 正确</span>
    200. 

  200. <span class="token keyword">export</span> <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    201. 

  201. 

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

  203. <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    204. 

  204. <span class="token keyword">export</span> <span class="token keyword">default</span> a<span class="token punctuation">;</span>
    205. 

  205. 

  206. <span class="token comment">// 错误</span>
    207. 

  207. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</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></div></div><p>上面代码中，<code>export default a</code>的含义是将变量<code>a</code>的值赋给变量<code>default</code>。所以，最后一种写法会报错。</p> <p>同样地，因为<code>export default</code>命令的本质是将后面的值，赋给<code>default</code>变量，所以可以直接将一个值写在<code>export default</code>之后。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 正确</span>
    209. 

  209. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token number">42</span><span class="token punctuation">;</span>
    210. 

  210. 

  211. <span class="token comment">// 报错</span>
    212. 

  212. <span class="token keyword">export</span> <span class="token number">42</span><span class="token punctuation">;</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></div></div><p>上面代码中，后一句报错是因为没有指定对外的接口，而前一句指定对外接口为<code>default</code>。</p> <p>有了<code>export default</code>命令，输入模块时就非常直观了，以输入 lodash 模块为例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> _ <span class="token keyword">from</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    214. 

  214. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果想在一条<code>import</code>语句中，同时输入默认方法和其他接口，可以写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> _<span class="token punctuation">,</span> <span class="token punctuation">{</span> each<span class="token punctuation">,</span> forEach <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    215. 

  215. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>对应上面代码的<code>export</code>语句如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    216. 

  216.   <span class="token comment">// ···</span>
    217. 

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

  218. 

  219. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">each</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> iterator<span class="token punctuation">,</span> context</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    220. 

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

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

  222. 

  223. <span class="token keyword">export</span> <span class="token punctuation">{</span> each <span class="token keyword">as</span> forEach <span class="token punctuation">}</span><span class="token punctuation">;</span>
    224. 

  224. </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>上面代码的最后一行的意思是，暴露出<code>forEach</code>接口，默认指向<code>each</code>接口，即<code>forEach</code>和<code>each</code>指向同一个方法。</p> <p><code>export default</code>也可以用来输出类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// MyClass.js</span>
    225. 

  225. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">class</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
    226. 

  226. 

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

  228. <span class="token keyword">import</span> MyClass <span class="token keyword">from</span> <span class="token string">'MyClass'</span><span class="token punctuation">;</span>
    229. 

  229. <span class="token keyword">let</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    230. 

  230. </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><h2 id="export-与-import-的复合写法"><a href="#export-与-import-的复合写法" class="header-anchor">#</a> export 与 import 的复合写法</h2> <p>如果在一个模块之中，先输入后输出同一个模块，<code>import</code>语句可以与<code>export</code>语句写在一起。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    231. 

  231. 

  232. <span class="token comment">// 可以简单理解为</span>
    233. 

  233. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    234. 

  234. <span class="token keyword">export</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span><span class="token punctuation">;</span>
    235. 

  235. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>export</code>和<code>import</code>语句可以结合在一起，写成一行。但需要注意的是，写成一行以后，<code>foo</code>和<code>bar</code>实际上并没有被导入当前模块，只是相当于对外转发了这两个接口，导致当前模块不能直接使用<code>foo</code>和<code>bar</code>。</p> <p>模块的接口改名和整体输出，也可以采用这种写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 接口改名</span>
    236. 

  236. <span class="token keyword">export</span> <span class="token punctuation">{</span> foo <span class="token keyword">as</span> myFoo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    237. 

  237. 

  238. <span class="token comment">// 整体输出</span>
    239. 

  239. <span class="token keyword">export</span> <span class="token operator">*</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    240. 

  240. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>默认接口的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> <span class="token keyword">default</span> <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'foo'</span><span class="token punctuation">;</span>
    241. 

  241. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>具名接口改为默认接口的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> es6 <span class="token keyword">as</span> <span class="token keyword">default</span> <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./someModule'</span><span class="token punctuation">;</span>
    242. 

  242. 

  243. <span class="token comment">// 等同于</span>
    244. 

  244. <span class="token keyword">import</span> <span class="token punctuation">{</span> es6 <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./someModule'</span><span class="token punctuation">;</span>
    245. 

  245. <span class="token keyword">export</span> <span class="token keyword">default</span> es6<span class="token punctuation">;</span>
    246. 

  246. </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>同样地，默认接口也可以改名为具名接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> <span class="token keyword">default</span> <span class="token keyword">as</span> es6 <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./someModule'</span><span class="token punctuation">;</span>
    247. 

