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href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">Module 的加载实现<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="module-的加载实现"><a href="#module-的加载实现" class="header-anchor">#</a> Module 的加载实现</h1> <p>上一章介绍了模块的语法，本章介绍如何在浏览器和 Node.js 之中加载 ES6 模块，以及实际开发中经常遇到的一些问题（比如循环加载）。
    26. 

  26. </p> <h2 id="浏览器加载"><a href="#浏览器加载" class="header-anchor">#</a> 浏览器加载</h2> <h3 id="传统方法"><a href="#传统方法" class="header-anchor">#</a> 传统方法</h3> <p>HTML 网页中，浏览器通过<code>&lt;script&gt;</code>标签加载 JavaScript 脚本。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token comment">&lt;!-- 页面内嵌的脚本 --&gt;</span>
    27. 

  27. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>application/javascript<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    28. 

  28.   <span class="token comment">// module code</span>
    29. 

  29. </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    30. 

  30. 

  31. <span class="token comment">&lt;!-- 外部脚本 --&gt;</span>
    32. 

  32. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>application/javascript<span class="token punctuation">&quot;</span></span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>path/to/myModule.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    33. 

  33. </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    34. 

  34. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，由于浏览器脚本的默认语言是 JavaScript，因此<code>type=&quot;application/javascript&quot;</code>可以省略。</p> <p>默认情况下，浏览器是同步加载 JavaScript 脚本，即渲染引擎遇到<code>&lt;script&gt;</code>标签就会停下来，等到执行完脚本，再继续向下渲染。如果是外部脚本，还必须加入脚本下载的时间。</p> <p>如果脚本体积很大，下载和执行的时间就会很长，因此造成浏览器堵塞，用户会感觉到浏览器“卡死”了，没有任何响应。这显然是很不好的体验，所以浏览器允许脚本异步加载，下面就是两种异步加载的语法。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>path/to/myModule.js<span class="token punctuation">&quot;</span></span> <span class="token attr-name">defer</span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    35. 

  35. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>path/to/myModule.js<span class="token punctuation">&quot;</span></span> <span class="token attr-name">async</span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    36. 

  36. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>&lt;script&gt;</code>标签打开<code>defer</code>或<code>async</code>属性，脚本就会异步加载。渲染引擎遇到这一行命令，就会开始下载外部脚本，但不会等它下载和执行，而是直接执行后面的命令。</p> <p><code>defer</code>与<code>async</code>的区别是：<code>defer</code>要等到整个页面在内存中正常渲染结束（DOM 结构完全生成，以及其他脚本执行完成），才会执行；<code>async</code>一旦下载完，渲染引擎就会中断渲染，执行这个脚本以后，再继续渲染。一句话，<strong><code>defer</code>是“渲染完再执行”，<code>async</code>是“下载完就执行”</strong>。另外，如果有多个<code>defer</code>脚本，会按照它们在页面出现的顺序加载，而多个<code>async</code>脚本是不能保证加载顺序的。</p> <h3 id="加载规则"><a href="#加载规则" class="header-anchor">#</a> 加载规则</h3> <p>浏览器加载 ES6 模块，也使用<code>&lt;script&gt;</code>标签，但是要加入<code>type=&quot;module&quot;</code>属性。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>./foo.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    37. 

  37. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码在网页中插入一个模块<code>foo.js</code>，由于<code>type</code>属性设为<code>module</code>，所以浏览器知道这是一个 ES6 模块。</p> <p>浏览器对于<strong>带有<code>type=&quot;module&quot;</code>的<code>&lt;script&gt;</code>，都是异步加载</strong>，不会造成堵塞浏览器，即等到整个页面渲染完，再执行模块脚本，<strong>等同于打开了<code>&lt;script&gt;</code>标签的<code>defer</code>属性。</strong></p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>./foo.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    38. 

  38. <span class="token comment">&lt;!-- 等同于 --&gt;</span>
    39. 

  39. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>./foo.js<span class="token punctuation">&quot;</span></span> <span class="token attr-name">defer</span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    40. 

  40. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>如果网页有多个<code>&lt;script type=&quot;module&quot;&gt;</code>，它们会按照在页面出现的顺序依次执行。</p> <p><code>&lt;script&gt;</code>标签的<code>async</code>属性也可以打开，这时只要加载完成，渲染引擎就会中断渲染立即执行。执行完成后，再恢复渲染。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>./foo.js<span class="token punctuation">&quot;</span></span> <span class="token attr-name">async</span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    41. 

  41. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>一旦使用了<code>async</code>属性，<code>&lt;script type=&quot;module&quot;&gt;</code>就不会按照在页面出现的顺序执行，而是只要该模块加载完成，就执行该模块。</p> <p>ES6 模块也允许内嵌在网页中，语法行为与加载外部脚本完全一致。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    42. 

  42.   <span class="token keyword">import</span> utils <span class="token keyword">from</span> <span class="token string">&quot;./utils.js&quot;</span><span class="token punctuation">;</span>
    43. 

  43. 

  44.   <span class="token comment">// other code</span>
    45. 

  45. </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    46. 

  46. </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>举例来说，jQuery 就支持模块加载。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    47. 

  47.   <span class="token keyword">import</span> $ <span class="token keyword">from</span> <span class="token string">&quot;./jquery/src/jquery.js&quot;</span><span class="token punctuation">;</span>
    48. 

  48.   <span class="token function">$</span><span class="token punctuation">(</span><span class="token string">'#message'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token string">'Hi from jQuery!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    49. 

  49. </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></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><span class="line-number">4</span><br></div></div><p>对于外部的模块脚本（上例是<code>foo.js</code>），有几点需要注意。</p> <ul><li>代码是<strong>在模块作用域之中运行</strong>，而不是在全局作用域运行。模块内部的顶层变量，外部不可见。</li> <li>模块脚本<strong>自动采用严格模式</strong>，不管有没有声明<code>use strict</code>。</li> <li>模块之中，可以使用<code>import</code>命令加载其他模块（<code>.js</code>后缀不可省略，需要提供绝对 URL 或相对 URL），也可以使用<code>export</code>命令输出对外接口。</li> <li>模块之中，<strong>顶层的<code>this</code>关键字返回<code>undefined</code></strong>，而不是指向<code>window</code>。也就是说，在模块顶层使用<code>this</code>关键字，是无意义的。</li> <li>同一个模块如果加载多次，将只执行一次。</li></ul> <p>下面是一个示例模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> utils <span class="token keyword">from</span> <span class="token string">'https://example.com/js/utils.js'</span><span class="token punctuation">;</span>
    51. 

