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class="active sidebar-link">最新提案</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#do-表达式" class="sidebar-link">do 表达式</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#throw-表达式" class="sidebar-link">throw 表达式</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#函数的部分执行" class="sidebar-link">函数的部分执行</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/7188882b8d65af1b/#语法" class="sidebar-link">语法</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/7188882b8d65af1b/#注意点" class="sidebar-link">注意点</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#管道运算符" class="sidebar-link">管道运算符</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#数值分隔符" class="sidebar-link">数值分隔符</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#math-signbit" class="sidebar-link">Math.signbit()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#双冒号运算符" class="sidebar-link">双冒号运算符</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#realm-api" class="sidebar-link">Realm API</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#命令" class="sidebar-link">#!命令</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7188882b8d65af1b/#import-meta" class="sidebar-link">import.meta</a></li></ul></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==">最新提案<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="最新提案"><a href="#最新提案" class="header-anchor">#</a> 最新提案</h1> <p>本章介绍一些尚未进入标准、但很有希望的最新提案。</p> <h2 id="do-表达式"><a href="#do-表达式" class="header-anchor">#</a> do 表达式</h2> <p>本质上，块级作用域是一个语句，将多个操作封装在一起，没有返回值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    26. 

  26.   <span class="token keyword">let</span> t <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>
    27. 

  27.   t <span class="token operator">=</span> t <span class="token operator">*</span> t <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
    28. 

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

  29. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div> <p>上面代码中，块级作用域将两个语句封装在一起。但是，在块级作用域以外，没有办法得到<code>t</code>的值，因为块级作用域不返回值，除非<code>t</code>是全局变量。</p> <p>现在有一个<a href="https://github.com/tc39/proposal-do-expressions" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，使得块级作用域可以变为表达式，也就是说可以返回值，办法就是在块级作用域之前加上<code>do</code>，使它变为<code>do</code>表达式，然后就会返回内部最后执行的表达式的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token keyword">do</span> <span class="token punctuation">{</span>
    30. 

  30.   <span class="token keyword">let</span> t <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>
    31. 

  31.   t <span class="token operator">*</span> t <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
    32. 

  32. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    33. 

  33. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，变量<code>x</code>会得到整个块级作用域的返回值（<code>t * t + 1</code>）。</p> <p><code>do</code>表达式的逻辑非常简单：封装的是什么，就会返回什么。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 等同于 &lt;表达式&gt;</span>
    34. 

  34. <span class="token keyword">do</span> <span class="token punctuation">{</span> <span class="token operator">&lt;</span>表达式<span class="token operator">&gt;</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    35. 

  35. 

  36. <span class="token comment">// 等同于 &lt;语句&gt;</span>
    37. 

  37. <span class="token keyword">do</span> <span class="token punctuation">{</span> <span class="token operator">&lt;</span>语句<span class="token operator">&gt;</span> <span class="token punctuation">}</span>
    38. 

  38. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><code>do</code>表达式的好处是可以封装多个语句，让程序更加模块化，就像乐高积木那样一块块拼装起来。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token keyword">do</span> <span class="token punctuation">{</span>
    39. 

  39.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">}</span>
    40. 

  40.   <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">g</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">}</span>
    41. 

  41.   <span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token function">h</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">}</span>
    42. 

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

  43. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码的本质，就是根据函数<code>foo</code>的执行结果，调用不同的函数，将返回结果赋给变量<code>x</code>。使用<code>do</code>表达式，就将这个操作的意图表达得非常简洁清晰。而且，<code>do</code>块级作用域提供了单独的作用域，内部操作可以与全局作用域隔绝。</p> <p>值得一提的是，<code>do</code>表达式在 JSX 语法中非常好用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">return</span> <span class="token punctuation">(</span>
    44. 

  44.   <span class="token operator">&lt;</span>nav<span class="token operator">&gt;</span>
    45. 

  45.     <span class="token operator">&lt;</span>Home <span class="token operator">/</span><span class="token operator">&gt;</span>
    46. 

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

  47.       <span class="token keyword">do</span> <span class="token punctuation">{</span>
    48. 

  48.         <span class="token keyword">if</span> <span class="token punctuation">(</span>loggedIn<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    49. 

  49.           <span class="token operator">&lt;</span>LogoutButton <span class="token operator">/</span><span class="token operator">&gt;</span>
    50. 