  247. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>下面三种<code>import</code>语句，没有对应的复合写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    248. 

  248. <span class="token keyword">import</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    249. 

  249. <span class="token keyword">import</span> someIdentifier<span class="token punctuation">,</span> <span class="token punctuation">{</span> namedIdentifier <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    250. 

  250. </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>为了做到形式的对称，现在有<a href="https://github.com/leebyron/ecmascript-export-default-from" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，提出补上这三种复合写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token operator">*</span> <span class="token keyword">as</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    251. 

  251. <span class="token keyword">export</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    252. 

  252. <span class="token keyword">export</span> someIdentifier<span class="token punctuation">,</span> <span class="token punctuation">{</span> namedIdentifier <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    253. 

  253. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="模块的继承"><a href="#模块的继承" class="header-anchor">#</a> 模块的继承</h2> <p>模块之间也可以继承。</p> <p>假设有一个<code>circleplus</code>模块，继承了<code>circle</code>模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// circleplus.js</span>
    254. 

  254. 

  255. <span class="token keyword">export</span> <span class="token operator">*</span> <span class="token keyword">from</span> <span class="token string">'circle'</span><span class="token punctuation">;</span>
    256. 

  256. <span class="token keyword">export</span> <span class="token keyword">var</span> e <span class="token operator">=</span> <span class="token number">2.71828182846</span><span class="token punctuation">;</span>
    257. 

  257. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    258. 

  258.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">exp</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    259. 

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

  260. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中的<code>export *</code>，表示再输出<code>circle</code>模块的所有属性和方法。注意，<code>export *</code>命令会忽略<code>circle</code>模块的<code>default</code>方法。然后，上面代码又输出了自定义的<code>e</code>变量和默认方法。</p> <p>这时，也可以将<code>circle</code>的属性或方法，改名后再输出。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// circleplus.js</span>
    261. 

  261. 

  262. <span class="token keyword">export</span> <span class="token punctuation">{</span> area <span class="token keyword">as</span> circleArea <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'circle'</span><span class="token punctuation">;</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></div></div><p>上面代码表示，只输出<code>circle</code>模块的<code>area</code>方法，且将其改名为<code>circleArea</code>。</p> <p>加载上面模块的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    264. 

  264. 

  265. <span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> math <span class="token keyword">from</span> <span class="token string">'circleplus'</span><span class="token punctuation">;</span>
    266. 

  266. <span class="token keyword">import</span> exp <span class="token keyword">from</span> <span class="token string">'circleplus'</span><span class="token punctuation">;</span>
    267. 

  267. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">exp</span><span class="token punctuation">(</span>math<span class="token punctuation">.</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    268. 

  268. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中的<code>import exp</code>表示，将<code>circleplus</code>模块的默认方法加载为<code>exp</code>方法。</p> <h2 id="跨模块常量"><a href="#跨模块常量" class="header-anchor">#</a> 跨模块常量</h2> <p>本书介绍<code>const</code>命令的时候说过，<code>const</code>声明的常量只在当前代码块有效。如果想设置跨模块的常量（即跨多个文件），或者说一个值要被多个模块共享，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">// constants.js 模块</span>
    269. 

  269. <span class="token keyword">export</span> <span class="token keyword">const</span> <span class="token constant">A</span> <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    270. 

  270. <span class="token keyword">export</span> <span class="token keyword">const</span> <span class="token constant">B</span> <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    271. 

  271. <span class="token keyword">export</span> <span class="token keyword">const</span> <span class="token constant">C</span> <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span>
    272. 

  272. 

  273. <span class="token comment">// test1.js 模块</span>
    274. 

  274. <span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> constants <span class="token keyword">from</span> <span class="token string">'./constants'</span><span class="token punctuation">;</span>
    275. 

  275. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>constants<span class="token punctuation">.</span><span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1</span>
    276. 

  276. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>constants<span class="token punctuation">.</span><span class="token constant">B</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    277. 

  277. 

  278. <span class="token comment">// test2.js 模块</span>
    279. 

  279. <span class="token keyword">import</span> <span class="token punctuation">{</span><span class="token constant">A</span><span class="token punctuation">,</span> <span class="token constant">B</span><span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./constants'</span><span class="token punctuation">;</span>
    280. 

  280. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1</span>
    281. 

  281. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token constant">B</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    282. 

  282. </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>如果要使用的常量非常多，可以建一个专门的<code>constants</code>目录，将各种常量写在不同的文件里面，保存在该目录下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// constants/db.js</span>
    283. 

  283. <span class="token keyword">export</span> <span class="token keyword">const</span> db <span class="token operator">=</span> <span class="token punctuation">{</span>
    284. 