  51. 

  52. <span class="token keyword">const</span> x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    53. 

  53. 

  54. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x <span class="token operator">===</span> window<span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//false</span>
    55. 

  55. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">===</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    56. 

  56. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>利用顶层的<code>this</code>等于<code>undefined</code>这个语法点，可以侦测当前代码是否在 ES6 模块之中。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> isNotModuleScript <span class="token operator">=</span> <span class="token keyword">this</span> <span class="token operator">!==</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span>
    57. 

  57. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h2 id="es6-模块与-commonjs-模块的差异"><a href="#es6-模块与-commonjs-模块的差异" class="header-anchor">#</a> ES6 模块与 CommonJS 模块的差异</h2> <p>讨论 Node.js 加载 ES6 模块之前，必须了解 ES6 模块与 CommonJS 模块完全不同。</p> <p>它们有<strong>两个重大差异</strong>。</p> <ul><li><strong>CommonJS 模块输出的是一个值的拷贝，ES6 模块输出的是值的引用。</strong></li> <li><strong>CommonJS 模块是运行时加载，ES6 模块是编译时输出接口。</strong></li></ul> <p>第二个差异是因为 CommonJS 加载的是一个对象（即<code>module.exports</code>属性），该对象只有在脚本运行完才会生成。而 ES6 模块不是对象，它的对外接口只是一种静态定义，在代码静态解析阶段就会生成。</p> <p>下面重点解释第一个差异。</p> <p>CommonJS 模块输出的是值的拷贝，也就是说，一旦输出一个值，模块内部的变化就影响不到这个值。请看下面这个模块文件<code>lib.js</code>的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// lib.js</span>
    58. 

  58. <span class="token keyword">var</span> counter <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    59. 

  59. <span class="token keyword">function</span> <span class="token function">incCounter</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    60. 

  60.   counter<span class="token operator">++</span><span class="token punctuation">;</span>
    61. 

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

  62. module<span class="token punctuation">.</span>exports <span class="token operator">=</span> <span class="token punctuation">{</span>
    63. 

  63.   <span class="token literal-property property">counter</span><span class="token operator">:</span> counter<span class="token punctuation">,</span>
    64. 

  64.   <span class="token literal-property property">incCounter</span><span class="token operator">:</span> incCounter<span class="token punctuation">,</span>
    65. 

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

  66. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码输出内部变量<code>counter</code>和改写这个变量的内部方法<code>incCounter</code>。然后，在<code>main.js</code>里面加载这个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    67. 

  67. <span class="token keyword">var</span> mod <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./lib'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    68. 

  68. 

  69. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>mod<span class="token punctuation">.</span>counter<span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 3</span>
    70. 

  70. mod<span class="token punctuation">.</span><span class="token function">incCounter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    71. 

  71. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>mod<span class="token punctuation">.</span>counter<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    72. 

  72. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码说明，<code>lib.js</code>模块加载以后，它的内部变化就影响不到输出的<code>mod.counter</code>了。这是因为<code>mod.counter</code>是一个原始类型的值，会被缓存。除非写成一个函数，才能得到内部变动后的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// lib.js</span>
    73. 

  73. <span class="token keyword">var</span> counter <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    74. 

  74. <span class="token keyword">function</span> <span class="token function">incCounter</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    75. 

  75.   counter<span class="token operator">++</span><span class="token punctuation">;</span>
    76. 

  76. <span class="token punctuation">}</span>
    77. 

  77. module<span class="token punctuation">.</span>exports <span class="token operator">=</span> <span class="token punctuation">{</span>
    78. 

  78.   <span class="token keyword">get</span> <span class="token function">counter</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    79. 

  79.     <span class="token keyword">return</span> counter
    80. 

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

  81.   <span class="token literal-property property">incCounter</span><span class="token operator">:</span> incCounter<span class="token punctuation">,</span>
    82. 

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

  83. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，输出的<code>counter</code>属性实际上是一个取值器函数。现在再执行<code>main.js</code>，就可以正确读取内部变量<code>counter</code>的变动了。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">node</span> main.js
    84. 

  84. <span class="token number">3</span>
    85. 

  85. <span class="token number">4</span>
    86. 

  86. </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>ES6 模块的运行机制与 CommonJS 不一样。JS 引擎对脚本静态分析的时候，遇到模块加载命令<code>import</code>，就会生成一个只读引用。等到脚本真正执行时，再根据这个只读引用，到被加载的那个模块里面去取值。换句话说，ES6 的<code>import</code>有点像 Unix 系统的“符号连接”，原始值变了，<code>import</code>加载的值也会跟着变。因此，<strong>ES6 模块是动态引用，并且不会缓存值</strong>，模块里面的变量绑定其所在的模块。</p> <p>还是举上面的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// lib.js</span>
    87. 

  87. <span class="token keyword">export</span> <span class="token keyword">let</span> counter <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    88. 

  88. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">incCounter</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    89. 

  89.   counter<span class="token operator">++</span><span class="token punctuation">;</span>
    90. 

  90. <span class="token punctuation">}</span>
    91. 

  91. 

  92. <span class="token comment">// main.js</span>
    93. 

  93. <span class="token keyword">import</span> <span class="token punctuation">{</span> counter<span class="token punctuation">,</span> incCounter <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./lib'</span><span class="token punctuation">;</span>
    94. 

  94. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>counter<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    95. 

  95. <span class="token function">incCounter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    96. 

  96. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>counter<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 4</span>
    97. 

  97. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码说明，ES6 模块输入的变量<code>counter</code>是活的，完全反应其所在模块<code>lib.js</code>内部的变化。</p> <p>再举一个出现在<code>export</code>一节中的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// m1.js</span>
    98. 

  98. <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>
    99. 

  99. <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>
    100. 

  100. 

  101. <span class="token comment">// m2.js</span>
    102. 

  102. <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">'./m1.js'</span><span class="token punctuation">;</span>
    103. 

  103. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span>
    104. 

  104. <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> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">500</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    105. 