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

  51.           <span class="token operator">&lt;</span>LoginButton <span class="token operator">/</span><span class="token operator">&gt;</span>
    52. 

  52.         <span class="token punctuation">}</span>
    53. 

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

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

  55.   <span class="token operator">&lt;</span><span class="token operator">/</span>nav<span class="token operator">&gt;</span>
    56. 

  56. <span class="token punctuation">)</span>
    57. 

  57. </code></pre> <div class="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，如果不用<code>do</code>表达式，就只能用三元判断运算符（<code>?:</code>）。那样的话，一旦判断逻辑复杂，代码就会变得很不易读。</p> <h2 id="throw-表达式"><a href="#throw-表达式" class="header-anchor">#</a> throw 表达式</h2> <p>JavaScript 语法规定<code>throw</code>是一个命令，用来抛出错误，不能用于表达式之中。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    58. 

  58. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><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><span class="token punctuation">;</span>
    59. 

  59. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>console.log</code>的参数必须是一个表达式，如果是一个<code>throw</code>语句就会报错。</p> <p>现在有一个<a href="https://github.com/tc39/proposal-throw-expressions" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，允许<code>throw</code>用于表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 参数的默认值</span>
    60. 

  60. <span class="token keyword">function</span> <span class="token function">save</span><span class="token punctuation">(</span>filename <span class="token operator">=</span> <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">TypeError</span><span class="token punctuation">(</span><span class="token string">&quot;Argument required&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    61. 

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

  62. 

  63. <span class="token comment">// 箭头函数的返回值</span>
    64. 

  64. <span class="token function">lint</span><span class="token punctuation">(</span>ast<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    65. 

  65.   <span class="token function-variable function">with</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">&quot;avoid using 'with' statements.&quot;</span><span class="token punctuation">)</span>
    66. 

  66. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    67. 

  67. 

  68. <span class="token comment">// 条件表达式</span>
    69. 

  69. <span class="token keyword">function</span> <span class="token function">getEncoder</span><span class="token punctuation">(</span><span class="token parameter">encoding</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    70. 

  70.   <span class="token keyword">const</span> encoder <span class="token operator">=</span> encoding <span class="token operator">===</span> <span class="token string">&quot;utf8&quot;</span> <span class="token operator">?</span>
    71. 

  71.     <span class="token keyword">new</span> <span class="token class-name">UTF8Encoder</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">:</span>
    72. 

  72.     encoding <span class="token operator">===</span> <span class="token string">&quot;utf16le&quot;</span> <span class="token operator">?</span>
    73. 

  73.       <span class="token keyword">new</span> <span class="token class-name">UTF16Encoder</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token operator">:</span>
    74. 

  74.       encoding <span class="token operator">===</span> <span class="token string">&quot;utf16be&quot;</span> <span class="token operator">?</span>
    75. 

  75.         <span class="token keyword">new</span> <span class="token class-name">UTF16Encoder</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token operator">:</span>
    76. 

  76.         <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">&quot;Unsupported encoding&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    77. 

  77. <span class="token punctuation">}</span>
    78. 

  78. 

  79. <span class="token comment">// 逻辑表达式</span>
    80. 

  80. <span class="token keyword">class</span> <span class="token class-name">Product</span> <span class="token punctuation">{</span>
    81. 

  81.   <span class="token keyword">get</span> <span class="token function">id</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    82. 

  82.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_id<span class="token punctuation">;</span>
    83. 

  83.   <span class="token punctuation">}</span>
    84. 

  84.   <span class="token keyword">set</span> <span class="token function">id</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    85. 

  85.     <span class="token keyword">this</span><span class="token punctuation">.</span>_id <span class="token operator">=</span> value <span class="token operator">||</span> <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">&quot;Invalid value&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    86. 

  86.   <span class="token punctuation">}</span>
    87. 

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

  88. </code></pre> <div class="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></div></div><p>上面代码中，<code>throw</code>都出现在表达式里面。</p> <p>语法上，<code>throw</code>表达式里面的<code>throw</code>不再是一个命令，而是一个运算符。为了避免与<code>throw</code>命令混淆，规定<code>throw</code>出现在行首，一律解释为<code>throw</code>语句，而不是<code>throw</code>表达式。</p> <h2 id="函数的部分执行"><a href="#函数的部分执行" class="header-anchor">#</a> 函数的部分执行</h2> <h3 id="语法"><a href="#语法" class="header-anchor">#</a> 语法</h3> <p>多参数的函数有时需要绑定其中的一个或多个参数，然后返回一个新函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    89. 

