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href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06225672>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAB4AAAAeCAYAAAA7MK6iAAAAAXNSR0IArs4c6QAABH1JREFUSA3tVl1oHFUUPmdmd2ltklqbpJDiNnXFmgbFktho7YMPNiJSSZM0+CAYSkUELVhM6YuwIPpgoOKDqOBDC0XE2CQoNtQXBUFTTcCi+Wlh1V2TQExsUzcltd3M9Tt3ZjZzZ2fT+OJTL8yeM+eee757fmeJbq//KQL8X3DUSFOcfr7cRsRtxNQMWueeVzOkaITIGqQHNg5y8+jNW9ldM7A6nTpAjuolUikAwq7CE3WcM2RRDz+XGVgN3FptU/aUSlvq9Pa3iZ1+sgAqJyyAFqkipd9dqiwHF3P65YycLWc/6sqGrvoEoIp6DOFaX5h6+dnfjkWprwqsPk0dUGq5vySwDImC10KxFHgGL1SWoc92O3eVht09qdXNH11I2SsTsJYqMWzihqGMi+A+Garf3BAuuLI5oGlULyNfyB/HYNujwktOfRrMr5t77NmevqaUopx0grnKAyvVpmwUDB4x6FPXuGvYLTDwWsejwgtgkYKPqRJg8SV6xaiZ3ZTppGneS4yfH5/66fZSDHv+QZci/+h5c5UHtpy67JUqGppM0sh0Nc1dW6/N1W5Yoqat8/TU/VnadmdeW2PLLSyh0cvxBs3KbqTmwYPpxN4do/mzE8nEpvX/UMu2Wbp74zUAK5q6WkHns7V0eWkdPbPzd3rxkTGybadYySumVzhcaJFbs5UrEkQ/+CK8gF5dnh/6ciIZ73gwQ927L1IitoxKLXYP3SjYdOrHHfTZhRRlFyrorafPk20B3HPD1y2G3qKZME5Jcf3t/HUC13/8tSd++vqFveMUTwAUxSUFI1QekR1+bIze3D9MF2aq6cPvG72CgnldWCFqyRw3lwH8ZMerjTD9ElRO7Gv44wNpC90aASqGfVlz/Rx17srQ57/UU26hkhQqUB7dBR71WmzQhHUnblGmVOEw0jhbV1n9OlXUDCIRGaNV5Jp43N516fN7JmnTHdfp7Hgy0luO4aMhtkLL8Bi3bUWYvzh5Mn1dTxrL6QmGuRhGL/TiTTxRoEdTszSaq9GR0NGA3KdkOz3hqSV3MIDhQ5IVX/Ivx3umBti2es2h4eZby7x8br1rkf7Mo90AqC8aQ3sJeNzqFRu+vSANAQe3PL7l0HGOAdwDCeZYvNKeoZp1Qfs6Aipndh86HmFRi0LAnEO47wsqM6cdfjh3jBPUzhZy7nvlUfFsamED1VQt6aISHVymXZ/B2aCtIG8AI8xfobj2d3en1wWVhOeHELKmLQ1s211s88comkv4UCwWyF787mJdYXtNfhKAXVqnKTq8QZvGAGGOfaTo5pGZ/PwbUCr5+DPr/1J92JNHr9aOl/F3iI5+O1nfybsGxoimvZ3ViWSluDITw3P37mypheDIPY0tw7+O/5ApbkYw+zpfaUVu32Pi98+defdUhEpZkRFq0aqyNh9FuL9hpYbEm6iwi0z2REd09ZmyENEbuhjDWzKvZXTqKYaBIr3tt5kuPtQBZFvEUwHt60vfCNu41XsksH9Ij1BMMz1Y0OOunHNShFIP5868g5zeXmuLwL9T4b6Q2+KejgAAAABJRU5ErkJggg==">Generator 函数的语法<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="generator-函数的语法"><a href="#generator-函数的语法" class="header-anchor">#</a> Generator 函数的语法</h1> <h2 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h2> <h3 id="基本概念"><a href="#基本概念" class="header-anchor">#</a> 基本概念</h3> <p>Generator 函数是 ES6 提供的一种<strong>异步编程解决方案</strong>，语法行为与传统函数完全不同。本章详细介绍 Generator 函数的语法和 API，它的异步编程应用请看《Generator 函数的异步应用》一章。
    26. 

  26. 

  27. Generator 函数有多种理解角度。语法上，首先可以把它理解成，Generator 函数是一个状态机，<strong>封装了多个内部状态</strong>。</p> <p>执行 Generator 函数会<strong>返回一个遍历器对象</strong>，也就是说，Generator 函数除了状态机，<strong>还是一个遍历器对象生成函数</strong>。返回的遍历器对象，可以依次遍历 Generator 函数内部的每一个状态。</p> <p>形式上，Generator 函数是一个普通函数，但是有<strong>两个特征</strong>。一是，<strong><code>function</code>关键字与函数名之间有一个星号</strong>；二是，函数体<strong>内部使用<code>yield</code>表达式</strong>，定义不同的内部状态（<code>yield</code>在英语里的意思就是“产出”）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">helloWorldGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    28. 

  28.   <span class="token keyword">yield</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    29. 

  29.   <span class="token keyword">yield</span> <span class="token string">'world'</span><span class="token punctuation">;</span>
    30. 

  30.   <span class="token keyword">return</span> <span class="token string">'ending'</span><span class="token punctuation">;</span>
    31. 

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

  32. 

  33. <span class="token keyword">var</span> hw <span class="token operator">=</span> <span class="token function">helloWorldGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</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></div></div><p>上面代码定义了一个 Generator 函数<code>helloWorldGenerator</code>，它内部有两个<code>yield</code>表达式（<code>hello</code>和<code>world</code>），即该函数有三个状态：hello，world 和 return 语句（结束执行）。</p> <p>然后，Generator 函数的调用方法与普通函数一样，也是在函数名后面加上一对圆括号。不同的是，<strong>调用 Generator 函数后，该函数并不执行，返回的也不是函数运行结果，而是一个指向内部状态的指针对象，也就是上一章介绍的遍历器对象</strong>（Iterator Object）。</p> <p>下一步，必须调用遍历器对象的<code>next</code>方法，使得指针移向下一个状态。也就是说，每次调用<code>next</code>方法，内部指针就从函数头部或上一次停下来的地方开始执行，直到遇到下一个<code>yield</code>表达式（或<code>return</code>语句）为止。换言之，Generator 函数是分段执行的，<code>yield</code>表达式是暂停执行的标记，而<code>next</code>方法可以恢复执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>hw<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    35. 

  35. <span class="token comment">// { value: 'hello', done: false }</span>
    36. 

  36. 

  37. hw<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    38. 

  38. <span class="token comment">// { value: 'world', done: false }</span>
    39. 

  39. 

  40. hw<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    41. 

  41. <span class="token comment">// { value: 'ending', done: true }</span>
    42. 

  42. 

  43. hw<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    44. 

  44. <span class="token comment">// { value: undefined, done: true }</span>
    45. 

  45. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码一共调用了四次<code>next</code>方法。</p> <p>第一次调用，Generator 函数开始执行，直到遇到第一个<code>yield</code>表达式为止。<code>next</code>方法返回一个对象，它的<code>value</code>属性就是当前<code>yield</code>表达式的值<code>hello</code>，<code>done</code>属性的值<code>false</code>，表示遍历还没有结束。</p> <p>第二次调用，Generator 函数从上次<code>yield</code>表达式停下的地方，一直执行到下一个<code>yield</code>表达式。<code>next</code>方法返回的对象的<code>value</code>属性就是当前<code>yield</code>表达式的值<code>world</code>，<code>done</code>属性的值<code>false</code>，表示遍历还没有结束。</p> <p>第三次调用，Generator 函数从上次<code>yield</code>表达式停下的地方，一直执行到<code>return</code>语句（如果没有<code>return</code>语句，就执行到函数结束）。<code>next</code>方法返回的对象的<code>value</code>属性，就是紧跟在<code>return</code>语句后面的表达式的值（如果没有<code>return</code>语句，则<code>value</code>属性的值为<code>undefined</code>），<code>done</code>属性的值<code>true</code>，表示遍历已经结束。</p> <p>第四次调用，此时 Generator 函数已经运行完毕，<code>next</code>方法返回对象的<code>value</code>属性为<code>undefined</code>，<code>done</code>属性为<code>true</code>。以后再调用<code>next</code>方法，返回的都是这个值。</p> <p><strong>总结一下，调用 Generator 函数，返回一个遍历器对象，代表 Generator 函数的内部指针。以后，每次调用遍历器对象的<code>next</code>方法，就会返回一个有着<code>value</code>和<code>done</code>两个属性的对象。<code>value</code>属性表示当前的内部状态的值，是<code>yield</code>表达式后面那个表达式的值；<code>done</code>属性是一个布尔值，表示是否遍历结束。</strong></p> <p>ES6 没有规定，<code>function</code>关键字与函数名之间的星号，写在哪个位置。这导致下面的写法都能通过。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> ··· <span class="token punctuation">}</span>
    46. 

  46. <span class="token keyword">function</span> <span class="token operator">*</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> ··· <span class="token punctuation">}</span>
    47. 

  47. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> ··· <span class="token punctuation">}</span>
    48. 

  48. <span class="token keyword">function</span><span class="token operator">*</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> ··· <span class="token punctuation">}</span>
    49. 

  49. </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>由于 Generator 函数仍然是普通函数，所以一般的写法是上面的第三种，即星号紧跟在<code>function</code>关键字后面。本书也采用这种写法。</p> <h3 id="yield-表达式"><a href="#yield-表达式" class="header-anchor">#</a> yield 表达式</h3> <p>由于 Generator 函数返回的遍历器对象，只有调用<code>next</code>方法才会遍历下一个内部状态，所以其实提供了一种可以暂停执行的函数。<code>yield</code>表达式就是暂停标志。</p> <p>遍历器对象的<code>next</code>方法的运行逻辑如下。</p> <p>（1）遇到<code>yield</code>表达式，就暂停执行后面的操作，并将紧跟在<code>yield</code>后面的那个表达式的值，作为返回的对象的<code>value</code>属性值。</p> <p>（2）下一次调用<code>next</code>方法时，再继续往下执行，直到遇到下一个<code>yield</code>表达式。</p> <p>（3）如果没有再遇到新的<code>yield</code>表达式，就一直运行到函数结束，直到<code>return</code>语句为止，并将<code>return</code>语句后面的表达式的值，作为返回的对象的<code>value</code>属性值。</p> <p>（4）如果该函数没有<code>return</code>语句，则返回的对象的<code>value</code>属性值为<code>undefined</code>。</p> <p>需要注意的是，<strong><code>yield</code>表达式后面的表达式，只有当调用<code>next</code>方法、内部指针指向该语句时才会执行</strong>，因此等于为 JavaScript 提供了手动的“<strong>惰性求值</strong>”（Lazy Evaluation）的语法功能。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    50. 

  50.   <span class="token keyword">yield</span>  <span class="token number">123</span> <span class="token operator">+</span> <span class="token number">456</span><span class="token punctuation">;</span>
    51. 

  51. <span class="token punctuation">}</span>
    52. 

  52. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>yield</code>后面的表达式<code>123 + 456</code>，不会立即求值，只会在<code>next</code>方法将指针移到这一句时，才会求值。</p> <p><code>yield</code>表达式与<code>return</code>语句既有相似之处，也有区别。相似之处在于，都能返回紧跟在语句后面的那个表达式的值。区别在于每次遇到<code>yield</code>，函数暂停执行，下一次再从该位置继续向后执行，而**<code>return</code>语句不具备位置记忆的功能**。一个函数里面，只能执行一次（或者说一个）<code>return</code>语句，但是可以执行多次（或者说多个）<code>yield</code>表达式。正常函数只能返回一个值，因为只能执行一次<code>return</code>；<strong>Generator 函数可以返回一系列的值，因为可以有任意多个<code>yield</code></strong>。从另一个角度看，也可以说 Generator 生成了一系列的值，这也就是它的名称的来历（英语中，generator 这个词是“生成器”的意思）。</p> <p>Generator 函数可以不用<code>yield</code>表达式，这时就变成了一个单纯的暂缓执行函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    53. 