  284.   <span class="token literal-property property">url</span><span class="token operator">:</span> <span class="token string">'http://my.couchdbserver.local:5984'</span><span class="token punctuation">,</span>
    285. 

  285.   <span class="token literal-property property">admin_username</span><span class="token operator">:</span> <span class="token string">'admin'</span><span class="token punctuation">,</span>
    286. 

  286.   <span class="token literal-property property">admin_password</span><span class="token operator">:</span> <span class="token string">'admin password'</span>
    287. 

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

  288. 

  289. <span class="token comment">// constants/user.js</span>
    290. 

  290. <span class="token keyword">export</span> <span class="token keyword">const</span> users <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'root'</span><span class="token punctuation">,</span> <span class="token string">'admin'</span><span class="token punctuation">,</span> <span class="token string">'staff'</span><span class="token punctuation">,</span> <span class="token string">'ceo'</span><span class="token punctuation">,</span> <span class="token string">'chief'</span><span class="token punctuation">,</span> <span class="token string">'moderator'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    291. 

  291. </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>然后，将这些文件输出的常量，合并在<code>index.js</code>里面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// constants/index.js</span>
    292. 

  292. <span class="token keyword">export</span> <span class="token punctuation">{</span>db<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./db'</span><span class="token punctuation">;</span>
    293. 

  293. <span class="token keyword">export</span> <span class="token punctuation">{</span>users<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./users'</span><span class="token punctuation">;</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></div></div><p>使用的时候，直接加载<code>index.js</code>就可以了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// script.js</span>
    295. 

  295. <span class="token keyword">import</span> <span class="token punctuation">{</span>db<span class="token punctuation">,</span> users<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./constants/index'</span><span class="token punctuation">;</span>
    296. 

  296. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="import"><a href="#import" class="header-anchor">#</a> import()</h2> <h3 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h3> <p>前面介绍过，<code>import</code>命令会被 JavaScript 引擎静态分析，先于模块内的其他语句执行（<code>import</code>命令叫做“连接” binding 其实更合适）。所以，下面的代码会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    297. 

  297. <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    298. 

  298.   <span class="token keyword">import</span> MyModual <span class="token keyword">from</span> <span class="token string">'./myModual'</span><span class="token punctuation">;</span>
    299. 

  299. <span class="token punctuation">}</span>
    300. 

  300. </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，引擎处理<code>import</code>语句是在编译时，这时不会去分析或执行<code>if</code>语句，所以<code>import</code>语句放在<code>if</code>代码块之中毫无意义，因此会报句法错误，而不是执行时错误。也就是说，<code>import</code>和<code>export</code>命令只能在模块的顶层，不能在代码块之中（比如，在<code>if</code>代码块之中，或在函数之中）。</p> <p>这样的设计，固然有利于编译器提高效率，但也导致无法在运行时加载模块。在语法上，条件加载就不可能实现。如果<code>import</code>命令要取代 Node 的<code>require</code>方法，这就形成了一个障碍。因为<code>require</code>是运行时加载模块，<code>import</code>命令无法取代<code>require</code>的动态加载功能。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> path <span class="token operator">=</span> <span class="token string">'./'</span> <span class="token operator">+</span> fileName<span class="token punctuation">;</span>
    301. 

  301. <span class="token keyword">const</span> myModual <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span>path<span class="token punctuation">)</span><span class="token punctuation">;</span>
    302. 

  302. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的语句就是动态加载，<code>require</code>到底加载哪一个模块，只有运行时才知道。<code>import</code>命令做不到这一点。</p> <p><strong><a href="https://github.com/tc39/proposal-dynamic-import" target="_blank" rel="noopener noreferrer">ES2020提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 引入<code>import()</code>函数，支持动态加载模块。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span>specifier<span class="token punctuation">)</span>
    303. 

  303. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>import</code>函数的参数<code>specifier</code>，指定所要加载的模块的位置。<code>import</code>命令能够接受什么参数，<code>import()</code>函数就能接受什么参数，两者区别主要是后者为动态加载。</p> <p><strong><code>import()</code>返回一个 Promise 对象</strong>。下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> main <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelector</span><span class="token punctuation">(</span><span class="token string">'main'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    304. 

  304. 

  305. <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">./section-modules/</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>someVariable<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">.js</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span>
    306. 

  306.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">module</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    307. 

  307.     module<span class="token punctuation">.</span><span class="token function">loadPageInto</span><span class="token punctuation">(</span>main<span class="token punctuation">)</span><span class="token punctuation">;</span>
    308. 

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

  309.   <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">err</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    310. 