  89. <span class="token keyword">function</span> <span class="token function">add7</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x <span class="token operator">+</span> <span class="token number">7</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    90. 

  90. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>add7</code>函数其实是<code>add</code>函数的一个特殊版本，通过将一个参数绑定为<code>7</code>，就可以从<code>add</code>得到<code>add7</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bind 方法</span>
    91. 

  91. <span class="token keyword">const</span> add7 <span class="token operator">=</span> <span class="token function">add</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 number">7</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    92. 

  92. 

  93. <span class="token comment">// 箭头函数</span>
    94. 

  94. <span class="token keyword">const</span> <span class="token function-variable function">add7</span> <span class="token operator">=</span> <span class="token parameter">x</span> <span class="token operator">=&gt;</span> <span class="token function">add</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    95. 

  95. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面两种写法都有些冗余。其中，<code>bind</code>方法的局限更加明显，它必须提供<code>this</code>，并且只能从前到后一个个绑定参数，无法只绑定非头部的参数。</p> <p>现在有一个<a href="https://github.com/tc39/proposal-partial-application" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，使得绑定参数并返回一个新函数更加容易。这叫做函数的部分执行（partial application）。</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">add</span> <span class="token operator">=</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 operator">=&gt;</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
    96. 

  96. <span class="token keyword">const</span> addOne <span class="token operator">=</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token operator">?</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    97. 

  97. 

  98. <span class="token keyword">const</span> maxGreaterThanZero <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    99. 

  99. </code></pre> <div class="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>根据新提案，<code>?</code>是单个参数的占位符，<code>...</code>是多个参数的占位符。以下的形式都属于函数的部分执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">f</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> <span class="token operator">?</span><span class="token punctuation">)</span>
    100. 

  100. <span class="token function">f</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span>
    101. 

  101. <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> x<span class="token punctuation">)</span>
    102. 

  102. <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">,</span> x<span class="token punctuation">)</span>
    103. 

  103. <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> x<span class="token punctuation">,</span> <span class="token operator">?</span><span class="token punctuation">)</span>
    104. 

  104. <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">,</span> x<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span>
    105. 

  105. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><code>?</code>和<code>...</code>只能出现在函数的调用之中，并且会返回一个新函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    106. 

  106. <span class="token comment">// 等同于</span>
    107. 

  107. <span class="token keyword">const</span> <span class="token function-variable function">g</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> <span class="token operator">...</span>y</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">f</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token operator">...</span>y<span class="token punctuation">)</span><span class="token punctuation">;</span>
    108. 

  108. </code></pre> <div class="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>函数的部分执行，也可以用于对象的方法。</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>
    109. 

  109.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    110. 

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

  111. 

  112. <span class="token keyword">const</span> g <span class="token operator">=</span> obj<span class="token punctuation">.</span><span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    113. 

  113. <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 comment">// 4</span>
    114. 

  114. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h3 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h3> <p>函数的部分执行有一些特别注意的地方。</p> <p>（1）函数的部分执行是基于原函数的。如果原函数发生变化，部分执行生成的新函数也会立即反映这种变化。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token function-variable function">f</span> <span class="token operator">=</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 operator">=&gt;</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
    115. 

  115. 

  116. <span class="token keyword">const</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    117. 

  117. <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 punctuation">;</span> <span class="token comment">// 4</span>
    118. 

  118. 

  119. <span class="token comment">// 替换函数 f</span>
    120. 

  120. <span class="token function-variable function">f</span> <span class="token operator">=</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 operator">=&gt;</span> x <span class="token operator">*</span> y<span class="token punctuation">;</span>
    121. 

  121. 

  122. <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 punctuation">;</span> <span class="token comment">// 3</span>
    123. 

  123. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，定义了函数的部分执行以后，更换原函数会立即影响到新函数。</p> <p>（2）如果预先提供的那个值是一个表达式，那么这个表达式并不会在定义时求值，而是在每次调用时求值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    124. 

  124. <span class="token keyword">const</span> <span class="token function-variable function">f</span> <span class="token operator">=</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 operator">=&gt;</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
    125. 