  53.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'执行了！'</span><span class="token punctuation">)</span>
    54. 

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

  55. 

  56. <span class="token keyword">var</span> generator <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    57. 

  57. 

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

  59.   generator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    60. 

  60. <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">2000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    61. 

  61. </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，函数<code>f</code>如果是普通函数，在为变量<code>generator</code>赋值时就会执行。但是，函数<code>f</code>是一个 Generator 函数，就变成只有调用<code>next</code>方法时，函数<code>f</code>才会执行。</p> <p>另外需要注意，<code>yield</code>表达式只能用在 Generator 函数里面，用在其他地方都会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    62. 

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

  63. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    64. 

  64. <span class="token comment">// SyntaxError: Unexpected number</span>
    65. 

  65. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码在一个普通函数中使用<code>yield</code>表达式，结果产生一个句法错误。</p> <p>下面是另外一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    66. 

  66. 

  67. <span class="token keyword">var</span> <span class="token function-variable function">flat</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    68. 

  68.   a<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    69. 

  69.     <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> item <span class="token operator">!==</span> <span class="token string">'number'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    70. 

  70.       <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">flat</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
    71. 

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

  72.       <span class="token keyword">yield</span> item<span class="token punctuation">;</span>
    73. 

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

  74.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    75. 

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

  76. 

  77. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> f <span class="token keyword">of</span> <span class="token function">flat</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    78. 

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

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

  80. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码也会<strong>产生句法错误，因为<code>forEach</code>方法的参数是一个普通函数</strong>，但是在里面使用了<code>yield</code>表达式（这个函数里面还使用了<code>yield*</code>表达式，详细介绍见后文）。一种修改方法是改用<code>for</code>循环。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    81. 

  81. 

  82. <span class="token keyword">var</span> <span class="token function-variable function">flat</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    83. 

  83.   <span class="token keyword">var</span> length <span class="token operator">=</span> a<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
    84. 

  84.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    85. 

  85.     <span class="token keyword">var</span> item <span class="token operator">=</span> a<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
    86. 

  86.     <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> item <span class="token operator">!==</span> <span class="token string">'number'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    87. 

  87.       <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">flat</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
    88. 

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

  89.       <span class="token keyword">yield</span> item<span class="token punctuation">;</span>
    90. 

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

  91.   <span class="token punctuation">}</span>
    92. 

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

  93. 

  94. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> f <span class="token keyword">of</span> <span class="token function">flat</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    95. 

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

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

  97. <span class="token comment">// 1, 2, 3, 4, 5, 6</span>
    98. 

  98. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>另外，<strong><code>yield</code>表达式如果用在另一个表达式之中，必须放在圆括号里面。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">demo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    99. 

  99.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Hello'</span> <span class="token operator">+</span> <span class="token keyword">yield</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// SyntaxError</span>
    100. 

  100.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Hello'</span> <span class="token operator">+</span> <span class="token keyword">yield</span> <span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// SyntaxError</span>
    101. 

  101. 

  102.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Hello'</span> <span class="token operator">+</span> <span class="token punctuation">(</span><span class="token keyword">yield</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// OK</span>
    103. 

  103.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Hello'</span> <span class="token operator">+</span> <span class="token punctuation">(</span><span class="token keyword">yield</span> <span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// OK</span>
    104. 

  104. <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></div></div><p><code>yield</code>表达式用作函数参数或放在赋值表达式的右边，可以不加括号。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">demo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    106. 

  106.   <span class="token function">foo</span><span class="token punctuation">(</span><span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// OK</span>
    107. 

  107.   <span class="token keyword">let</span> input <span class="token operator">=</span> <span class="token keyword">yield</span><span class="token punctuation">;</span> <span class="token comment">// OK</span>
    108. 

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

  109. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="与-iterator-接口的关系"><a href="#与-iterator-接口的关系" class="header-anchor">#</a> 与 Iterator 接口的关系</h3> <p>上一章说过，任意一个对象的<code>Symbol.iterator</code>方法，等于该对象的遍历器生成函数，调用该函数会返回该对象的一个遍历器对象。</p> <p>由于 Generator 函数就是遍历器生成函数，因此<strong>可以把 Generator 赋值给对象的<code>Symbol.iterator</code>属性，从而使得该对象具有 Iterator 接口。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myIterable <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    110. 

  110. myIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    111. 

  111.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    112. 

  112.   <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    113. 

  113.   <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    114. 

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

  115. 

  116. <span class="token punctuation">[</span><span class="token operator">...</span>myIterable<span class="token punctuation">]</span> <span class="token comment">// [1, 2, 3]</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，Generator 函数赋值给<code>Symbol.iterator</code>属性，从而使得<code>myIterable</code>对象具有了 Iterator 接口，可以被<code>...</code>运算符遍历了。</p> <p>Generator 函数执行后，返回一个遍历器对象。该对象本身也具有<code>Symbol.iterator</code>属性，执行后返回自身。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    118. 

  118.   <span class="token comment">// some code</span>
    119. 

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

  120. 

  121. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    122. 

  122. 

  123. g<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">===</span> g
    124. 

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

  125. </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>gen</code>是一个 Generator 函数，调用它会生成一个遍历器对象<code>g</code>。它的<code>Symbol.iterator</code>属性，也是一个遍历器对象生成函数，执行后返回它自己。</p> <h2 id="next-方法的参数"><a href="#next-方法的参数" class="header-anchor">#</a> next 方法的参数</h2> <p><code>yield</code>表达式本身没有返回值，或者说总是返回<code>undefined</code>。<code>next</code>方法可以带一个参数，该参数就会被当作上一个<code>yield</code>表达式的返回值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    126. 

  126.   <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token boolean">true</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    127. 

  127.     <span class="token keyword">var</span> reset <span class="token operator">=</span> <span class="token keyword">yield</span> i<span class="token punctuation">;</span>
    128. 

  128.     <span class="token keyword">if</span><span class="token punctuation">(</span>reset<span class="token punctuation">)</span> <span class="token punctuation">{</span> i <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    129. 

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

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

  131. 

  132. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    133. 

  133. 

  134. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 0, done: false }</span>
    135. 

  135. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 1, done: false }</span>
    136. 

  136. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// { value: 0, done: false } // 参数true 传给上面的变量reset</span>
    137. 

  137. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码先定义了一个可以无限运行的 Generator 函数<code>f</code>，如果<code>next</code>方法没有参数，每次运行到<code>yield</code>表达式，变量<code>reset</code>的值总是<code>undefined</code>。当<code>next</code>方法带一个参数<code>true</code>时，变量<code>reset</code>就被重置为这个参数（即<code>true</code>），因此<code>i</code>会等于<code>-1</code>，下一轮循环就会从<code>-1</code>开始递增。</p> <p>这个功能有很重要的语法意义。Generator 函数从暂停状态到恢复运行，它的上下文状态（context）是不变的。通过<code>next</code>方法的参数，就有办法在 Generator 函数开始运行之后，继续向函数体内部注入值。也就是说，可以在 Generator 函数运行的不同阶段，<strong>从外部向内部注入不同的值，从而调整函数行为</strong>。</p> <p>再看一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    138. 

  138.   <span class="token keyword">var</span> y <span class="token operator">=</span> <span class="token number">2</span> <span class="token operator">*</span> <span class="token punctuation">(</span><span class="token keyword">yield</span> <span class="token punctuation">(</span>x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    139. 

  139.   <span class="token keyword">var</span> z <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token punctuation">(</span>y <span class="token operator">/</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    140. 

  140.   <span class="token keyword">return</span> <span class="token punctuation">(</span>x <span class="token operator">+</span> y <span class="token operator">+</span> z<span class="token punctuation">)</span><span class="token punctuation">;</span>
    141. 

  141. <span class="token punctuation">}</span>
    142. 

  142. 

  143. <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    144. 

  144. a<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Object{value:6, done:false}</span>
    145. 

  145. a<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Object{value:NaN, done:false}</span>
    146. 

  146. a<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Object{value:NaN, done:true}</span>
    147. 

  147. 

  148. <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    149. 

  149. b<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value:6, done:false }</span>
    150. 

  150. b<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token number">12</span><span class="token punctuation">)</span> <span class="token comment">// { value:8, done:false }</span>
    151. 

  151. b<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token number">13</span><span class="token punctuation">)</span> <span class="token comment">// { value:42, done:true }</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><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，第二次运行<code>next</code>方法的时候不带参数，导致 y 的值等于<code>2 * undefined</code>（即<code>NaN</code>），除以 3 以后还是<code>NaN</code>，因此返回对象的<code>value</code>属性也等于<code>NaN</code>。第三次运行<code>Next</code>方法的时候不带参数，所以<code>z</code>等于<code>undefined</code>，返回对象的<code>value</code>属性等于<code>5 + NaN + undefined</code>，即<code>NaN</code>。</p> <p>如果向<code>next</code>方法提供参数，返回结果就完全不一样了。上面代码第一次调用<code>b</code>的<code>next</code>方法时，返回<code>x+1</code>的值<code>6</code>；第二次调用<code>next</code>方法，将上一次<code>yield</code>表达式的值设为<code>12</code>，因此<code>y</code>等于<code>24</code>，返回<code>y / 3</code>的值<code>8</code>；第三次调用<code>next</code>方法，将上一次<code>yield</code>表达式的值设为<code>13</code>，因此<code>z</code>等于<code>13</code>，这时<code>x</code>等于<code>5</code>，<code>y</code>等于<code>24</code>，所以<code>return</code>语句的值等于<code>42</code>。</p> <p>注意，<strong>由于<code>next</code>方法的参数表示上一个<code>yield</code>表达式的返回值，所以在第一次使用<code>next</code>方法时，传递参数是无效的</strong>。V8 引擎直接忽略第一次使用<code>next</code>方法时的参数，只有从第二次使用<code>next</code>方法开始，参数才是有效的。从语义上讲，第一个<code>next</code>方法用来启动遍历器对象，所以不用带有参数。</p> <p>再看一个通过<code>next</code>方法的参数，向 Generator 函数内部输入值的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">dataConsumer</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    153. 

  153.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Started'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    154. 

  154.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">1. </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span><span class="token keyword">yield</span><span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    155. 

  155.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">2. </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span><span class="token keyword">yield</span><span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    156. 

  156.   <span class="token keyword">return</span> <span class="token string">'result'</span><span class="token punctuation">;</span>
    157. 

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

  158. 

  159. <span class="token keyword">let</span> genObj <span class="token operator">=</span> <span class="token function">dataConsumer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    160. 

  160. genObj<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    161. 

  161. <span class="token comment">// Started</span>
    162. 

  162. genObj<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span>
    163. 

  163. <span class="token comment">// 1. a</span>
    164. 

  164. genObj<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span>
    165. 

  165. <span class="token comment">// 2. b</span>
    166. 

  166. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码是一个很直观的例子，每次通过<code>next</code>方法向 Generator 函数输入值，然后打印出来。</p> <p>如果想要第一次调用<code>next</code>方法时，就能够输入值，可以在 Generator 函数外面再包一层。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">wrapper</span><span class="token punctuation">(</span><span class="token parameter">generatorFunction</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    167. 

  167.   <span class="token keyword">return</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    168. 

  168.     <span class="token keyword">let</span> generatorObject <span class="token operator">=</span> <span class="token function">generatorFunction</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
    169. 

  169.     generatorObject<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    170. 

  170.     <span class="token keyword">return</span> generatorObject<span class="token punctuation">;</span>
    171. 

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

  172. <span class="token punctuation">}</span>
    173. 

  173. 