  105. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>m1.js</code>的变量<code>foo</code>，在刚加载时等于<code>bar</code>，过了 500 毫秒，又变为等于<code>baz</code>。</p> <p>让我们看看，<code>m2.js</code>能否正确读取这个变化。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ babel-node m2.js
    106. 

  106. 

  107. bar
    108. 

  108. baz
    109. 

  109. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码表明，ES6 模块不会缓存运行结果，而是动态地去被加载的模块取值，并且变量总是绑定其所在的模块。</p> <p>由于 ES6 输入的模块变量，只是一个“符号连接”，所以<strong>这个变量是只读的，对它进行重新赋值会报错</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// lib.js</span>
    110. 

  110. <span class="token keyword">export</span> <span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    111. 

  111. 

  112. <span class="token comment">// main.js</span>
    113. 

  113. <span class="token keyword">import</span> <span class="token punctuation">{</span> obj <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./lib'</span><span class="token punctuation">;</span>
    114. 

  114. 

  115. obj<span class="token punctuation">.</span>prop <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span> <span class="token comment">// OK</span>
    116. 

  116. obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// TypeError</span>
    117. 

  117. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>main.js</code>从<code>lib.js</code>输入变量<code>obj</code>，可以对<code>obj</code>添加属性，但是重新赋值就会报错。因为变量<code>obj</code>指向的地址是只读的，不能重新赋值，这就好比<code>main.js</code>创造了一个名为<code>obj</code>的<code>const</code>变量。</p> <p>最后，<code>export</code>通过接口，输出的是同一个值。不同的脚本加载这个接口，得到的都是同样的实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// mod.js</span>
    118. 

  118. <span class="token keyword">function</span> <span class="token constant">C</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    119. 

  119.   <span class="token keyword">this</span><span class="token punctuation">.</span>sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    120. 

  120.   <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function-variable function">add</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>
    121. 

  121.     <span class="token keyword">this</span><span class="token punctuation">.</span>sum <span class="token operator">+=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    122. 

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

  123.   <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function-variable function">show</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>
    124. 

  124.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
    125. 

  125.   <span class="token punctuation">}</span><span class="token punctuation">;</span>
    126. 

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

  127. 

  128. <span class="token keyword">export</span> <span class="token keyword">let</span> c <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">C</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    129. 

  129. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面的脚本<code>mod.js</code>，输出的是一个<code>C</code>的实例。不同的脚本加载这个模块，得到的都是同一个实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// x.js</span>
    130. 

  130. <span class="token keyword">import</span> <span class="token punctuation">{</span>c<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./mod'</span><span class="token punctuation">;</span>
    131. 

  131. c<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    132. 

  132. 

  133. <span class="token comment">// y.js</span>
    134. 

  134. <span class="token keyword">import</span> <span class="token punctuation">{</span>c<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./mod'</span><span class="token punctuation">;</span>
    135. 

  135. c<span class="token punctuation">.</span><span class="token function">show</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    136. 

  136. 

  137. <span class="token comment">// main.js</span>
    138. 

  138. <span class="token keyword">import</span> <span class="token string">'./x'</span><span class="token punctuation">;</span>
    139. 

  139. <span class="token keyword">import</span> <span class="token string">'./y'</span><span class="token punctuation">;</span>
    140. 

  140. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>现在执行<code>main.js</code>，输出的是<code>1</code>。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ babel-node main.js
    141. 

  141. <span class="token number">1</span>
    142. 

  142. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这就证明了<code>x.js</code>和<code>y.js</code>加载的都是<code>C</code>的同一个实例。</p> <h2 id="node-js-加载"><a href="#node-js-加载" class="header-anchor">#</a> Node.js 加载</h2> <h3 id="概述"><a href="#概述" class="header-anchor">#</a> 概述</h3> <p>Node.js 对 ES6 模块的处理比较麻烦，因为它有自己的 CommonJS 模块格式，与 ES6 模块格式是不兼容的。目前的解决方案是，将两者分开，ES6 模块和 CommonJS 采用各自的加载方案。从 v13.2 版本开始，Node.js 已经默认打开了 ES6 模块支持。</p> <p>Node.js 要求 ES6 模块<strong>采用<code>.mjs</code>后缀文件名</strong>。也就是说，只要脚本文件里面使用<code>import</code>或者<code>export</code>命令，那么就必须采用<code>.mjs</code>后缀名。Node.js 遇到<code>.mjs</code>文件，就认为它是 ES6 模块，默认启用严格模式，不必在每个模块文件顶部指定<code>&quot;use strict&quot;</code>。</p> <p><strong>如果不希望将后缀名改成<code>.mjs</code>，可以在项目的<code>package.json</code>文件中，指定<code>type</code>字段为<code>module</code>。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    143. 

  143.    <span class="token string-property property">&quot;type&quot;</span><span class="token operator">:</span> <span class="token string">&quot;module&quot;</span>
    144. 

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

  145. </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>一旦设置了以后，该目录里面的 JS 脚本，就被解释用 ES6 模块。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code><span class="token comment"># 解释成 ES6 模块</span>
    146. 

  146. $ <span class="token function">node</span> my-app.js
    147. 

  147. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果这时还要使用 CommonJS 模块，那么<strong>需要将 CommonJS 脚本的后缀名都改成<code>.cjs</code>。如果没有<code>type</code>字段，或者<code>type</code>字段为<code>commonjs</code>，则<code>.js</code>脚本会被解释成 CommonJS 模块</strong>。</p> <p>总结为一句话：<strong><code>.mjs</code>文件总是以 ES6 模块加载，<code>.cjs</code>文件总是以 CommonJS 模块加载，<code>.js</code>文件的加载取决于<code>package.json</code>里面<code>type</code>字段的设置</strong>。</p> <p>注意，ES6 模块与 CommonJS 模块尽量<strong>不要混用</strong>。<code>require</code>命令不能加载<code>.mjs</code>文件，会报错，只有<code>import</code>命令才可以加载<code>.mjs</code>文件。反过来，<code>.mjs</code>文件里面也不能使用<code>require</code>命令，必须使用<code>import</code>。</p> <h3 id="main-字段"><a href="#main-字段" class="header-anchor">#</a> main 字段</h3> <p><code>package.json</code>文件有两个字段可以指定模块的入口文件：<code>main</code>和<code>exports</code>。比较简单的模块，可以只使用<code>main</code>字段，指定模块加载的入口文件。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./node_modules/es-module-package/package.json</span>
    148. 