  125. 

  126. <span class="token keyword">const</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span><span class="token punctuation">;</span>
    127. 

  127. <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 punctuation">;</span> <span class="token comment">// 4</span>
    128. 

  128. 

  129. <span class="token comment">// 改变 a 的值</span>
    130. 

  130. a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    131. 

  131. <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 punctuation">;</span> <span class="token comment">// 11</span>
    132. 

  132. </code></pre> <div class="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，预先提供的参数是变量<code>a</code>，那么每次调用函数<code>g</code>的时候，才会对<code>a</code>进行求值。</p> <p>（3）如果新函数的参数多于占位符的数量，那么多余的参数将被忽略。</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">f</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> <span class="token operator">...</span>y</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> <span class="token operator">...</span>y<span class="token punctuation">]</span><span class="token punctuation">;</span>
    133. 

  133. <span class="token keyword">const</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><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>
    134. 

  134. <span class="token function">g</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 number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [2, 1]</span>
    135. 

  135. </code></pre> <div class="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，函数<code>g</code>只有一个占位符，也就意味着它只能接受一个参数，多余的参数都会被忽略。</p> <p>写成下面这样，多余的参数就没有问题。</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">f</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> <span class="token operator">...</span>y</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> <span class="token operator">...</span>y<span class="token punctuation">]</span><span class="token punctuation">;</span>
    136. 

  136. <span class="token keyword">const</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">?</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    137. 

  137. <span class="token function">g</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 number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [2, 1, 3, 4];</span>
    138. 

  138. </code></pre> <div class="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>（4）<code>...</code>只会被采集一次，如果函数的部分执行使用了多个<code>...</code>，那么每个<code>...</code>的值都将相同。</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">f</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>x</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">;</span>
    139. 

  139. <span class="token keyword">const</span> g <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">,</span> <span class="token number">9</span><span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    140. 

  140. <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 number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [1, 2, 3, 9, 1, 2, 3]</span>
    141. 

  141. </code></pre> <div class="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>g</code>定义了两个<code>...</code>占位符，真正执行的时候，它们的值是一样的。</p> <h2 id="管道运算符"><a href="#管道运算符" class="header-anchor">#</a> 管道运算符</h2> <p>Unix 操作系统有一个管道机制（pipeline），可以把前一个操作的值传给后一个操作。这个机制非常有用，使得简单的操作可以组合成为复杂的操作。许多语言都有管道的实现，现在有一个<a href="https://github.com/tc39/proposal-pipeline-operator" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，让 JavaScript 也拥有管道机制。</p> <p>JavaScript 的管道是一个运算符，写作<code>|&gt;</code>。它的左边是一个表达式，右边是一个函数。管道运算符把左边表达式的值，传入右边的函数进行求值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>x <span class="token operator">|</span><span class="token operator">&gt;</span> f
    142. 

  142. <span class="token comment">// 等同于</span>
    143. 

  143. <span class="token function">f</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span>
    144. 

  144. </code></pre> <div class="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>管道运算符最大的好处，就是可以把嵌套的函数，写成从左到右的链式表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">doubleSay</span> <span class="token punctuation">(</span><span class="token parameter">str</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    145. 

  145.   <span class="token keyword">return</span> str <span class="token operator">+</span> <span class="token string">&quot;, &quot;</span> <span class="token operator">+</span> str<span class="token punctuation">;</span>
    146. 

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

  147. 

  148. <span class="token keyword">function</span> <span class="token function">capitalize</span> <span class="token punctuation">(</span><span class="token parameter">str</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    149. 

  149.   <span class="token keyword">return</span> str<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">toUpperCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> str<span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    150. 

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

  151. 

  152. <span class="token keyword">function</span> <span class="token function">exclaim</span> <span class="token punctuation">(</span><span class="token parameter">str</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    153. 

  153.   <span class="token keyword">return</span> str <span class="token operator">+</span> <span class="token string">'!'</span><span class="token punctuation">;</span>
    154. 

  154. <span class="token punctuation">}</span>
    155. 

  155. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><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>上面是三个简单的函数。如果要嵌套执行，传统的写法和管道的写法分别如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 传统的写法</span>
    156. 

  156. <span class="token function">exclaim</span><span class="token punctuation">(</span><span class="token function">capitalize</span><span class="token punctuation">(</span><span class="token function">doubleSay</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    157. 