  174. <span class="token keyword">const</span> wrapped <span class="token operator">=</span> <span class="token function">wrapper</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    175. 

  175.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">First input: </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span><span class="token keyword">yield</span><span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    176. 

  176.   <span class="token keyword">return</span> <span class="token string">'DONE'</span><span class="token punctuation">;</span>
    177. 

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

  178. 

  179. <span class="token function">wrapped</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'hello!'</span><span class="token punctuation">)</span>
    180. 

  180. <span class="token comment">// First input: hello!</span>
    181. 

  181. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，Generator 函数如果不用<code>wrapper</code>先包一层，是无法第一次调用<code>next</code>方法，就输入参数的。</p> <h2 id="for-of-循环"><a href="#for-of-循环" class="header-anchor">#</a> for...of 循环</h2> <p><code>for...of</code>循环可以自动遍历 Generator 函数运行时生成的<code>Iterator</code>对象，且此时不再需要调用<code>next</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    182. 

  182.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    183. 

  183.   <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    184. 

  184.   <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    185. 

  185.   <span class="token keyword">yield</span> <span class="token number">4</span><span class="token punctuation">;</span>
    186. 

  186.   <span class="token keyword">yield</span> <span class="token number">5</span><span class="token punctuation">;</span>
    187. 

  187.   <span class="token keyword">return</span> <span class="token number">6</span><span class="token punctuation">;</span>
    188. 

  188. <span class="token punctuation">}</span>
    189. 

  189. 

  190. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> v <span class="token keyword">of</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>
    191. 

  191.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    192. 

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

  193. <span class="token comment">// 1 2 3 4 5</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><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>上面代码使用<code>for...of</code>循环，依次显示 5 个<code>yield</code>表达式的值。这里需要注意，一旦<code>next</code>方法的返回对象的<code>done</code>属性为<code>true</code>，<code>for...of</code>循环就会中止，且不包含该返回对象，所以上面代码的<code>return</code>语句返回的<code>6</code>，不包括在<code>for...of</code>循环之中。</p> <p>下面是一个利用 Generator 函数和<code>for...of</code>循环，实现斐波那契数列的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">fibonacci</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    195. 

  195.   <span class="token keyword">let</span> <span class="token punctuation">[</span>prev<span class="token punctuation">,</span> curr<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    196. 

  196.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token punctuation">;</span><span class="token punctuation">;</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    197. 

  197.     <span class="token keyword">yield</span> curr<span class="token punctuation">;</span>
    198. 

  198.     <span class="token punctuation">[</span>prev<span class="token punctuation">,</span> curr<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span>curr<span class="token punctuation">,</span> prev <span class="token operator">+</span> curr<span class="token punctuation">]</span><span class="token punctuation">;</span>
    199. 

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

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

  201. 

  202. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> n <span class="token keyword">of</span> <span class="token function">fibonacci</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    203. 

  203.   <span class="token keyword">if</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;</span> <span class="token number">1000</span><span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    204. 

  204.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span>
    205. 

  205. <span class="token punctuation">}</span>
    206. 

  206. </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>从上面代码可见，使用<code>for...of</code>语句时不需要使用<code>next</code>方法。</p> <p>利用<code>for...of</code>循环，可以写出遍历任意对象（object）的方法。原生的 JavaScript 对象没有遍历接口，无法使用<code>for...of</code>循环，通过 Generator 函数为它加上这个接口，就可以用了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">objectEntries</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    207. 

  207.   <span class="token keyword">let</span> propKeys <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    208. 

  208. 

  209.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> propKey <span class="token keyword">of</span> propKeys<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    210. 

  210.     <span class="token keyword">yield</span> <span class="token punctuation">[</span>propKey<span class="token punctuation">,</span> obj<span class="token punctuation">[</span>propKey<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    211. 

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

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

  213. 

  214. <span class="token keyword">let</span> jane <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">first</span><span class="token operator">:</span> <span class="token string">'Jane'</span><span class="token punctuation">,</span> <span class="token literal-property property">last</span><span class="token operator">:</span> <span class="token string">'Doe'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    215. 

  215. 

  216. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> <span class="token function">objectEntries</span><span class="token punctuation">(</span>jane<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    217. 

  217.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>key<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">: </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>value<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    218. 

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

  219. <span class="token comment">// first: Jane</span>
    220. 

  220. <span class="token comment">// last: Doe</span>
    221. 

  221. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，对象<code>jane</code>原生不具备 Iterator 接口，无法用<code>for...of</code>遍历。这时，我们通过 Generator 函数<code>objectEntries</code>为它加上遍历器接口，就可以用<code>for...of</code>遍历了。加上遍历器接口的另一种写法是，将 Generator 函数加到对象的<code>Symbol.iterator</code>属性上面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">objectEntries</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    222. 

  222.   <span class="token keyword">let</span> propKeys <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    223. 

  223. 

  224.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> propKey <span class="token keyword">of</span> propKeys<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    225. 

  225.     <span class="token keyword">yield</span> <span class="token punctuation">[</span>propKey<span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">[</span>propKey<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    226. 

  226.   <span class="token punctuation">}</span>
    227. 

  227. <span class="token punctuation">}</span>
    228. 

  228. 

  229. <span class="token keyword">let</span> jane <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">first</span><span class="token operator">:</span> <span class="token string">'Jane'</span><span class="token punctuation">,</span> <span class="token literal-property property">last</span><span class="token operator">:</span> <span class="token string">'Doe'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    230. 

  230. 

  231. jane<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> objectEntries<span class="token punctuation">;</span>
    232. 

  232. 

  233. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> jane<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    234. 

  234.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>key<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">: </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>value<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    235. 

  235. <span class="token punctuation">}</span>
    236. 

  236. <span class="token comment">// first: Jane</span>
    237. 

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

  238. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>除了<code>for...of</code>循环以外，扩展运算符（<code>...</code>）、解构赋值和<code>Array.from</code>方法内部调用的，都是遍历器接口。这意味着，它们都可以将 Generator 函数返回的 Iterator 对象，作为参数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">numbers</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    239. 

  239.   <span class="token keyword">yield</span> <span class="token number">1</span>
    240. 

  240.   <span class="token keyword">yield</span> <span class="token number">2</span>
    241. 

  241.   <span class="token keyword">return</span> <span class="token number">3</span>
    242. 

  242.   <span class="token keyword">yield</span> <span class="token number">4</span>
    243. 

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

  244. 

  245. <span class="token comment">// 扩展运算符</span>
    246. 

  246. <span class="token punctuation">[</span><span class="token operator">...</span><span class="token function">numbers</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// [1, 2]</span>
    247. 

  247. 

  248. <span class="token comment">// Array.from 方法</span>
    249. 

  249. Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token function">numbers</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// [1, 2]</span>
    250. 

  250. 

  251. <span class="token comment">// 解构赋值</span>
    252. 

  252. <span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">numbers</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    253. 

  253. x <span class="token comment">// 1</span>
    254. 

  254. y <span class="token comment">// 2</span>
    255. 

  255. 

  256. <span class="token comment">// for...of 循环</span>
    257. 

  257. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> n <span class="token keyword">of</span> <span class="token function">numbers</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    258. 

  258.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span>
    259. 

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

  260. <span class="token comment">// 1</span>
    261. 

  261. <span class="token comment">// 2</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><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br></div></div><h2 id="generator-prototype-throw"><a href="#generator-prototype-throw" class="header-anchor">#</a> Generator.prototype.throw()</h2> <p>Generator 函数返回的遍历器对象，都有一个<code>throw</code>方法，可以在函数体外抛出错误，然后在 Generator 函数体内捕获。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">g</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    263. 

  263.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    264. 

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

  265.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    266. 

  266.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'内部捕获'</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    267. 

  267.   <span class="token punctuation">}</span>
    268. 

  268. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    269. 

  269. 

  270. <span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    271. 

  271. i<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    272. 

  272. 

  273. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    274. 

  274.   i<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    275. 

  275.   i<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    276. 

  276. <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    277. 

  277.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'外部捕获'</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    278. 

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

  279. <span class="token comment">// 内部捕获 a</span>
    280. 

  280. <span class="token comment">// 外部捕获 b</span>
    281. 

  281. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>上面代码中，遍历器对象<code>i</code>连续抛出两个错误。第一个错误被 Generator 函数体内的<code>catch</code>语句捕获。<code>i</code>第二次抛出错误，由于 Generator 函数内部的<code>catch</code>语句已经执行过了，不会再捕捉到这个错误了，所以这个错误就被抛出了 Generator 函数体，被函数体外的<code>catch</code>语句捕获。</p> <p><code>throw</code>方法可以接受一个参数，该参数会被<code>catch</code>语句接收，建议抛出<code>Error</code>对象的实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">g</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    282. 

  282.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    283. 

  283.     <span class="token keyword">yield</span><span class="token punctuation">;</span>
    284. 

  284.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    285. 

  285.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    286. 

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

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

  288. 

  289. <span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    290. 

  290. i<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    291. 

  291. i<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了！'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    292. 

  292. <span class="token comment">// Error: 出错了！(…)</span>
    293. 

  293. </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>注意，不要混淆遍历器对象的<code>throw</code>方法和全局的<code>throw</code>命令。上面代码的错误，是用遍历器对象的<code>throw</code>方法抛出的，而不是用<code>throw</code>命令抛出的。后者只能被函数体外的<code>catch</code>语句捕获。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">g</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    294. 

  294.   <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    295. 

  295.     <span class="token keyword">try</span> <span class="token punctuation">{</span>
    296. 

  296.       <span class="token keyword">yield</span><span class="token punctuation">;</span>
    297. 

  297.     <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    298. 

  298.       <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token operator">!=</span> <span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token keyword">throw</span> e<span class="token punctuation">;</span>
    299. 

  299.       console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'内部捕获'</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    300. 

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

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

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

  303. 

  304. <span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    305. 

  305. i<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    306. 

  306. 

  307. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    308. 

  308.   <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    309. 

  309.   <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    310. 

  310. <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    311. 

  311.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'外部捕获'</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    312. 

  312. <span class="token punctuation">}</span>
    313. 

  313. <span class="token comment">// 外部捕获 [Error: a]</span>
    314. 

  314. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>上面代码之所以只捕获了<code>a</code>，是因为函数体外的<code>catch</code>语句块，捕获了抛出的<code>a</code>错误以后，就不会再继续<code>try</code>代码块里面剩余的语句了。</p> <p>如果 Generator 函数内部没有部署<code>try...catch</code>代码块，那么<code>throw</code>方法抛出的错误，将被外部<code>try...catch</code>代码块捕获。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">g</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    315. 

  315.   <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    316. 

  316.     <span class="token keyword">yield</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><span class="token string">'内部捕获'</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    318. 

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

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

  320. 

  321. <span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    322. 

  322. i<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    323. 

  323. 

  324. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    325. 

  325.   i<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    326. 

  326.   i<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    327. 

  327. <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    328. 

  328.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'外部捕获'</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    329. 

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

  330. <span class="token comment">// 外部捕获 a</span>
    331. 

  331. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，Generator 函数<code>g</code>内部没有部署<code>try...catch</code>代码块，所以抛出的错误直接被外部<code>catch</code>代码块捕获。</p> <p>如果 Generator 函数内部和外部，都没有部署<code>try...catch</code>代码块，那么程序将报错，直接中断执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">gen</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    332. 

  332.   <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    333. 

  333.   <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    334. 

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

  335. 

  336. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    337. 

  337. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    338. 

  338. g<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    339. 

  339. <span class="token comment">// hello</span>
    340. 

  340. <span class="token comment">// Uncaught undefined</span>
    341. 

  341. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>g.throw</code>抛出错误以后，没有任何<code>try...catch</code>代码块可以捕获这个错误，导致程序报错，中断执行。</p> <p><code>throw</code>方法抛出的错误要被内部捕获，前提是必须至少执行过一次<code>next</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    342. 

  342.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    343. 