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

  149.   <span class="token string-property property">&quot;type&quot;</span><span class="token operator">:</span> <span class="token string">&quot;module&quot;</span><span class="token punctuation">,</span>
    150. 

  150.   <span class="token string-property property">&quot;main&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./src/index.js&quot;</span>
    151. 

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

  152. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码指定项目的入口脚本为<code>./src/index.js</code>，它的格式为 ES6 模块。如果没有<code>type</code>字段，<code>index.js</code>就会被解释为 CommonJS 模块。</p> <p>然后，<code>import</code>命令就可以加载这个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./my-app.mjs</span>
    153. 

  153. 

  154. <span class="token keyword">import</span> <span class="token punctuation">{</span> something <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'es-module-package'</span><span class="token punctuation">;</span>
    155. 

  155. <span class="token comment">// 实际加载的是 ./node_modules/es-module-package/src/index.js</span>
    156. 

  156. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<strong>运行该脚本以后，Node.js 就会到<code>./node_modules</code>目录下面，寻找<code>es-module-package</code>模块，然后根据该模块<code>package.json</code>的<code>main</code>字段去执行入口文件。</strong></p> <p>这时，如果用 CommonJS 模块的<code>require()</code>命令去加载<code>es-module-package</code>模块会报错，因为 CommonJS 模块不能处理<code>export</code>命令。</p> <h3 id="exports-字段"><a href="#exports-字段" class="header-anchor">#</a> exports 字段</h3> <p><strong><code>exports</code>字段的优先级高于<code>main</code>字段</strong>。它有多种用法。</p> <p>（1）<strong>子目录别名</strong></p> <p><code>package.json</code>文件的<code>exports</code>字段可以指定脚本或子目录的别名。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./node_modules/es-module-package/package.json</span>
    157. 

  157. <span class="token punctuation">{</span>
    158. 

  158.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    159. 

  159.     <span class="token string-property property">&quot;./submodule&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./src/submodule.js&quot;</span>
    160. 

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

  161. <span class="token punctuation">}</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><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面的代码指定<code>src/submodule.js</code>别名为<code>submodule</code>，然后就可以从别名加载这个文件。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> submodule <span class="token keyword">from</span> <span class="token string">'es-module-package/submodule'</span><span class="token punctuation">;</span>
    163. 

  163. <span class="token comment">// 加载 ./node_modules/es-module-package/src/submodule.js</span>
    164. 

  164. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>下面是子目录别名的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./node_modules/es-module-package/package.json</span>
    165. 

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

  166.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    167. 

  167.     <span class="token string-property property">&quot;./features/&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./src/features/&quot;</span>
    168. 

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

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

  170. 

  171. <span class="token keyword">import</span> feature <span class="token keyword">from</span> <span class="token string">'es-module-package/features/x.js'</span><span class="token punctuation">;</span>
    172. 

  172. <span class="token comment">// 加载 ./node_modules/es-module-package/src/features/x.js</span>
    173. 

  173. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>如果没有指定别名，就不能用“模块+脚本名”这种形式加载脚本。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    174. 

  174. <span class="token keyword">import</span> submodule <span class="token keyword">from</span> <span class="token string">'es-module-package/private-module.js'</span><span class="token punctuation">;</span>
    175. 

  175. 

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

  177. <span class="token keyword">import</span> submodule <span class="token keyword">from</span> <span class="token string">'./node_modules/es-module-package/private-module.js'</span><span class="token punctuation">;</span>
    178. 

  178. </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>（2）main 的别名</p> <p><code>exports</code>字段的别名如果是<code>.</code>，就代表模块的主入口，优先级高于<code>main</code>字段，并且可以直接简写成<code>exports</code>字段的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    179. 

  179.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    180. 

  180.     <span class="token string-property property">&quot;.&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.js&quot;</span>
    181. 

  181.   <span class="token punctuation">}</span>
    182. 

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

  183. 

  184. <span class="token comment">// 等同于</span>
    185. 

  185. <span class="token punctuation">{</span>
    186. 

  186.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.js&quot;</span>
    187. 

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

  188. </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><strong>由于<code>exports</code>字段只有支持 ES6 的 Node.js 才认识，所以可以用来兼容旧版本的 Node.js</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    189. 

  189.   <span class="token string-property property">&quot;main&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main-legacy.cjs&quot;</span><span class="token punctuation">,</span>
    190. 

  190.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    191. 

  191.     <span class="token string-property property">&quot;.&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main-modern.cjs&quot;</span>
    192. 

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

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

  194. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，老版本的 Node.js （不支持 ES6 模块）的入口文件是<code>main-legacy.cjs</code>，新版本的 Node.js 的入口文件是<code>main-modern.cjs</code>。</p> <p><strong>（3）条件加载</strong></p> <p>利用<code>.</code>这个别名，可以为 ES6 模块和 CommonJS 指定不同的入口。目前，这个功能需要在 Node.js 运行的时候，打开<code>--experimental-conditional-exports</code>标志。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    195. 

  195.   <span class="token string-property property">&quot;type&quot;</span><span class="token operator">:</span> <span class="token string">&quot;module&quot;</span><span class="token punctuation">,</span>
    196. 

  196.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    197. 

  197.     <span class="token string-property property">&quot;.&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    198. 

  198.       <span class="token string-property property">&quot;require&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.cjs&quot;</span><span class="token punctuation">,</span>
    199. 

  199.       <span class="token string-property property">&quot;default&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.js&quot;</span>
    200. 

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

  201.   <span class="token punctuation">}</span>
    202. 

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

  203. </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，别名<code>.</code>的<code>require</code>条件指定<code>require()</code>命令的入口文件（即 CommonJS 的入口），<code>default</code>条件指定其他情况的入口（即 ES6 的入口）。</p> <p>上面的写法可以简写如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    204. 

  204.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    205. 

  205.     <span class="token string-property property">&quot;require&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.cjs&quot;</span><span class="token punctuation">,</span>
    206. 

  206.     <span class="token string-property property">&quot;default&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.js&quot;</span>
    207. 

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

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

  209. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>注意，如果同时还有其他别名，就不能采用简写，否则或报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    210. 

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

  211.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    212. 

  212.     <span class="token string-property property">&quot;./feature&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./lib/feature.js&quot;</span><span class="token punctuation">,</span>
    213. 