  157. <span class="token comment">// &quot;Hello, hello!&quot;</span>
    158. 

  158. 

  159. <span class="token comment">// 管道的写法</span>
    160. 

  160. <span class="token string">'hello'</span>
    161. 

  161.   <span class="token operator">|</span><span class="token operator">&gt;</span> doubleSay
    162. 

  162.   <span class="token operator">|</span><span class="token operator">&gt;</span> capitalize
    163. 

  163.   <span class="token operator">|</span><span class="token operator">&gt;</span> exclaim
    164. 

  164. <span class="token comment">// &quot;Hello, hello!&quot;</span>
    165. 

  165. </code></pre> <div class="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>管道运算符只能传递一个值，这意味着它右边的函数必须是一个单参数函数。如果是多参数函数，就必须进行柯里化，改成单参数的版本。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">double</span> <span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x <span class="token operator">+</span> x<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    166. 

  166. <span class="token keyword">function</span> <span class="token function">add</span> <span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span> <span class="token punctuation">}</span>
    167. 

  167. 

  168. <span class="token keyword">let</span> person <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">score</span><span class="token operator">:</span> <span class="token number">25</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    169. 

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

  170.   <span class="token operator">|</span><span class="token operator">&gt;</span> double
    171. 

  171.   <span class="token operator">|</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span><span class="token parameter">_</span> <span class="token operator">=&gt;</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">,</span> _<span class="token punctuation">)</span><span class="token punctuation">)</span>
    172. 

  172. <span class="token comment">// 57</span>
    173. 

  173. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>add</code>函数需要两个参数。但是，管道运算符只能传入一个值，因此需要事先提供另一个参数，并将其改成单参数的箭头函数<code>_ =&gt; add(7, _)</code>。这个函数里面的下划线并没有特别的含义，可以用其他符号代替，使用下划线只是因为，它能够形象地表示这里是占位符。</p> <p>管道运算符对于<code>await</code>函数也适用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>x <span class="token operator">|</span><span class="token operator">&gt;</span> <span class="token keyword">await</span> f
    174. 

  174. <span class="token comment">// 等同于</span>
    175. 

  175. <span class="token keyword">await</span> <span class="token function">f</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span>
    176. 

  176. 

  177. <span class="token keyword">const</span> userAge <span class="token operator">=</span> userId <span class="token operator">|</span><span class="token operator">&gt;</span> <span class="token keyword">await</span> fetchUserById <span class="token operator">|</span><span class="token operator">&gt;</span> getAgeFromUser<span class="token punctuation">;</span>
    178. 

  178. <span class="token comment">// 等同于</span>
    179. 

  179. <span class="token keyword">const</span> userAge <span class="token operator">=</span> <span class="token function">getAgeFromUser</span><span class="token punctuation">(</span><span class="token keyword">await</span> <span class="token function">fetchUserById</span><span class="token punctuation">(</span>userId<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    180. 

  180. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h2 id="数值分隔符"><a href="#数值分隔符" class="header-anchor">#</a> 数值分隔符</h2> <p>欧美语言中，较长的数值允许每三位添加一个分隔符（通常是一个逗号），增加数值的可读性。比如，<code>1000</code>可以写作<code>1,000</code>。</p> <p>现在有一个<a href="https://github.com/tc39/proposal-numeric-separator" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，允许 JavaScript 的数值使用下划线（<code>_</code>）作为分隔符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> budget <span class="token operator">=</span> <span class="token number">1_000_000_000_000</span><span class="token punctuation">;</span>
    181. 

  181. budget <span class="token operator">===</span> <span class="token number">10</span> <span class="token operator">**</span> <span class="token number">12</span> <span class="token comment">// true</span>
    182. 

  182. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>JavaScript 的数值分隔符没有指定间隔的位数，也就是说，可以每三位添加一个分隔符，也可以每一位、每两位、每四位添加一个。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">123_00</span> <span class="token operator">===</span> <span class="token number">12_300</span> <span class="token comment">// true</span>
    183. 

  183. 

  184. <span class="token number">12345_00</span> <span class="token operator">===</span> <span class="token number">123_4500</span> <span class="token comment">// true</span>
    185. 

  185. <span class="token number">12345_00</span> <span class="token operator">===</span> <span class="token number">1_234_500</span> <span class="token comment">// true</span>
    186. 