  343.     <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    344. 

  344.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    345. 

  345.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'内部捕获'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    346. 

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

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

  348. 

  349. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    350. 

  350. g<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    351. 

  351. <span class="token comment">// Uncaught 1</span>
    352. 

  352. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>g.throw(1)</code>执行时，<code>next</code>方法一次都没有执行过。这时，抛出的错误不会被内部捕获，而是直接在外部抛出，导致程序出错。这种行为其实很好理解，因为第一次执行<code>next</code>方法，等同于启动执行 Generator 函数的内部代码，否则 Generator 函数还没有开始执行，这时<code>throw</code>方法抛错只可能抛出在函数外部。</p> <p><code>throw</code>方法被捕获以后，会附带执行下一条<code>yield</code>表达式。也就是说，会附带执行一次<code>next</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">gen</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    353. 

  353.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    354. 

  354.     <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    355. 

  355.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    356. 

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

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

  358.   <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    359. 

  359.   <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'c'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    360. 

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

  361. 

  362. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    363. 

  363. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// a</span>
    364. 

  364. g<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// b</span>
    365. 

  365. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// c</span>
    366. 

  366. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><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>g.throw</code>方法被捕获以后，自动执行了一次<code>next</code>方法，所以会打印<code>b</code>。另外，也可以看到，只要 Generator 函数内部部署了<code>try...catch</code>代码块，那么遍历器的<code>throw</code>方法抛出的错误，不影响下一次遍历。</p> <p>另外，<code>throw</code>命令与<code>g.throw</code>方法是无关的，两者互不影响。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">gen</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    367. 

  367.   <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    368. 

  368.   <span class="token keyword">yield</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    369. 

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

  370. 

  371. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    372. 

  372. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    373. 

  373. 

  374. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    375. 

  375.   <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    376. 

  376. <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    377. 

  377.   g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    378. 

  378. <span class="token punctuation">}</span>
    379. 

  379. <span class="token comment">// hello</span>
    380. 

  380. <span class="token comment">// world</span>
    381. 

  381. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，<code>throw</code>命令抛出的错误不会影响到遍历器的状态，所以两次执行<code>next</code>方法，都进行了正确的操作。</p> <p>这种函数体内捕获错误的机制，大大方便了对错误的处理。多个<code>yield</code>表达式，可以只用一个<code>try...catch</code>代码块来捕获错误。如果使用回调函数的写法，想要捕获多个错误，就不得不为每个函数内部写一个错误处理语句，现在只在 Generator 函数内部写一次<code>catch</code>语句就可以了。</p> <p>Generator 函数体外抛出的错误，可以在函数体内捕获；反过来，Generator 函数体内抛出的错误，也可以被函数体外的<code>catch</code>捕获。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    382. 

  382.   <span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    383. 

  383.   <span class="token keyword">var</span> y <span class="token operator">=</span> x<span class="token punctuation">.</span><span class="token function">toUpperCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    384. 

  384.   <span class="token keyword">yield</span> y<span class="token punctuation">;</span>
    385. 

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

  386. 

  387. <span class="token keyword">var</span> it <span class="token operator">=</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    388. 

  388. 

  389. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value:3, done:false }</span>
    390. 

  390. 

  391. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    392. 

  392.   it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    393. 

  393. <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    394. 

  394.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">;</span>
    395. 

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

  396. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，第二个<code>next</code>方法向函数体内传入一个参数 42，数值是没有<code>toUpperCase</code>方法的，所以会抛出一个 TypeError 错误，被函数体外的<code>catch</code>捕获。</p> <p>一旦 Generator 执行过程中抛出错误，且没有被内部捕获，就不会再执行下去了。如果此后还调用<code>next</code>方法，将返回一个<code>value</code>属性等于<code>undefined</code>、<code>done</code>属性等于<code>true</code>的对象，即 JavaScript 引擎认为这个 Generator 已经运行结束了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    397. 

  397.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    398. 

  398.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'throwing an exception'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    399. 

  399.   <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'generator broke!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    400. 

  400.   <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    401. 

  401.   <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    402. 

  402. <span class="token punctuation">}</span>
    403. 

  403. 

  404. <span class="token keyword">function</span> <span class="token function">log</span><span class="token punctuation">(</span><span class="token parameter">generator</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    405. 

  405.   <span class="token keyword">var</span> v<span class="token punctuation">;</span>
    406. 

  406.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'starting generator'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    407. 

  407.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    408. 

  408.     v <span class="token operator">=</span> generator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    409. 

  409.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'第一次运行next方法'</span><span class="token punctuation">,</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    410. 

  410.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    411. 

  411.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'捕捉错误'</span><span class="token punctuation">,</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    412. 

  412.   <span class="token punctuation">}</span>
    413. 

  413.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    414. 

  414.     v <span class="token operator">=</span> generator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    415. 

  415.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'第二次运行next方法'</span><span class="token punctuation">,</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    416. 

  416.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    417. 

  417.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'捕捉错误'</span><span class="token punctuation">,</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    418. 

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

  419.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    420. 

  420.     v <span class="token operator">=</span> generator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    421. 

  421.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'第三次运行next方法'</span><span class="token punctuation">,</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    422. 

  422.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    423. 

  423.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'捕捉错误'</span><span class="token punctuation">,</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    424. 

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

  425.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'caller done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    426. 

  426. <span class="token punctuation">}</span>
    427. 

  427. 

  428. <span class="token function">log</span><span class="token punctuation">(</span><span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    429. 

  429. <span class="token comment">// starting generator</span>
    430. 

  430. <span class="token comment">// 第一次运行next方法 { value: 1, done: false }</span>
    431. 

  431. <span class="token comment">// throwing an exception</span>
    432. 

  432. <span class="token comment">// 捕捉错误 { value: 1, done: false }</span>
    433. 

  433. <span class="token comment">// 第三次运行next方法 { value: undefined, done: true }</span>
    434. 

  434. <span class="token comment">// caller done</span>
    435. 

  435. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br></div></div><p>上面代码一共三次运行<code>next</code>方法，第二次运行的时候会抛出错误，然后第三次运行的时候，Generator 函数就已经结束了，不再执行下去了。</p> <h2 id="generator-prototype-return"><a href="#generator-prototype-return" class="header-anchor">#</a> Generator.prototype.return()</h2> <p>Generator 函数返回的遍历器对象，还有一个**<code>return</code>方法，可以返回给定的值，并且终结遍历 Generator 函数。**</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    436. 

  436.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    437. 

  437.   <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    438. 

  438.   <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    439. 

  439. <span class="token punctuation">}</span>
    440. 

  440. 

  441. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    442. 

  442. 

  443. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>        <span class="token comment">// { value: 1, done: false }</span>
    444. 

  444. g<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// { value: &quot;foo&quot;, done: true }</span>
    445. 

  445. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>        <span class="token comment">// { value: undefined, done: true }</span>
    446. 

  446. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><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，遍历器对象<code>g</code>调用<code>return</code>方法后，返回值的<code>value</code>属性就是<code>return</code>方法的参数<code>foo</code>。并且，Generator 函数的遍历就终止了，返回值的<code>done</code>属性为<code>true</code>，以后再调用<code>next</code>方法，<code>done</code>属性总是返回<code>true</code>。</p> <p>如果<code>return</code>方法调用时，不提供参数，则返回值的<code>value</code>属性为<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    447. 

  447.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    448. 

  448.   <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    449. 

  449.   <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    450. 

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

  451. 

  452. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    453. 

  453. 

  454. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>        <span class="token comment">// { value: 1, done: false }</span>
    455. 

  455. g<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: undefined, done: true }</span>
    456. 

  456. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>如果 Generator 函数内部有<code>try...finally</code>代码块，且正在执行<code>try</code>代码块，那么<code>return</code>方法会导致立刻进入<code>finally</code>代码块，执行完以后，整个函数才会结束。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">numbers</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    457. 

  457.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    458. 

  458.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    459. 

  459.     <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    460. 

  460.     <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    461. 

  461.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    462. 

  462.     <span class="token keyword">yield</span> <span class="token number">4</span><span class="token punctuation">;</span>
    463. 

  463.     <span class="token keyword">yield</span> <span class="token number">5</span><span class="token punctuation">;</span>
    464. 

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

  465.   <span class="token keyword">yield</span> <span class="token number">6</span><span class="token punctuation">;</span>
    466. 

  466. <span class="token punctuation">}</span>
    467. 

  467. <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token function">numbers</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    468. 

  468. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 1, done: false }</span>
    469. 

  469. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 2, done: false }</span>
    470. 

  470. g<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">)</span> <span class="token comment">// { value: 4, done: false }</span>
    471. 

  471. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 5, done: false }</span>
    472. 

  472. g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 7, done: true }</span>
    473. 

  473. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，调用<code>return()</code>方法后，就开始执行<code>finally</code>代码块，不执行<code>try</code>里面剩下的代码了，然后等到<code>finally</code>代码块执行完，再返回<code>return()</code>方法指定的返回值。</p> <h2 id="next-、throw-、return-的共同点"><a href="#next-、throw-、return-的共同点" class="header-anchor">#</a> next()、throw()、return() 的共同点</h2> <p><code>next()</code>、<code>throw()</code>、<code>return()</code>这三个方法本质上是同一件事，可以放在一起理解。它们的作用都是让 Generator 函数恢复执行，并且使用不同的语句替换<code>yield</code>表达式。</p> <p><code>next()</code>是将<code>yield</code>表达式替换成一个值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token function-variable function">g</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    474. 

  474.   <span class="token keyword">let</span> result <span class="token operator">=</span> <span class="token keyword">yield</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
    475. 

  475.   <span class="token keyword">return</span> result<span class="token punctuation">;</span>
    476. 

  476. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    477. 

  477. 

  478. <span class="token keyword">const</span> gen <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    479. 

  479. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Object {value: 3, done: false}</span>
    480. 

  480. 

  481. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Object {value: 1, done: true}</span>
    482. 

  482. <span class="token comment">// 相当于将 let result = yield x + y</span>
    483. 

  483. <span class="token comment">// 替换成 let result = 1;</span>
    484. 

  484. </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，第二个<code>next(1)</code>方法就相当于将<code>yield</code>表达式替换成一个值<code>1</code>。如果<code>next</code>方法没有参数，就相当于替换成<code>undefined</code>。</p> <p><code>throw()</code>是将<code>yield</code>表达式替换成一个<code>throw</code>语句。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>gen<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Uncaught Error: 出错了</span>
    485. 

  485. <span class="token comment">// 相当于将 let result = yield x + y</span>
    486. 

  486. <span class="token comment">// 替换成 let result = throw(new Error('出错了'));</span>
    487. 

  487. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>return()</code>是将<code>yield</code>表达式替换成一个<code>return</code>语句。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>gen<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Object {value: 2, done: true}</span>
    488. 

  488. <span class="token comment">// 相当于将 let result = yield x + y</span>
    489. 

  489. <span class="token comment">// 替换成 let result = return 2;</span>
    490. 

  490. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="yield-表达式-2"><a href="#yield-表达式-2" class="header-anchor">#</a> yield* 表达式</h2> <p>如果在 Generator 函数内部，调用另一个 Generator 函数。需要在前者的函数体内部，自己手动完成遍历。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    491. 

  491.   <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
    492. 

  492.   <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    493. 

  493. <span class="token punctuation">}</span>
    494. 

  494. 

  495. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    496. 

  496.   <span class="token keyword">yield</span> <span class="token string">'x'</span><span class="token punctuation">;</span>
    497. 

  497.   <span class="token comment">// 手动遍历 foo()</span>
    498. 

  498.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token keyword">of</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>
    499. 

  499.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    500. 

  500.   <span class="token punctuation">}</span>
    501. 

  501.   <span class="token keyword">yield</span> <span class="token string">'y'</span><span class="token punctuation">;</span>
    502. 

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

  503. 

  504. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> v <span class="token keyword">of</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>
    505. 