  213.     <span class="token string-property property">&quot;require&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.cjs&quot;</span><span class="token punctuation">,</span>
    214. 

  214.     <span class="token string-property property">&quot;default&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./main.js&quot;</span>
    215. 

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

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

  217. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="es6-模块加载-commonjs-模块"><a href="#es6-模块加载-commonjs-模块" class="header-anchor">#</a> ES6 模块加载 CommonJS 模块</h3> <p>目前，一个模块同时支持 ES6 和 CommonJS 两种格式的常见方法是，<code>package.json</code>文件的<code>main</code>字段指定 CommonJS 入口，给 Node.js 使用；<code>module</code>字段指定 ES6 模块入口，给打包工具使用，因为 Node.js 不认识<code>module</code>字段。</p> <p>有了上一节的条件加载以后，Node.js 本身就可以同时处理两种模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./node_modules/pkg/package.json</span>
    218. 

  218. <span class="token punctuation">{</span>
    219. 

  219.   <span class="token string-property property">&quot;type&quot;</span><span class="token operator">:</span> <span class="token string">&quot;module&quot;</span><span class="token punctuation">,</span>
    220. 

  220.   <span class="token string-property property">&quot;main&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./index.cjs&quot;</span><span class="token punctuation">,</span>
    221. 

  221.   <span class="token string-property property">&quot;exports&quot;</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    222. 

  222.     <span class="token string-property property">&quot;require&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./index.cjs&quot;</span><span class="token punctuation">,</span>
    223. 

  223.     <span class="token string-property property">&quot;default&quot;</span><span class="token operator">:</span> <span class="token string">&quot;./wrapper.mjs&quot;</span>
    224. 

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

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

  226. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><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>上面代码指定了 CommonJS 入口文件<code>index.cjs</code>，下面是这个文件的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./node_modules/pkg/index.cjs</span>
    227. 

  227. exports<span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">'value'</span><span class="token punctuation">;</span>
    228. 

  228. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>然后，ES6 模块可以加载这个文件。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ./node_modules/pkg/wrapper.mjs</span>
    229. 

  229. <span class="token keyword">import</span> cjsModule <span class="token keyword">from</span> <span class="token string">'./index.cjs'</span><span class="token punctuation">;</span>
    230. 

  230. <span class="token keyword">export</span> <span class="token keyword">const</span> name <span class="token operator">=</span> cjsModule<span class="token punctuation">.</span>name<span class="token punctuation">;</span>
    231. 

  231. </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>注意，<code>import</code>命令加载 CommonJS 模块，只能整体加载，不能只加载单一的输出项。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 正确</span>
    232. 

  232. <span class="token keyword">import</span> packageMain <span class="token keyword">from</span> <span class="token string">'commonjs-package'</span><span class="token punctuation">;</span>
    233. 

  233. 

  234. <span class="token comment">// 报错</span>
    235. 

  235. <span class="token keyword">import</span> <span class="token punctuation">{</span> method <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'commonjs-package'</span><span class="token punctuation">;</span>
    236. 

  236. </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>还有一种变通的加载方法，就是使用 Node.js 内置的<code>module.createRequire()</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// cjs.cjs</span>
    237. 

  237. module<span class="token punctuation">.</span>exports <span class="token operator">=</span> <span class="token string">'cjs'</span><span class="token punctuation">;</span>
    238. 

  238. 

  239. <span class="token comment">// esm.mjs</span>
    240. 

  240. <span class="token keyword">import</span> <span class="token punctuation">{</span> createRequire <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'module'</span><span class="token punctuation">;</span>
    241. 

  241. 

  242. <span class="token keyword">const</span> require <span class="token operator">=</span> <span class="token function">createRequire</span><span class="token punctuation">(</span><span class="token keyword">import</span><span class="token punctuation">.</span>meta<span class="token punctuation">.</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
    243. 

  243. 

  244. <span class="token keyword">const</span> cjs <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./cjs.cjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    245. 

  245. cjs <span class="token operator">===</span> <span class="token string">'cjs'</span><span class="token punctuation">;</span> <span class="token comment">// true</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><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，ES6 模块通过<code>module.createRequire()</code>方法可以加载 CommonJS 模块</p> <h3 id="commonjs-模块加载-es6-模块"><a href="#commonjs-模块加载-es6-模块" class="header-anchor">#</a> CommonJS 模块加载 ES6 模块</h3> <p>CommonJS 的<code>require</code>命令不能加载 ES6 模块，会报错，只能使用<code>import()</code>这个方法加载。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    247. 

  247.   <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./my-app.mjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    248. 

  248. <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>
    249. 

  249. </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>上面代码可以在 CommonJS 模块中运行。</p> <h3 id="node-js-的内置模块"><a href="#node-js-的内置模块" class="header-anchor">#</a> Node.js 的内置模块</h3> <p>Node.js 的内置模块可以整体加载，也可以加载指定的输出项。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 整体加载</span>
    250. 

  250. <span class="token keyword">import</span> EventEmitter <span class="token keyword">from</span> <span class="token string">'events'</span><span class="token punctuation">;</span>
    251. 

  251. <span class="token keyword">const</span> e <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">EventEmitter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    252. 

  252. 

  253. <span class="token comment">// 加载指定的输出项</span>
    254. 

  254. <span class="token keyword">import</span> <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>
    255. 

  255. <span class="token function">readFile</span><span class="token punctuation">(</span><span class="token string">'./foo.txt'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token parameter">err<span class="token punctuation">,</span> source</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    256. 

  256.   <span class="token keyword">if</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    257. 

  257.     console<span class="token punctuation">.</span><span class="token function">error</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">;</span>
    258. 

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

  259.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>source<span class="token punctuation">)</span><span class="token punctuation">;</span>
    260. 

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

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

  262. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><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><h3 id="加载路径"><a href="#加载路径" class="header-anchor">#</a> 加载路径</h3> <p>ES6 模块的加载路径必须给出脚本的完整路径，不能省略脚本的后缀名。<code>import</code>命令和<code>package.json</code>文件的<code>main</code>字段如果省略脚本的后缀名，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES6 模块中将报错</span>
    263. 

  263. <span class="token keyword">import</span> <span class="token punctuation">{</span> something <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./index'</span><span class="token punctuation">;</span>
    264. 