  310.     main<span class="token punctuation">.</span>textContent <span class="token operator">=</span> err<span class="token punctuation">.</span>message<span class="token punctuation">;</span>
    311. 

  311.   <span class="token punctuation">}</span><span class="token punctuation">)</span><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><span class="line-number">9</span><br></div></div><p><strong><code>import()</code>函数可以用在任何地方，不仅仅是模块，非模块的脚本也可以使用</strong>。它是运行时执行，也就是说，什么时候运行到这一句，就会加载指定的模块。另外，<code>import()</code>函数与所加载的模块没有静态连接关系，这点也是与<code>import</code>语句不相同。<code>import()</code>类似于 Node 的<code>require</code>方法，区别主要是前者是异步加载，后者是同步加载。</p> <h3 id="适用场合"><a href="#适用场合" class="header-anchor">#</a> 适用场合</h3> <p>下面是<code>import()</code>的一些适用场合。</p> <p>（1）按需加载。</p> <p><code>import()</code>可以在需要的时候，再加载某个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>button<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'click'</span><span class="token punctuation">,</span> <span class="token parameter">event</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    313. 

  313.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./dialogBox.js'</span><span class="token punctuation">)</span>
    314. 

  314.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">dialogBox</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    315. 

  315.     dialogBox<span class="token punctuation">.</span><span class="token function">open</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    316. 

  316.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    317. 

  317.   <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">error</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    318. 

  318.     <span class="token comment">/* Error handling */</span>
    319. 

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

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

  321. </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>import()</code>方法放在<code>click</code>事件的监听函数之中，只有用户点击了按钮，才会加载这个模块。</p> <p>（2）条件加载</p> <p><code>import()</code>可以放在<code>if</code>代码块，根据不同的情况，加载不同的模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>condition<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    322. 

  322.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'moduleA'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    323. 

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

  324.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'moduleB'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    325. 

  325. <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></div></div><p>上面代码中，如果满足条件，就加载模块 A，否则加载模块 B。</p> <p>（3）动态的模块路径</p> <p><code>import()</code>允许模块路径动态生成。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    327. 

  327. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    328. 

  328. </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，根据函数<code>f</code>的返回结果，加载不同的模块。</p> <h3 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h3> <p><code>import()</code>加载模块成功以后，这个模块会作为一个对象，当作<code>then</code>方法的参数。因此，可以使用对象解构赋值的语法，获取输出接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span>
    329. 

  329. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>export1<span class="token punctuation">,</span> export2<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    330. 

  330.   <span class="token comment">// ...·</span>
    331. 

  331. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    332. 

  332. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>export1</code>和<code>export2</code>都是<code>myModule.js</code>的输出接口，可以解构获得。</p> <p>如果模块有<code>default</code>输出接口，可以用参数直接获得。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span>
    333. 

  333. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">myModule</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    334. 

  334.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>myModule<span class="token punctuation">.</span>default<span class="token punctuation">)</span><span class="token punctuation">;</span>
    335. 

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

  336. </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>上面的代码也可以使用具名输入的形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span>
    337. 

  337. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span><span class="token keyword">default</span><span class="token operator">:</span> theDefault<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    338. 

  338.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>theDefault<span class="token punctuation">)</span><span class="token punctuation">;</span>
    339. 

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

  340. </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>如果想同时加载多个模块，可以采用下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
    341. 

  341.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module1.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    342. 

  342.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module2.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    343. 

  343.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module3.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    344. 

  344. <span class="token punctuation">]</span><span class="token punctuation">)</span>
    345. 

  345. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span>module1<span class="token punctuation">,</span> module2<span class="token punctuation">,</span> module3<span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    346. 

  346.    ···
    347. 

  347. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    348. 

  348. </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><code>import()</code>也可以用在 async 函数之中。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    349. 

  349.   <span class="token keyword">const</span> myModule <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    350. 

  350.   <span class="token keyword">const</span> <span class="token punctuation">{</span>export1<span class="token punctuation">,</span> export2<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    351. 

  351.   <span class="token keyword">const</span> <span class="token punctuation">[</span>module1<span class="token punctuation">,</span> module2<span class="token punctuation">,</span> module3<span class="token punctuation">]</span> <span class="token operator">=</span>
    352. 

  352.     <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
    353. 

  353.       <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module1.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    354. 

  354.       <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module2.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    355. 

  355.       <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module3.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    356. 

  356.     <span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    357. 

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

  358. <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    359. 

  359. </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/23.Module 的语法.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/83f8c3a0cd87dd83/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Class 的继承</div></a> <a href="/blog/pages/a79ca2e64ceae213/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Module 的加载实现</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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