  505.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    506. 

  506. <span class="token punctuation">}</span>
    507. 

  507. <span class="token comment">// x</span>
    508. 

  508. <span class="token comment">// a</span>
    509. 

  509. <span class="token comment">// b</span>
    510. 

  510. <span class="token comment">// y</span>
    511. 

  511. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>上面代码中，<code>foo</code>和<code>bar</code>都是 Generator 函数，在<code>bar</code>里面调用<code>foo</code>，就需要手动遍历<code>foo</code>。如果有多个 Generator 函数嵌套，写起来就非常麻烦。</p> <p>ES6 提供了<code>yield*</code>表达式，作为解决办法，用来在一个 Generator 函数里面执行另一个 Generator 函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    512. 

  512.   <span class="token keyword">yield</span> <span class="token string">'x'</span><span class="token punctuation">;</span>
    513. 

  513.   <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    514. 

  514.   <span class="token keyword">yield</span> <span class="token string">'y'</span><span class="token punctuation">;</span>
    515. 

  515. <span class="token punctuation">}</span>
    516. 

  516. 

  517. <span class="token comment">// 等同于</span>
    518. 

  518. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    519. 

  519.   <span class="token keyword">yield</span> <span class="token string">'x'</span><span class="token punctuation">;</span>
    520. 

  520.   <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
    521. 

  521.   <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    522. 

  522.   <span class="token keyword">yield</span> <span class="token string">'y'</span><span class="token punctuation">;</span>
    523. 

  523. <span class="token punctuation">}</span>
    524. 

  524. 

  525. <span class="token comment">// 等同于</span>
    526. 

  526. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    527. 

  527.   <span class="token keyword">yield</span> <span class="token string">'x'</span><span class="token punctuation">;</span>
    528. 

  528.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> v <span class="token keyword">of</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>
    529. 

  529.     <span class="token keyword">yield</span> v<span class="token punctuation">;</span>
    530. 

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

  531.   <span class="token keyword">yield</span> <span class="token string">'y'</span><span class="token punctuation">;</span>
    532. 

  532. <span class="token punctuation">}</span>
    533. 

  533. 

  534. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> v <span class="token keyword">of</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>
    535. 

  535.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    536. 

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

  537. <span class="token comment">// &quot;x&quot;</span>
    538. 

  538. <span class="token comment">// &quot;a&quot;</span>
    539. 

  539. <span class="token comment">// &quot;b&quot;</span>
    540. 

  540. <span class="token comment">// &quot;y&quot;</span>
    541. 

  541. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br></div></div><p>再来看一个对比的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">inner</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    542. 

  542.   <span class="token keyword">yield</span> <span class="token string">'hello!'</span><span class="token punctuation">;</span>
    543. 

  543. <span class="token punctuation">}</span>
    544. 

  544. 

  545. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">outer1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    546. 

  546.   <span class="token keyword">yield</span> <span class="token string">'open'</span><span class="token punctuation">;</span>
    547. 

  547.   <span class="token keyword">yield</span> <span class="token function">inner</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    548. 

  548.   <span class="token keyword">yield</span> <span class="token string">'close'</span><span class="token punctuation">;</span>
    549. 

  549. <span class="token punctuation">}</span>
    550. 

  550. 

  551. <span class="token keyword">var</span> gen <span class="token operator">=</span> <span class="token function">outer1</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    552. 

  552. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;open&quot;</span>
    553. 

  553. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 返回一个遍历器对象</span>
    554. 

  554. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;close&quot;</span>
    555. 

  555. 

  556. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">outer2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    557. 

  557.   <span class="token keyword">yield</span> <span class="token string">'open'</span>
    558. 

  558.   <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">inner</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    559. 

  559.   <span class="token keyword">yield</span> <span class="token string">'close'</span>
    560. 

  560. <span class="token punctuation">}</span>
    561. 

  561. 

  562. <span class="token keyword">var</span> gen <span class="token operator">=</span> <span class="token function">outer2</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    563. 

  563. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;open&quot;</span>
    564. 

  564. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;hello!&quot;</span>
    565. 

  565. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;close&quot;</span>
    566. 

  566. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br></div></div><p>上面例子中，<code>outer2</code>使用了<code>yield*</code>，<code>outer1</code>没使用。结果就是，<code>outer1</code>返回一个遍历器对象，<code>outer2</code>返回该遍历器对象的内部值。</p> <p>从语法角度看，如果<code>yield</code>表达式后面跟的是一个遍历器对象，需要在<code>yield</code>表达式后面加上星号，表明它返回的是一个遍历器对象。这被称为<code>yield*</code>表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> delegatedIterator <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    567. 

  567.   <span class="token keyword">yield</span> <span class="token string">'Hello!'</span><span class="token punctuation">;</span>
    568. 

  568.   <span class="token keyword">yield</span> <span class="token string">'Bye!'</span><span class="token punctuation">;</span>
    569. 

  569. <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>
    570. 

  570. 

  571. <span class="token keyword">let</span> delegatingIterator <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    572. 

  572.   <span class="token keyword">yield</span> <span class="token string">'Greetings!'</span><span class="token punctuation">;</span>
    573. 

  573.   <span class="token keyword">yield</span><span class="token operator">*</span> delegatedIterator<span class="token punctuation">;</span>
    574. 

  574.   <span class="token keyword">yield</span> <span class="token string">'Ok, bye.'</span><span class="token punctuation">;</span>
    575. 

  575. <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>
    576. 

  576. 

  577. <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> value <span class="token keyword">of</span> delegatingIterator<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    578. 

  578.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    579. 

  579. <span class="token punctuation">}</span>
    580. 

  580. <span class="token comment">// &quot;Greetings!</span>
    581. 

  581. <span class="token comment">// &quot;Hello!&quot;</span>
    582. 

  582. <span class="token comment">// &quot;Bye!&quot;</span>
    583. 

  583. <span class="token comment">// &quot;Ok, bye.&quot;</span>
    584. 

  584. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>上面代码中，<code>delegatingIterator</code>是代理者，<code>delegatedIterator</code>是被代理者。由于<code>yield* delegatedIterator</code>语句得到的值，是一个遍历器，所以要用星号表示。运行结果就是使用一个遍历器，遍历了多个 Generator 函数，有递归的效果。</p> <p><code>yield*</code>后面的 Generator 函数（没有<code>return</code>语句时），等同于在 Generator 函数内部，部署一个<code>for...of</code>循环。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">concat</span><span class="token punctuation">(</span><span class="token parameter">iter1<span class="token punctuation">,</span> iter2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    585. 

  585.   <span class="token keyword">yield</span><span class="token operator">*</span> iter1<span class="token punctuation">;</span>
    586. 

  586.   <span class="token keyword">yield</span><span class="token operator">*</span> iter2<span class="token punctuation">;</span>
    587. 

  587. <span class="token punctuation">}</span>
    588. 

  588. 

  589. <span class="token comment">// 等同于</span>
    590. 

  590. 

  591. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">concat</span><span class="token punctuation">(</span><span class="token parameter">iter1<span class="token punctuation">,</span> iter2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    592. 

  592.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> value <span class="token keyword">of</span> iter1<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    593. 

  593.     <span class="token keyword">yield</span> value<span class="token punctuation">;</span>
    594. 

  594.   <span class="token punctuation">}</span>
    595. 

  595.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> value <span class="token keyword">of</span> iter2<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    596. 

  596.     <span class="token keyword">yield</span> value<span class="token punctuation">;</span>
    597. 

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

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

  599. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码说明，<code>yield*</code>后面的 Generator 函数（没有<code>return</code>语句时），不过是<code>for...of</code>的一种简写形式，完全可以用后者替代前者。反之，在有<code>return</code>语句时，则需要用<code>var value = yield* iterator</code>的形式获取<code>return</code>语句的值。</p> <p>如果<code>yield*</code>后面跟着一个数组，由于数组原生支持遍历器，因此就会遍历数组成员。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    600. 

  600.   <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token punctuation">[</span><span class="token string">&quot;a&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;c&quot;</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    601. 

  601. <span class="token punctuation">}</span>
    602. 

  602. 

  603. <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value:&quot;a&quot;, done:false }</span>
    604. 

  604. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>yield</code>命令后面如果不加星号，返回的是整个数组，加了星号就表示返回的是数组的遍历器对象。</p> <p>实际上，任何数据结构只要有 Iterator 接口，就可以被<code>yield*</code>遍历。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> read <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    605. 

  605.   <span class="token keyword">yield</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    606. 

  606.   <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    607. 

  607. <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>
    608. 

  608. 

  609. read<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;hello&quot;</span>
    610. 

  610. read<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// &quot;h&quot;</span>
    611. 

  611. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>yield</code>表达式返回整个字符串，<code>yield*</code>语句返回单个字符。因为字符串具有 Iterator 接口，所以被<code>yield*</code>遍历。</p> <p>如果被代理的 Generator 函数有<code>return</code>语句，那么就可以向代理它的 Generator 函数返回数据。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    612. 

  612.   <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    613. 

  613.   <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
    614. 

  614.   <span class="token keyword">return</span> <span class="token string">&quot;foo&quot;</span><span class="token punctuation">;</span>
    615. 

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

  616. 

  617. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    618. 

  618.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    619. 

  619.   <span class="token keyword">var</span> v <span class="token operator">=</span> <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    620. 

  620.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">&quot;v: &quot;</span> <span class="token operator">+</span> v<span class="token punctuation">)</span><span class="token punctuation">;</span>
    621. 

  621.   <span class="token keyword">yield</span> <span class="token number">4</span><span class="token punctuation">;</span>
    622. 

  622. <span class="token punctuation">}</span>
    623. 

  623. 

  624. <span class="token keyword">var</span> it <span class="token operator">=</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    625. 

  625. 

  626. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    627. 

  627. <span class="token comment">// {value: 1, done: false}</span>
    628. 

  628. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    629. 

  629. <span class="token comment">// {value: 2, done: false}</span>
    630. 

  630. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    631. 

  631. <span class="token comment">// {value: 3, done: false}</span>
    632. 

  632. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    633. 

  633. <span class="token comment">// &quot;v: foo&quot;</span>
    634. 

  634. <span class="token comment">// {value: 4, done: false}</span>
    635. 

  635. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    636. 

  636. <span class="token comment">// {value: undefined, done: true}</span>
    637. 

  637. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br></div></div><p>上面代码在第四次调用<code>next</code>方法的时候，屏幕上会有输出，这是因为函数<code>foo</code>的<code>return</code>语句，向函数<code>bar</code>提供了返回值。</p> <p>再看一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">genFuncWithReturn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    638. 

  638.   <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
    639. 

  639.   <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    640. 

  640.   <span class="token keyword">return</span> <span class="token string">'The result'</span><span class="token punctuation">;</span>
    641. 

  641. <span class="token punctuation">}</span>
    642. 

  642. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">logReturned</span><span class="token punctuation">(</span><span class="token parameter">genObj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    643. 

  643.   <span class="token keyword">let</span> result <span class="token operator">=</span> <span class="token keyword">yield</span><span class="token operator">*</span> genObj<span class="token punctuation">;</span>
    644. 

  644.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    645. 

  645. <span class="token punctuation">}</span>
    646. 

  646. 

  647. <span class="token punctuation">[</span><span class="token operator">...</span><span class="token function">logReturned</span><span class="token punctuation">(</span><span class="token function">genFuncWithReturn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">]</span>
    648. 

  648. <span class="token comment">// The result</span>
    649. 

  649. <span class="token comment">// 值为 [ 'a', 'b' ]</span>
    650. 