  264. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>为了与浏览器的<code>import</code>加载规则相同，Node.js 的<code>.mjs</code>文件支持 URL 路径。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token string">'./foo.mjs?query=1'</span><span class="token punctuation">;</span> <span class="token comment">// 加载 ./foo 传入参数 ?query=1</span>
    265. 

  265. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，脚本路径带有参数<code>?query=1</code>，Node 会按 URL 规则解读。同一个脚本只要参数不同，就会被加载多次，并且保存成不同的缓存。由于这个原因，只要文件名中含有<code>:</code>、<code>%</code>、<code>#</code>、<code>?</code>等特殊字符，最好对这些字符进行转义。</p> <p>目前，Node.js 的<code>import</code>命令只支持加载本地模块（<code>file:</code>协议）和<code>data:</code>协议，不支持加载远程模块。另外，脚本路径只支持相对路径，不支持绝对路径（即以<code>/</code>或<code>//</code>开头的路径）。</p> <p>最后，Node 的<code>import</code>命令是异步加载，这一点与浏览器的处理方法相同。</p> <h3 id="内部变量"><a href="#内部变量" class="header-anchor">#</a> 内部变量</h3> <p>ES6 模块应该是通用的，同一个模块不用修改，就可以用在浏览器环境和服务器环境。为了达到这个目标，Node 规定 ES6 模块之中不能使用 CommonJS 模块的特有的一些内部变量。</p> <p>首先，就是<code>this</code>关键字。ES6 模块之中，顶层的<code>this</code>指向<code>undefined</code>；CommonJS 模块的顶层<code>this</code>指向当前模块，这是两者的一个重大差异。</p> <p>其次，以下这些顶层变量在 ES6 模块之中都是不存在的。</p> <ul><li><code>arguments</code></li> <li><code>require</code></li> <li><code>module</code></li> <li><code>exports</code></li> <li><code>__filename</code></li> <li><code>__dirname</code></li></ul> <h2 id="循环加载"><a href="#循环加载" class="header-anchor">#</a> 循环加载</h2> <p>“循环加载”（circular dependency）指的是，<code>a</code>脚本的执行依赖<code>b</code>脚本，而<code>b</code>脚本的执行又依赖<code>a</code>脚本。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// a.js</span>
    266. 

  266. <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    267. 

  267. 

  268. <span class="token comment">// b.js</span>
    269. 

  269. <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    270. 

  270. </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>通常，“循环加载”表示存在强耦合，如果处理不好，还可能导致递归加载，使得程序无法执行，因此应该避免出现。</p> <p>但是实际上，这是很难避免的，尤其是依赖关系复杂的大项目，很容易出现<code>a</code>依赖<code>b</code>，<code>b</code>依赖<code>c</code>，<code>c</code>又依赖<code>a</code>这样的情况。这意味着，模块加载机制必须考虑“循环加载”的情况。</p> <p>对于 JavaScript 语言来说，目前最常见的两种模块格式 CommonJS 和 ES6，处理“循环加载”的方法是不一样的，返回的结果也不一样。</p> <h3 id="commonjs-模块的加载原理"><a href="#commonjs-模块的加载原理" class="header-anchor">#</a> CommonJS 模块的加载原理</h3> <p>介绍 ES6 如何处理“循环加载”之前，先介绍目前最流行的 CommonJS 模块格式的加载原理。</p> <p>CommonJS 的一个模块，就是一个脚本文件。<code>require</code>命令第一次加载该脚本，就会执行整个脚本，然后在内存生成一个对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    271. 

  271.   <span class="token literal-property property">id</span><span class="token operator">:</span> <span class="token string">'...'</span><span class="token punctuation">,</span>
    272. 

  272.   <span class="token literal-property property">exports</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    273. 

  273.   <span class="token literal-property property">loaded</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    274. 

  274.   <span class="token operator">...</span>
    275. 

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

  276. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码就是 Node 内部加载模块后生成的一个对象。该对象的<code>id</code>属性是模块名，<code>exports</code>属性是模块输出的各个接口，<code>loaded</code>属性是一个布尔值，表示该模块的脚本是否执行完毕。其他还有很多属性，这里都省略了。</p> <p>以后需要用到这个模块的时候，就会到<code>exports</code>属性上面取值。即使再次执行<code>require</code>命令，也不会再次执行该模块，而是到缓存之中取值。也就是说，CommonJS 模块无论加载多少次，都只会在第一次加载时运行一次，以后再加载，就返回第一次运行的结果，除非手动清除系统缓存。</p> <h3 id="commonjs-模块的循环加载"><a href="#commonjs-模块的循环加载" class="header-anchor">#</a> CommonJS 模块的循环加载</h3> <p>CommonJS 模块的重要特性是加载时执行，即脚本代码在<code>require</code>的时候，就会全部执行。一旦出现某个模块被&quot;循环加载&quot;，就只输出已经执行的部分，还未执行的部分不会输出。</p> <p>让我们来看，Node <a href="https://nodejs.org/api/modules.html#modules_cycles" target="_blank" rel="noopener noreferrer">官方文档<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>里面的例子。脚本文件<code>a.js</code>代码如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>exports<span class="token punctuation">.</span>done <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    277. 

  277. <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./b.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    278. 

  278. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'在 a.js 之中，b.done = %j'</span><span class="token punctuation">,</span> b<span class="token punctuation">.</span>done<span class="token punctuation">)</span><span class="token punctuation">;</span>
    279. 

  279. exports<span class="token punctuation">.</span>done <span class="token operator">=</span> <span class="token boolean">true</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 string">'a.js 执行完毕'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    281. 

  281. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码之中，<code>a.js</code>脚本先输出一个<code>done</code>变量，然后加载另一个脚本文件<code>b.js</code>。注意，此时<code>a.js</code>代码就停在这里，等待<code>b.js</code>执行完毕，再往下执行。</p> <p>再看<code>b.js</code>的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>exports<span class="token punctuation">.</span>done <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    282. 

  282. <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./a.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    283. 

  283. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'在 b.js 之中，a.done = %j'</span><span class="token punctuation">,</span> a<span class="token punctuation">.</span>done<span class="token punctuation">)</span><span class="token punctuation">;</span>
    284. 

  284. exports<span class="token punctuation">.</span>done <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    285. 

  285. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'b.js 执行完毕'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    286. 