  650. </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，存在两次遍历。第一次是扩展运算符遍历函数<code>logReturned</code>返回的遍历器对象，第二次是<code>yield*</code>语句遍历函数<code>genFuncWithReturn</code>返回的遍历器对象。这两次遍历的效果是叠加的，最终表现为扩展运算符遍历函数<code>genFuncWithReturn</code>返回的遍历器对象。所以，最后的数据表达式得到的值等于<code>[ 'a', 'b' ]</code>。但是，函数<code>genFuncWithReturn</code>的<code>return</code>语句的返回值<code>The result</code>，会返回给函数<code>logReturned</code>内部的<code>result</code>变量，因此会有终端输出。</p> <p><code>yield*</code>命令可以很方便地取出嵌套数组的所有成员。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">iterTree</span><span class="token punctuation">(</span><span class="token parameter">tree</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    651. 

  651.   <span class="token keyword">if</span> <span class="token punctuation">(</span>Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>tree<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    652. 

  652.     <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> tree<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    653. 

  653.       <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">iterTree</span><span class="token punctuation">(</span>tree<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    654. 

  654.     <span class="token punctuation">}</span>
    655. 

  655.   <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    656. 

  656.     <span class="token keyword">yield</span> tree<span class="token punctuation">;</span>
    657. 

  657.   <span class="token punctuation">}</span>
    658. 

  658. <span class="token punctuation">}</span>
    659. 

  659. 

  660. <span class="token keyword">const</span> tree <span class="token operator">=</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 string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token string">'d'</span><span class="token punctuation">,</span> <span class="token string">'e'</span><span class="token punctuation">]</span> <span class="token punctuation">]</span><span class="token punctuation">;</span>
    661. 

  661. 

  662. <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> <span class="token function">iterTree</span><span class="token punctuation">(</span>tree<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    663. 

  663.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    664. 

  664. <span class="token punctuation">}</span>
    665. 

  665. <span class="token comment">// a</span>
    666. 

  666. <span class="token comment">// b</span>
    667. 

  667. <span class="token comment">// c</span>
    668. 

  668. <span class="token comment">// d</span>
    669. 

  669. <span class="token comment">// e</span>
    670. 

  670. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>由于扩展运算符<code>...</code>默认调用 Iterator 接口，所以上面这个函数也可以用于嵌套数组的平铺。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token operator">...</span><span class="token function">iterTree</span><span class="token punctuation">(</span>tree<span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;, &quot;d&quot;, &quot;e&quot;]</span>
    671. 

  671. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>下面是一个稍微复杂的例子，使用<code>yield*</code>语句遍历完全二叉树。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 下面是二叉树的构造函数，</span>
    672. 

  672. <span class="token comment">// 三个参数分别是左树、当前节点和右树</span>
    673. 

  673. <span class="token keyword">function</span> <span class="token function">Tree</span><span class="token punctuation">(</span><span class="token parameter">left<span class="token punctuation">,</span> label<span class="token punctuation">,</span> right</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    674. 

  674.   <span class="token keyword">this</span><span class="token punctuation">.</span>left <span class="token operator">=</span> left<span class="token punctuation">;</span>
    675. 

  675.   <span class="token keyword">this</span><span class="token punctuation">.</span>label <span class="token operator">=</span> label<span class="token punctuation">;</span>
    676. 

  676.   <span class="token keyword">this</span><span class="token punctuation">.</span>right <span class="token operator">=</span> right<span class="token punctuation">;</span>
    677. 

  677. <span class="token punctuation">}</span>
    678. 

  678. 

  679. <span class="token comment">// 下面是中序（inorder）遍历函数。</span>
    680. 

  680. <span class="token comment">// 由于返回的是一个遍历器，所以要用generator函数。</span>
    681. 

  681. <span class="token comment">// 函数体内采用递归算法，所以左树和右树要用yield*遍历</span>
    682. 

  682. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">inorder</span><span class="token punctuation">(</span><span class="token parameter">t</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    683. 

  683.   <span class="token keyword">if</span> <span class="token punctuation">(</span>t<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    684. 

  684.     <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">inorder</span><span class="token punctuation">(</span>t<span class="token punctuation">.</span>left<span class="token punctuation">)</span><span class="token punctuation">;</span>
    685. 

  685.     <span class="token keyword">yield</span> t<span class="token punctuation">.</span>label<span class="token punctuation">;</span>
    686. 

  686.     <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">inorder</span><span class="token punctuation">(</span>t<span class="token punctuation">.</span>right<span class="token punctuation">)</span><span class="token punctuation">;</span>
    687. 

  687.   <span class="token punctuation">}</span>
    688. 

  688. <span class="token punctuation">}</span>
    689. 

  689. 

  690. <span class="token comment">// 下面生成二叉树</span>
    691. 

  691. <span class="token keyword">function</span> <span class="token function">make</span><span class="token punctuation">(</span><span class="token parameter">array</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    692. 

  692.   <span class="token comment">// 判断是否为叶节点</span>
    693. 

  693.   <span class="token keyword">if</span> <span class="token punctuation">(</span>array<span class="token punctuation">.</span>length <span class="token operator">==</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Tree</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> array<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    694. 

  694.   <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Tree</span><span class="token punctuation">(</span><span class="token function">make</span><span class="token punctuation">(</span>array<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">,</span> array<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token function">make</span><span class="token punctuation">(</span>array<span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    695. 

  695. <span class="token punctuation">}</span>
    696. 

  696. <span class="token keyword">let</span> tree <span class="token operator">=</span> <span class="token function">make</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">'d'</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token string">'e'</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">'f'</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token string">'g'</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    697. 

  697. 

  698. <span class="token comment">// 遍历二叉树</span>
    699. 

  699. <span class="token keyword">var</span> result <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    700. 

  700. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> node <span class="token keyword">of</span> <span class="token function">inorder</span><span class="token punctuation">(</span>tree<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    701. 

  701.   result<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>node<span class="token punctuation">)</span><span class="token punctuation">;</span>
    702. 

  702. <span class="token punctuation">}</span>
    703. 

  703. 

  704. result
    705. 

  705. <span class="token comment">// ['a', 'b', 'c', 'd', 'e', 'f', 'g']</span>
    706. 

  706. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br></div></div><h2 id="作为对象属性的-generator-函数"><a href="#作为对象属性的-generator-函数" class="header-anchor">#</a> 作为对象属性的 Generator 函数</h2> <p>如果一个对象的属性是 Generator 函数，可以简写成下面的形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    707. 

  707.   <span class="token operator">*</span> <span class="token function">myGeneratorMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    708. 

  708.     ···
    709. 

  709.   <span class="token punctuation">}</span>
    710. 

  710. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    711. 

  711. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>myGeneratorMethod</code>属性前面有一个星号，表示这个属性是一个 Generator 函数。</p> <p>它的完整形式如下，与上面的写法是等价的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    712. 

  712.   <span class="token function-variable function">myGeneratorMethod</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    713. 

  713.     <span class="token comment">// ···</span>
    714. 

  714.   <span class="token punctuation">}</span>
    715. 

  715. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    716. 

  716. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="generator-函数的this"><a href="#generator-函数的this" class="header-anchor">#</a> Generator 函数的<code>this</code></h2> <p>Generator 函数总是返回一个遍历器，ES6 规定这个遍历器是 Generator 函数的实例，也继承了 Generator 函数的<code>prototype</code>对象上的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    717. 

  717. 

  718. g<span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">hello</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>
    719. 

  719.   <span class="token keyword">return</span> <span class="token string">'hi!'</span><span class="token punctuation">;</span>
    720. 

  720. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    721. 

  721. 

  722. <span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    723. 

  723. 

  724. obj <span class="token keyword">instanceof</span> <span class="token class-name">g</span> <span class="token comment">// true</span>
    725. 

  725. obj<span class="token punctuation">.</span><span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 'hi!'</span>
    726. 

  726. </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>上面代码表明，Generator 函数<code>g</code>返回的遍历器<code>obj</code>，是<code>g</code>的实例，而且继承了<code>g.prototype</code>。但是，如果把<code>g</code>当作普通的构造函数，并不会生效，因为<code>g</code>返回的总是遍历器对象，而不是<code>this</code>对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    727. 

  727.   <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">11</span><span class="token punctuation">;</span>
    728. 

  728. <span class="token punctuation">}</span>
    729. 

  729. 

  730. <span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    731. 

  731. obj<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    732. 

  732. obj<span class="token punctuation">.</span>a <span class="token comment">// undefined</span>
    733. 

  733. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，Generator 函数<code>g</code>在<code>this</code>对象上面添加了一个属性<code>a</code>，但是<code>obj</code>对象拿不到这个属性。</p> <p>Generator 函数也不能跟<code>new</code>命令一起用，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token constant">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    734. 

  734.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    735. 

  735.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    736. 

  736. <span class="token punctuation">}</span>
    737. 

  737. 

  738. <span class="token keyword">new</span> <span class="token class-name">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    739. 

  739. <span class="token comment">// TypeError: F is not a constructor</span>
    740. 

  740. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>new</code>命令跟构造函数<code>F</code>一起使用，结果报错，因为<code>F</code>不是构造函数。</p> <p>那么，有没有办法让 Generator 函数返回一个正常的对象实例，既可以用<code>next</code>方法，又可以获得正常的<code>this</code>？</p> <p>下面是一个变通方法。首先，生成一个空对象，使用<code>call</code>方法绑定 Generator 函数内部的<code>this</code>。这样，构造函数调用以后，这个空对象就是 Generator 函数的实例对象了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token constant">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    741. 

  741.   <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    742. 

  742.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    743. 

  743.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>c <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    744. 

  744. <span class="token punctuation">}</span>
    745. 

  745. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    746. 

  746. <span class="token keyword">var</span> f <span class="token operator">=</span> <span class="token constant">F</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    747. 

  747. 

  748. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: 2, done: false}</span>
    749. 

  749. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: 3, done: false}</span>
    750. 

  750. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: undefined, done: true}</span>
    751. 

  751. 

  752. obj<span class="token punctuation">.</span>a <span class="token comment">// 1</span>
    753. 

  753. obj<span class="token punctuation">.</span>b <span class="token comment">// 2</span>
    754. 

  754. obj<span class="token punctuation">.</span>c <span class="token comment">// 3</span>
    755. 

  755. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，首先是<code>F</code>内部的<code>this</code>对象绑定<code>obj</code>对象，然后调用它，返回一个 Iterator 对象。这个对象执行三次<code>next</code>方法（因为<code>F</code>内部有两个<code>yield</code>表达式），完成 F 内部所有代码的运行。这时，所有内部属性都绑定在<code>obj</code>对象上了，因此<code>obj</code>对象也就成了<code>F</code>的实例。</p> <p>上面代码中，执行的是遍历器对象<code>f</code>，但是生成的对象实例是<code>obj</code>，有没有办法将这两个对象统一呢？</p> <p>一个办法就是将<code>obj</code>换成<code>F.prototype</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token constant">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    756. 

  756.   <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    757. 

  757.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    758. 

  758.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>c <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    759. 

  759. <span class="token punctuation">}</span>
    760. 

  760. <span class="token keyword">var</span> f <span class="token operator">=</span> <span class="token constant">F</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token class-name">F</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
    761. 

  761. 

  762. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: 2, done: false}</span>
    763. 

  763. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: 3, done: false}</span>
    764. 

  764. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: undefined, done: true}</span>
    765. 

  765. 

  766. f<span class="token punctuation">.</span>a <span class="token comment">// 1</span>
    767. 

  767. f<span class="token punctuation">.</span>b <span class="token comment">// 2</span>
    768. 

  768. f<span class="token punctuation">.</span>c <span class="token comment">// 3</span>
    769. 

  769. </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>再将<code>F</code>改成构造函数，就可以对它执行<code>new</code>命令了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    770. 

  770.   <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    771. 

  771.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    772. 

  772.   <span class="token keyword">yield</span> <span class="token keyword">this</span><span class="token punctuation">.</span>c <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    773. 

  773. <span class="token punctuation">}</span>
    774. 

  774. 

  775. <span class="token keyword">function</span> <span class="token constant">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    776. 

  776.   <span class="token keyword">return</span> <span class="token function">gen</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>gen<span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
    777. 

  777. <span class="token punctuation">}</span>
    778. 

  778. 

  779. <span class="token keyword">var</span> f <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    780. 

  780. 

  781. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: 2, done: false}</span>
    782. 

  782. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: 3, done: false}</span>
    783. 

  783. f<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// Object {value: undefined, done: true}</span>
    784. 

  784. 

  785. f<span class="token punctuation">.</span>a <span class="token comment">// 1</span>
    786. 

  786. f<span class="token punctuation">.</span>b <span class="token comment">// 2</span>
    787. 

  787. f<span class="token punctuation">.</span>c <span class="token comment">// 3</span>
    788. 

  788. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><h2 id="含义"><a href="#含义" class="header-anchor">#</a> 含义</h2> <h3 id="generator-与状态机"><a href="#generator-与状态机" class="header-anchor">#</a> Generator 与状态机</h3> <p>Generator 是实现状态机的最佳结构。比如，下面的<code>clock</code>函数就是一个状态机。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> ticking <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    789. 

  789. <span class="token keyword">var</span> <span class="token function-variable function">clock</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>
    790. 

  790.   <span class="token keyword">if</span> <span class="token punctuation">(</span>ticking<span class="token punctuation">)</span>
    791. 

  791.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Tick!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    792. 

  792.   <span class="token keyword">else</span>
    793. 

  793.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Tock!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    794. 

  794.   ticking <span class="token operator">=</span> <span class="token operator">!</span>ticking<span class="token punctuation">;</span>
    795. 

  795. <span class="token punctuation">}</span>
    796. 

  796. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码的<code>clock</code>函数一共有两种状态（<code>Tick</code>和<code>Tock</code>），每运行一次，就改变一次状态。这个函数如果用 Generator 实现，就是下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token function-variable function">clock</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    797. 

  797.   <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    798. 

  798.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Tick!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    799. 

  799.     <span class="token keyword">yield</span><span class="token punctuation">;</span>
    800. 

  800.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Tock!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    801. 

  801.     <span class="token keyword">yield</span><span class="token punctuation">;</span>
    802. 

  802.   <span class="token punctuation">}</span>
    803. 

  803. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    804. 

  804. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面的 Generator 实现与 ES5 实现对比，可以看到少了用来保存状态的外部变量<code>ticking</code>，这样就更简洁，更安全（状态不会被非法篡改）、更符合函数式编程的思想，在写法上也更优雅。Generator 之所以可以不用外部变量保存状态，是因为它本身就包含了一个状态信息，即目前是否处于暂停态。</p> <h3 id="generator-与协程"><a href="#generator-与协程" class="header-anchor">#</a> Generator 与协程</h3> <p>协程（coroutine）是一种程序运行的方式，可以理解成“协作的线程”或“协作的函数”。协程既可以用单线程实现，也可以用多线程实现。前者是一种特殊的子例程，后者是一种特殊的线程。</p> <p><strong>（1）协程与子例程的差异</strong></p> <p>传统的“子例程”（subroutine）采用堆栈式“后进先出”的执行方式，只有当调用的子函数完全执行完毕，才会结束执行父函数。协程与其不同，多个线程（单线程情况下，即多个函数）可以并行执行，但是只有一个线程（或函数）处于正在运行的状态，其他线程（或函数）都处于暂停态（suspended），线程（或函数）之间可以交换执行权。也就是说，一个线程（或函数）执行到一半，可以暂停执行，将执行权交给另一个线程（或函数），等到稍后收回执行权的时候，再恢复执行。这种可以并行执行、交换执行权的线程（或函数），就称为协程。</p> <p>从实现上看，在内存中，子例程只使用一个栈（stack），而协程是同时存在多个栈，但只有一个栈是在运行状态，也就是说，协程是以多占用内存为代价，实现多任务的并行。</p> <p><strong>（2）协程与普通线程的差异</strong></p> <p>不难看出，协程适合用于多任务运行的环境。在这个意义上，它与普通的线程很相似，都有自己的执行上下文、可以分享全局变量。它们的不同之处在于，同一时间可以有多个线程处于运行状态，但是运行的协程只能有一个，其他协程都处于暂停状态。此外，普通的线程是抢先式的，到底哪个线程优先得到资源，必须由运行环境决定，但是协程是合作式的，执行权由协程自己分配。</p> <p>由于 JavaScript 是单线程语言，只能保持一个调用栈。引入协程以后，每个任务可以保持自己的调用栈。这样做的最大好处，就是抛出错误的时候，可以找到原始的调用栈。不至于像异步操作的回调函数那样，一旦出错，原始的调用栈早就结束。</p> <p>Generator 函数是 ES6 对协程的实现，但属于不完全实现。Generator 函数被称为“半协程”（semi-coroutine），意思是只有 Generator 函数的调用者，才能将程序的执行权还给 Generator 函数。如果是完全执行的协程，任何函数都可以让暂停的协程继续执行。</p> <p>如果将 Generator 函数当作协程，完全可以将多个需要互相协作的任务写成 Generator 函数，它们之间使用<code>yield</code>表达式交换控制权。</p> <h3 id="generator-与上下文"><a href="#generator-与上下文" class="header-anchor">#</a> Generator 与上下文</h3> <p>JavaScript 代码运行时，会产生一个全局的上下文环境（context，又称运行环境），包含了当前所有的变量和对象。然后，执行函数（或块级代码）的时候，又会在当前上下文环境的上层，产生一个函数运行的上下文，变成当前（active）的上下文，由此形成一个上下文环境的堆栈（context stack）。</p> <p>这个堆栈是“后进先出”的数据结构，最后产生的上下文环境首先执行完成，退出堆栈，然后再执行完成它下层的上下文，直至所有代码执行完成，堆栈清空。</p> <p>Generator 函数不是这样，它执行产生的上下文环境，一旦遇到<code>yield</code>命令，就会暂时退出堆栈，但是并不消失，里面的所有变量和对象会冻结在当前状态。等到对它执行<code>next</code>命令时，这个上下文环境又会重新加入调用栈，冻结的变量和对象恢复执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    805. 

  805.   <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    806. 

  806.   <span class="token keyword">return</span> <span class="token number">2</span><span class="token punctuation">;</span>
    807. 

  807. <span class="token punctuation">}</span>
    808. 

  808. 

  809. <span class="token keyword">let</span> g <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    810. 

  810. 

  811. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>
    812. 

  812.   g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">,</span>
    813. 

  813.   g<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">,</span>
    814. 

  814. <span class="token punctuation">)</span><span class="token punctuation">;</span>
    815. 

  815. </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，第一次执行<code>g.next()</code>时，Generator 函数<code>gen</code>的上下文会加入堆栈，即开始运行<code>gen</code>内部的代码。等遇到<code>yield 1</code>时，<code>gen</code>上下文退出堆栈，内部状态冻结。第二次执行<code>g.next()</code>时，<code>gen</code>上下文重新加入堆栈，变成当前的上下文，重新恢复执行。</p> <h2 id="应用"><a href="#应用" class="header-anchor">#</a> 应用</h2> <p>Generator 可以暂停函数执行，返回任意表达式的值。这种特点使得 Generator 有多种应用场景。</p> <h3 id="_1-异步操作的同步化表达"><a href="#_1-异步操作的同步化表达" class="header-anchor">#</a> （1）异步操作的同步化表达</h3> <p>Generator 函数的暂停执行的效果，意味着可以把异步操作写在<code>yield</code>表达式里面，等到调用<code>next</code>方法时再往后执行。这实际上等同于不需要写回调函数了，因为异步操作的后续操作可以放在<code>yield</code>表达式下面，反正要等到调用<code>next</code>方法时再执行。所以，Generator 函数的一个重要实际意义就是用来处理异步操作，改写回调函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">loadUI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    816. 

  816.   <span class="token function">showLoadingScreen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    817. 

  817.   <span class="token keyword">yield</span> <span class="token function">loadUIDataAsynchronously</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    818. 

  818.   <span class="token function">hideLoadingScreen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    819. 

  819. <span class="token punctuation">}</span>
    820. 

  820. <span class="token keyword">var</span> loader <span class="token operator">=</span> <span class="token function">loadUI</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    821. 

  821. <span class="token comment">// 加载UI</span>
    822. 

  822. loader<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    823. 

  823. 

  824. <span class="token comment">// 卸载UI</span>
    825. 

  825. loader<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    826. 

  826. </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，第一次调用<code>loadUI</code>函数时，该函数不会执行，仅返回一个遍历器。下一次对该遍历器调用<code>next</code>方法，则会显示<code>Loading</code>界面（<code>showLoadingScreen</code>），并且异步加载数据（<code>loadUIDataAsynchronously</code>）。等到数据加载完成，再一次使用<code>next</code>方法，则会隐藏<code>Loading</code>界面。可以看到，这种写法的好处是所有<code>Loading</code>界面的逻辑，都被封装在一个函数，按部就班非常清晰。</p> <p>Ajax 是典型的异步操作，通过 Generator 函数部署 Ajax 操作，可以用同步的方式表达。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    827. 

  827.   <span class="token keyword">var</span> result <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token function">request</span><span class="token punctuation">(</span><span class="token string">&quot;http://some.url&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    828. 

  828.   <span class="token keyword">var</span> resp <span class="token operator">=</span> <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    829. 

  829.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>resp<span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    830. 

  830. <span class="token punctuation">}</span>
    831. 

  831. 

  832. <span class="token keyword">function</span> <span class="token function">request</span><span class="token punctuation">(</span><span class="token parameter">url</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    833. 

  833.   <span class="token function">makeAjaxCall</span><span class="token punctuation">(</span>url<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">response</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    834. 

  834.     it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span>response<span class="token punctuation">)</span><span class="token punctuation">;</span>
    835. 

  835.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    836. 

  836. <span class="token punctuation">}</span>
    837. 

  837. 

  838. <span class="token keyword">var</span> it <span class="token operator">=</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    839. 

  839. it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    840. 

  840. </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>上面代码的<code>main</code>函数，就是通过 Ajax 操作获取数据。可以看到，除了多了一个<code>yield</code>，它几乎与同步操作的写法完全一样。注意，<code>makeAjaxCall</code>函数中的<code>next</code>方法，必须加上<code>response</code>参数，因为<code>yield</code>表达式，本身是没有值的，总是等于<code>undefined</code>。</p> <p>下面是另一个例子，通过 Generator 函数逐行读取文本文件。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">numbers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    841. 

  841.   <span class="token keyword">let</span> file <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileReader</span><span class="token punctuation">(</span><span class="token string">&quot;numbers.txt&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    842. 

  842.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    843. 

  843.     <span class="token keyword">while</span><span class="token punctuation">(</span><span class="token operator">!</span>file<span class="token punctuation">.</span>eof<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    844. 

  844.       <span class="token keyword">yield</span> <span class="token function">parseInt</span><span class="token punctuation">(</span>file<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    845. 

  845.     <span class="token punctuation">}</span>
    846. 

  846.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    847. 

  847.     file<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    848. 

  848.   <span class="token punctuation">}</span>
    849. 

  849. <span class="token punctuation">}</span>
    850. 

  850. </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>上面代码打开文本文件，使用<code>yield</code>表达式可以手动逐行读取文件。</p> <h3 id="_2-控制流管理"><a href="#_2-控制流管理" class="header-anchor">#</a> （2）控制流管理</h3> <p>如果有一个多步操作非常耗时，采用回调函数，可能会写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">step1</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    851. 

  851.   <span class="token function">step2</span><span class="token punctuation">(</span>value1<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">value2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    852. 

  852.     <span class="token function">step3</span><span class="token punctuation">(</span>value2<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">value3</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    853. 

  853.       <span class="token function">step4</span><span class="token punctuation">(</span>value3<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">value4</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    854. 

  854.         <span class="token comment">// Do something with value4</span>
    855. 

  855.       <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    856. 

  856.     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    857. 

  857.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    858. 