  286. </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>上面代码之中，<code>b.js</code>执行到第二行，就会去加载<code>a.js</code>，这时，就发生了“循环加载”。系统会去<code>a.js</code>模块对应对象的<code>exports</code>属性取值，可是因为<code>a.js</code>还没有执行完，从<code>exports</code>属性只能取回已经执行的部分，而不是最后的值。</p> <p><code>a.js</code>已经执行的部分，只有一行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>exports<span class="token punctuation">.</span>done <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    287. 

  287. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>因此，对于<code>b.js</code>来说，它从<code>a.js</code>只输入一个变量<code>done</code>，值为<code>false</code>。</p> <p>然后，<code>b.js</code>接着往下执行，等到全部执行完毕，再把执行权交还给<code>a.js</code>。于是，<code>a.js</code>接着往下执行，直到执行完毕。我们写一个脚本<code>main.js</code>，验证这个过程。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./a.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    288. 

  288. <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./b.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    289. 

  289. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'在 main.js 之中, a.done=%j, b.done=%j'</span><span class="token punctuation">,</span> a<span class="token punctuation">.</span>done<span class="token punctuation">,</span> b<span class="token punctuation">.</span>done<span class="token punctuation">)</span><span class="token punctuation">;</span>
    290. 

  290. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>执行<code>main.js</code>，运行结果如下。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">node</span> main.js
    291. 

  291. 

  292. 在 b.js 之中，a.done <span class="token operator">=</span> <span class="token boolean">false</span>
    293. 

  293. b.js 执行完毕
    294. 

  294. 在 a.js 之中，b.done <span class="token operator">=</span> <span class="token boolean">true</span>
    295. 

  295. a.js 执行完毕
    296. 

  296. 在 main.js 之中, <span class="token assign-left variable">a.done</span><span class="token operator">=</span>true, <span class="token assign-left variable">b.done</span><span class="token operator">=</span>true
    297. 

  297. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面的代码证明了两件事。一是，在<code>b.js</code>之中，<code>a.js</code>没有执行完毕，只执行了第一行。二是，<code>main.js</code>执行到第二行时，不会再次执行<code>b.js</code>，而是输出缓存的<code>b.js</code>的执行结果，即它的第四行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>exports<span class="token punctuation">.</span>done <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    298. 

  298. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>总之，CommonJS 输入的是被输出值的拷贝，不是引用。</p> <p>另外，由于 CommonJS 模块遇到循环加载时，返回的是当前已经执行的部分的值，而不是代码全部执行后的值，两者可能会有差异。所以，输入变量的时候，必须非常小心。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 安全的写法</span>
    299. 

  299. <span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>foo<span class="token punctuation">;</span> <span class="token comment">// 危险的写法</span>
    300. 

  300. 

  301. exports<span class="token punctuation">.</span><span class="token function-variable function">good</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">arg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    302. 

  302.   <span class="token keyword">return</span> a<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token string">'good'</span><span class="token punctuation">,</span> arg<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 使用的是 a.foo 的最新值</span>
    303. 

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

  304. 

  305. exports<span class="token punctuation">.</span><span class="token function-variable function">bad</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">arg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    306. 

  306.   <span class="token keyword">return</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token string">'bad'</span><span class="token punctuation">,</span> arg<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 使用的是一个部分加载时的值</span>
    307. 

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

  308. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，如果发生循环加载，<code>require('a').foo</code>的值很可能后面会被改写，改用<code>require('a')</code>会更保险一点。</p> <h3 id="es6-模块的循环加载"><a href="#es6-模块的循环加载" class="header-anchor">#</a> ES6 模块的循环加载</h3> <p>ES6 处理“循环加载”与 CommonJS 有本质的不同。ES6 模块是动态引用，如果使用<code>import</code>从一个模块加载变量（即<code>import foo from 'foo'</code>），那些变量不会被缓存，而是成为一个指向被加载模块的引用，需要开发者自己保证，真正取值的时候能够取到值。</p> <p>请看下面这个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// a.mjs</span>
    309. 

  309. <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">'./b'</span><span class="token punctuation">;</span>
    310. 

  310. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'a.mjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    311. 

  311. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>bar<span class="token punctuation">)</span><span class="token punctuation">;</span>
    312. 

  312. <span class="token keyword">export</span> <span class="token keyword">let</span> foo <span class="token operator">=</span> <span class="token string">'foo'</span><span class="token punctuation">;</span>
    313. 

  313. 

  314. <span class="token comment">// b.mjs</span>
    315. 

  315. <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">'./a'</span><span class="token punctuation">;</span>
    316. 

  316. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'b.mjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    317. 

  317. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span>
    318. 

  318. <span class="token keyword">export</span> <span class="token keyword">let</span> bar <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    319. 

  319. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>a.mjs</code>加载<code>b.mjs</code>，<code>b.mjs</code>又加载<code>a.mjs</code>，构成循环加载。执行<code>a.mjs</code>，结果如下。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">node</span> --experimental-modules a.mjs
    320. 

  320. b.mjs
    321. 

  321. ReferenceError: foo is not defined
    322. 

  322. </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，执行<code>a.mjs</code>以后会报错，<code>foo</code>变量未定义，这是为什么？</p> <p>让我们一行行来看，ES6 循环加载是怎么处理的。首先，执行<code>a.mjs</code>以后，引擎发现它加载了<code>b.mjs</code>，因此会优先执行<code>b.mjs</code>，然后再执行<code>a.mjs</code>。接着，执行<code>b.mjs</code>的时候，已知它从<code>a.mjs</code>输入了<code>foo</code>接口，这时不会去执行<code>a.mjs</code>，而是认为这个接口已经存在了，继续往下执行。执行到第三行<code>console.log(foo)</code>的时候，才发现这个接口根本没定义，因此报错。</p> <p>解决这个问题的方法，就是让<code>b.mjs</code>运行的时候，<code>foo</code>已经有定义了。这可以通过将<code>foo</code>写成函数来解决。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// a.mjs</span>
    323. 

  323. <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">'./b'</span><span class="token punctuation">;</span>
    324. 

  324. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'a.mjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    325. 

  325. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    326. 

  326. <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> <span class="token keyword">return</span> <span class="token string">'foo'</span> <span class="token punctuation">}</span>
    327. 

  327. <span class="token keyword">export</span> <span class="token punctuation">{</span>foo<span class="token punctuation">}</span><span class="token punctuation">;</span>
    328. 

  328. 