  858. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    859. 

  859. </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>采用 Promise 改写上面的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span>step1<span class="token punctuation">)</span>
    860. 

  860.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>step2<span class="token punctuation">)</span>
    861. 

  861.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>step3<span class="token punctuation">)</span>
    862. 

  862.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>step4<span class="token punctuation">)</span>
    863. 

  863.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value4</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    864. 

  864.     <span class="token comment">// Do something with value4</span>
    865. 

  865.   <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">error</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    866. 

  866.     <span class="token comment">// Handle any error from step1 through step4</span>
    867. 

  867.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    868. 

  868.   <span class="token punctuation">.</span><span class="token function">done</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    869. 

  869. </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>上面代码已经把回调函数，改成了直线执行的形式，但是加入了大量 Promise 的语法。Generator 函数可以进一步改善代码运行流程。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">longRunningTask</span><span class="token punctuation">(</span><span class="token parameter">value1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    870. 

  870.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    871. 

  871.     <span class="token keyword">var</span> value2 <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token function">step1</span><span class="token punctuation">(</span>value1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    872. 

  872.     <span class="token keyword">var</span> value3 <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token function">step2</span><span class="token punctuation">(</span>value2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    873. 

  873.     <span class="token keyword">var</span> value4 <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token function">step3</span><span class="token punctuation">(</span>value3<span class="token punctuation">)</span><span class="token punctuation">;</span>
    874. 

  874.     <span class="token keyword">var</span> value5 <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token function">step4</span><span class="token punctuation">(</span>value4<span class="token punctuation">)</span><span class="token punctuation">;</span>
    875. 

  875.     <span class="token comment">// Do something with value4</span>
    876. 

  876.   <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    877. 

  877.     <span class="token comment">// Handle any error from step1 through step4</span>
    878. 

  878.   <span class="token punctuation">}</span>
    879. 

  879. <span class="token punctuation">}</span>
    880. 

  880. </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>然后，使用一个函数，按次序自动执行所有步骤。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">scheduler</span><span class="token punctuation">(</span><span class="token function">longRunningTask</span><span class="token punctuation">(</span>initialValue<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    881. 

  881. 

  882. <span class="token keyword">function</span> <span class="token function">scheduler</span><span class="token punctuation">(</span><span class="token parameter">task</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    883. 

  883.   <span class="token keyword">var</span> taskObj <span class="token operator">=</span> task<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span>task<span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    884. 

  884.   <span class="token comment">// 如果Generator函数未结束，就继续调用</span>
    885. 

  885.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>taskObj<span class="token punctuation">.</span>done<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    886. 

  886.     task<span class="token punctuation">.</span>value <span class="token operator">=</span> taskObj<span class="token punctuation">.</span>value
    887. 

  887.     <span class="token function">scheduler</span><span class="token punctuation">(</span>task<span class="token punctuation">)</span><span class="token punctuation">;</span>
    888. 

  888.   <span class="token punctuation">}</span>
    889. 

  889. <span class="token punctuation">}</span>
    890. 

  890. </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>注意，上面这种做法，只适合同步操作，即所有的<code>task</code>都必须是同步的，不能有异步操作。因为这里的代码一得到返回值，就继续往下执行，没有判断异步操作何时完成。如果要控制异步的操作流程，详见后面的《异步操作》一章。</p> <p>下面，利用<code>for...of</code>循环会自动依次执行<code>yield</code>命令的特性，提供一种更一般的控制流管理的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> steps <span class="token operator">=</span> <span class="token punctuation">[</span>step1Func<span class="token punctuation">,</span> step2Func<span class="token punctuation">,</span> step3Func<span class="token punctuation">]</span><span class="token punctuation">;</span>
    891. 

  891. 

  892. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">iterateSteps</span><span class="token punctuation">(</span><span class="token parameter">steps</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    893. 

  893.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span> i<span class="token operator">&lt;</span> steps<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    894. 

  894.     <span class="token keyword">var</span> step <span class="token operator">=</span> steps<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
    895. 

  895.     <span class="token keyword">yield</span> <span class="token function">step</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    896. 

  896.   <span class="token punctuation">}</span>
    897. 

  897. <span class="token punctuation">}</span>
    898. 

  898. </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，数组<code>steps</code>封装了一个任务的多个步骤，Generator 函数<code>iterateSteps</code>则是依次为这些步骤加上<code>yield</code>命令。</p> <p>将任务分解成步骤之后，还可以将项目分解成多个依次执行的任务。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> jobs <span class="token operator">=</span> <span class="token punctuation">[</span>job1<span class="token punctuation">,</span> job2<span class="token punctuation">,</span> job3<span class="token punctuation">]</span><span class="token punctuation">;</span>
    899. 

  899. 

  900. <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">iterateJobs</span><span class="token punctuation">(</span><span class="token parameter">jobs</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    901. 

  901.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span> i<span class="token operator">&lt;</span> jobs<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    902. 

  902.     <span class="token keyword">var</span> job <span class="token operator">=</span> jobs<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
    903. 

  903.     <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">iterateSteps</span><span class="token punctuation">(</span>job<span class="token punctuation">.</span>steps<span class="token punctuation">)</span><span class="token punctuation">;</span>
    904. 

  904.   <span class="token punctuation">}</span>
    905. 

  905. <span class="token punctuation">}</span>
    906. 

  906. </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，数组<code>jobs</code>封装了一个项目的多个任务，Generator 函数<code>iterateJobs</code>则是依次为这些任务加上<code>yield*</code>命令。</p> <p>最后，就可以用<code>for...of</code>循环一次性依次执行所有任务的所有步骤。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> step <span class="token keyword">of</span> <span class="token function">iterateJobs</span><span class="token punctuation">(</span>jobs<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    907. 

  907.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>step<span class="token punctuation">.</span>id<span class="token punctuation">)</span><span class="token punctuation">;</span>
    908. 

  908. <span class="token punctuation">}</span>
    909. 

  909. </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>再次提醒，上面的做法只能用于所有步骤都是同步操作的情况，不能有异步操作的步骤。如果想要依次执行异步的步骤，必须使用后面的《异步操作》一章介绍的方法。</p> <p><code>for...of</code>的本质是一个<code>while</code>循环，所以上面的代码实质上执行的是下面的逻辑。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> it <span class="token operator">=</span> <span class="token function">iterateJobs</span><span class="token punctuation">(</span>jobs<span class="token punctuation">)</span><span class="token punctuation">;</span>
    910. 

  910. <span class="token keyword">var</span> res <span class="token operator">=</span> it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    911. 

  911. 

  912. <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>res<span class="token punctuation">.</span>done<span class="token punctuation">)</span><span class="token punctuation">{</span>
    913. 

  913.   <span class="token keyword">var</span> result <span class="token operator">=</span> res<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
    914. 

  914.   <span class="token comment">// ...</span>
    915. 

  915.   res <span class="token operator">=</span> it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    916. 

  916. <span class="token punctuation">}</span>
    917. 

  917. </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="_3-部署-iterator-接口"><a href="#_3-部署-iterator-接口" class="header-anchor">#</a> （3）部署 Iterator 接口</h3> <p>利用 Generator 函数，可以在任意对象上部署 Iterator 接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">iterEntries</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    918. 

  918.   <span class="token keyword">let</span> keys <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    919. 

  919.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> keys<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    920. 

  920.     <span class="token keyword">let</span> key <span class="token operator">=</span> keys<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
    921. 

  921.     <span class="token keyword">yield</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    922. 

  922.   <span class="token punctuation">}</span>
    923. 

  923. <span class="token punctuation">}</span>
    924. 

  924. 

  925. <span class="token keyword">let</span> myObj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token number">7</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    926. 

  926. 

  927. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> <span class="token function">iterEntries</span><span class="token punctuation">(</span>myObj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    928. 

  928.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    929. 

  929. <span class="token punctuation">}</span>
    930. 

  930. 

  931. <span class="token comment">// foo 3</span>
    932. 

  932. <span class="token comment">// bar 7</span>
    933. 

  933. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上述代码中，<code>myObj</code>是一个普通对象，通过<code>iterEntries</code>函数，就有了 Iterator 接口。也就是说，可以在任意对象上部署<code>next</code>方法。</p> <p>下面是一个对数组部署 Iterator 接口的例子，尽管数组原生具有这个接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">makeSimpleGenerator</span><span class="token punctuation">(</span><span class="token parameter">array</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    934. 

  934.   <span class="token keyword">var</span> nextIndex <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    935. 

  935. 

  936.   <span class="token keyword">while</span><span class="token punctuation">(</span>nextIndex <span class="token operator">&lt;</span> array<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">{</span>
    937. 

  937.     <span class="token keyword">yield</span> array<span class="token punctuation">[</span>nextIndex<span class="token operator">++</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    938. 

  938.   <span class="token punctuation">}</span>
    939. 

  939. <span class="token punctuation">}</span>
    940. 

  940. 

  941. <span class="token keyword">var</span> gen <span class="token operator">=</span> <span class="token function">makeSimpleGenerator</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'yo'</span><span class="token punctuation">,</span> <span class="token string">'ya'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    942. 

  942. 

  943. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 'yo'</span>
    944. 

  944. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 'ya'</span>
    945. 

  945. gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>done  <span class="token comment">// true</span>
    946. 

  946. </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="_4-作为数据结构"><a href="#_4-作为数据结构" class="header-anchor">#</a> （4）作为数据结构</h3> <p>Generator 可以看作是数据结构，更确切地说，可以看作是一个数组结构，因为 Generator 函数可以返回一系列的值，这意味着它可以对任意表达式，提供类似数组的接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">doStuff</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    947. 

  947.   <span class="token keyword">yield</span> fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'hello.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    948. 

  948.   <span class="token keyword">yield</span> fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'world.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    949. 

  949.   <span class="token keyword">yield</span> fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'and-such.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    950. 

  950. <span class="token punctuation">}</span>
    951. 

  951. </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>上面代码就是依次返回三个函数，但是由于使用了 Generator 函数，导致可以像处理数组那样，处理这三个返回的函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span>task <span class="token keyword">of</span> <span class="token function">doStuff</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    952. 

  952.   <span class="token comment">// task是一个函数，可以像回调函数那样使用它</span>
    953. 

  953. <span class="token punctuation">}</span>
    954. 

  954. </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>实际上，如果用 ES5 表达，完全可以用数组模拟 Generator 的这种用法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">doStuff</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    955. 

  955.   <span class="token keyword">return</span> <span class="token punctuation">[</span>
    956. 

  956.     fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'hello.txt'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    957. 

  957.     fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'world.txt'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    958. 

  958.     fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'and-such.txt'</span><span class="token punctuation">)</span>
    959. 

  959.   <span class="token punctuation">]</span><span class="token punctuation">;</span>
    960. 

  960. <span class="token punctuation">}</span>
    961. 

  961. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面的函数，可以用一模一样的<code>for...of</code>循环处理！两相一比较，就不难看出 Generator 使得数据或者操作，具备了类似数组的接口。</p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/18.Generator 函数的语法.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/48df907ad3570f3d/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Iterator 和 for-of 循环</div></a> <a href="/blog/pages/75af7031eb66847b/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Generator 函数的异步应用</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
    962. 

  962.         ←
    963. 

  963.         <a href="/blog/pages/48df907ad3570f3d/" class="prev">Iterator 和 for-of 循环</a></span> <span class="next"><a href="/blog/pages/75af7031eb66847b/">Generator 函数的异步应用</a>→
    964. 

  964.       </span></p></div></div></div> <div class="article-list"><div class="article-title"><a href="/blog/archives/" class="iconfont icon-bi">最近更新</a></div> <div class="article-wrapper"><dl><dd>01</dd> <dt><a href="/blog/pages/922650/"><div>
    965. 

  965.             Git修改分支名
    966. 

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    967. 

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    968. 

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    969. 

  969.             我做了一个手写春联小网页，祝大家虎年暴富
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    976. 

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