  329. <span class="token comment">// b.mjs</span>
    330. 

  330. <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">'./a'</span><span class="token punctuation">;</span>
    331. 

  331. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'b.mjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    332. 

  332. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    333. 

  333. <span class="token keyword">function</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token string">'bar'</span> <span class="token punctuation">}</span>
    334. 

  334. <span class="token keyword">export</span> <span class="token punctuation">{</span>bar<span class="token punctuation">}</span><span class="token punctuation">;</span>
    335. 

  335. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>这时再执行<code>a.mjs</code>就可以得到预期结果。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">node</span> --experimental-modules a.mjs
    336. 

  336. b.mjs
    337. 

  337. foo
    338. 

  338. a.mjs
    339. 

  339. bar
    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><span class="line-number">5</span><br></div></div><p>这是因为函数具有提升作用，在执行<code>import {bar} from './b'</code>时，函数<code>foo</code>就已经有定义了，所以<code>b.mjs</code>加载的时候不会报错。这也意味着，如果把函数<code>foo</code>改写成函数表达式，也会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// a.mjs</span>
    341. 

  341. <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">'./b'</span><span class="token punctuation">;</span>
    342. 

  342. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'a.mjs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    343. 

  343. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    344. 

  344. <span class="token keyword">const</span> <span class="token function-variable function">foo</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token string">'foo'</span><span class="token punctuation">;</span>
    345. 

  345. <span class="token keyword">export</span> <span class="token punctuation">{</span>foo<span class="token punctuation">}</span><span class="token punctuation">;</span>
    346. 

  346. </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>上面代码的第四行，改成了函数表达式，就不具有提升作用，执行就会报错。</p> <p>我们再来看 ES6 模块加载器<a href="https://github.com/ModuleLoader/es6-module-loader/blob/master/docs/circular-references-bindings.md" target="_blank" rel="noopener noreferrer">SystemJS<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>给出的一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// even.js</span>
    347. 

  347. <span class="token keyword">import</span> <span class="token punctuation">{</span> odd <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./odd'</span>
    348. 

  348. <span class="token keyword">export</span> <span class="token keyword">var</span> counter <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    349. 

  349. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">even</span><span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    350. 

  350.   counter<span class="token operator">++</span><span class="token punctuation">;</span>
    351. 

  351.   <span class="token keyword">return</span> n <span class="token operator">===</span> <span class="token number">0</span> <span class="token operator">||</span> <span class="token function">odd</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    352. 

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

  353. 

  354. <span class="token comment">// odd.js</span>
    355. 

  355. <span class="token keyword">import</span> <span class="token punctuation">{</span> even <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./even'</span><span class="token punctuation">;</span>
    356. 

  356. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">odd</span><span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    357. 

  357.   <span class="token keyword">return</span> n <span class="token operator">!==</span> <span class="token number">0</span> <span class="token operator">&amp;&amp;</span> <span class="token function">even</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    358. 

  358. <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><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，<code>even.js</code>里面的函数<code>even</code>有一个参数<code>n</code>，只要不等于 0，就会减去 1，传入加载的<code>odd()</code>。<code>odd.js</code>也会做类似操作。</p> <p>运行上面这段代码，结果如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>$ babel<span class="token operator">-</span>node
    360. 

  360. <span class="token operator">&gt;</span> <span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> m <span class="token keyword">from</span> <span class="token string">'./even.js'</span><span class="token punctuation">;</span>
    361. 

  361. <span class="token operator">&gt;</span> m<span class="token punctuation">.</span><span class="token function">even</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    362. 

  362. <span class="token boolean">true</span>
    363. 

  363. <span class="token operator">&gt;</span> m<span class="token punctuation">.</span>counter
    364. 

  364. <span class="token number">6</span>
    365. 

  365. <span class="token operator">&gt;</span> m<span class="token punctuation">.</span><span class="token function">even</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span>
    366. 

  366. <span class="token boolean">true</span>
    367. 

  367. <span class="token operator">&gt;</span> m<span class="token punctuation">.</span>counter
    368. 

  368. <span class="token number">17</span>
    369. 

  369. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，参数<code>n</code>从 10 变为 0 的过程中，<code>even()</code>一共会执行 6 次，所以变量<code>counter</code>等于 6。第二次调用<code>even()</code>时，参数<code>n</code>从 20 变为 0，<code>even()</code>一共会执行 11 次，加上前面的 6 次，所以变量<code>counter</code>等于 17。</p> <p>这个例子要是改写成 CommonJS，就根本无法执行，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// even.js</span>
    370. 

  370. <span class="token keyword">var</span> odd <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./odd'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    371. 

  371. <span class="token keyword">var</span> counter <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    372. 

  372. exports<span class="token punctuation">.</span>counter <span class="token operator">=</span> counter<span class="token punctuation">;</span>
    373. 

  373. exports<span class="token punctuation">.</span><span class="token function-variable function">even</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    374. 

  374.   counter<span class="token operator">++</span><span class="token punctuation">;</span>
    375. 

  375.   <span class="token keyword">return</span> n <span class="token operator">==</span> <span class="token number">0</span> <span class="token operator">||</span> <span class="token function">odd</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    376. 

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

  377. 

  378. <span class="token comment">// odd.js</span>
    379. 

  379. <span class="token keyword">var</span> even <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'./even'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>even<span class="token punctuation">;</span>
    380. 

  380. module<span class="token punctuation">.</span><span class="token function-variable function">exports</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    381. 

  381.   <span class="token keyword">return</span> n <span class="token operator">!=</span> <span class="token number">0</span> <span class="token operator">&amp;&amp;</span> <span class="token function">even</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    382. 

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

  383. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，<code>even.js</code>加载<code>odd.js</code>，而<code>odd.js</code>又去加载<code>even.js</code>，形成“循环加载”。这时，执行引擎就会输出<code>even.js</code>已经执行的部分（不存在任何结果），所以在<code>odd.js</code>之中，变量<code>even</code>等于<code>undefined</code>，等到后面调用<code>even(n - 1)</code>就会报错。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">node</span>
    384. 

  384. <span class="token operator">&gt;</span> var m <span class="token operator">=</span> require<span class="token punctuation">(</span><span class="token string">'./even'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    385. 

  385. <span class="token operator">&gt;</span> m.even<span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span>
    386. 

  386. TypeError: even is not a <span class="token keyword">function</span>
    387. 

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