  186. </code></pre> <div class="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>小数和科学计数法也可以使用数值分隔符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 小数</span>
    187. 

  187. <span class="token number">0.000_001</span>
    188. 

  188. <span class="token comment">// 科学计数法</span>
    189. 

  189. <span class="token number">1e10_000</span>
    190. 

  190. </code></pre> <div class="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>数值分隔符有几个使用注意点。</p> <ul><li>不能在数值的最前面（leading）或最后面（trailing）。</li> <li>不能两个或两个以上的分隔符连在一起。</li> <li>小数点的前后不能有分隔符。</li> <li>科学计数法里面，表示指数的<code>e</code>或<code>E</code>前后不能有分隔符。</li></ul> <p>下面的写法都会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 全部报错</span>
    191. 

  191. 3_<span class="token punctuation">.</span><span class="token number">141</span>
    192. 

  192. <span class="token number">3</span><span class="token punctuation">.</span>_141
    193. 

  193. 1_e12
    194. 

  194. 1e_12
    195. 

  195. 123__456
    196. 

  196. _1464301
    197. 

  197. 1464301_
    198. 

  198. </code></pre> <div class="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>除了十进制，其他进制的数值也可以使用分隔符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 二进制</span>
    199. 

  199. <span class="token number">0b1010_0001_1000_0101</span>
    200. 

  200. <span class="token comment">// 十六进制</span>
    201. 

  201. <span class="token number">0xA0_B0_C0</span>
    202. 

  202. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>注意，分隔符不能紧跟着进制的前缀<code>0b</code>、<code>0B</code>、<code>0o</code>、<code>0O</code>、<code>0x</code>、<code>0X</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    203. 

  203. 0_b111111000
    204. 

  204. 0b_111111000
    205. 

  205. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>下面三个将字符串转成数值的函数，不支持数值分隔符。主要原因是提案的设计者认为，数值分隔符主要是为了编码时书写数值的方便，而不是为了处理外部输入的数据。</p> <ul><li>Number()</li> <li>parseInt()</li> <li>parseFloat()</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'123_456'</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    206. 

  206. <span class="token function">parseInt</span><span class="token punctuation">(</span><span class="token string">'123_456'</span><span class="token punctuation">)</span> <span class="token comment">// 123</span>
    207. 

  207. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="math-signbit"><a href="#math-signbit" class="header-anchor">#</a> Math.signbit()</h2> <p><code>Math.sign()</code>用来判断一个值的正负，但是如果参数是<code>-0</code>，它会返回<code>-0</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// -0</span>
    208. 

  208. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>这导致对于判断符号位的正负，<code>Math.sign()</code>不是很有用。JavaScript 内部使用 64 位浮点数（国际标准 IEEE 754）表示数值，IEEE 754 规定第一位是符号位，<code>0</code>表示正数，<code>1</code>表示负数。所以会有两种零，<code>+0</code>是符号位为<code>0</code>时的零值，<code>-0</code>是符号位为<code>1</code>时的零值。实际编程中，判断一个值是<code>+0</code>还是<code>-0</code>非常麻烦，因为它们是相等的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">+</span><span class="token number">0</span> <span class="token operator">===</span> <span class="token operator">-</span><span class="token number">0</span> <span class="token comment">// true</span>
    209. 

  209. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>目前，有一个<a href="http://jfbastien.github.io/papers/Math.signbit.html" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，引入了<code>Math.signbit()</code>方法判断一个数的符号位是否设置了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">signbit</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">//false</span>
    210. 

  210. Math<span class="token punctuation">.</span><span class="token function">signbit</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">//true</span>
    211. 

  211. Math<span class="token punctuation">.</span><span class="token function">signbit</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">//false</span>
    212. 

  212. Math<span class="token punctuation">.</span><span class="token function">signbit</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">//true</span>
    213. 

  213. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>可以看到，该方法正确返回了<code>-0</code>的符号位是设置了的。</p> <p>该方法的算法如下。</p> <ul><li>如果参数是<code>NaN</code>，返回<code>false</code></li> <li>如果参数是<code>-0</code>，返回<code>true</code></li> <li>如果参数是负值，返回<code>true</code></li> <li>其他情况返回<code>false</code></li></ul> <h2 id="双冒号运算符"><a href="#双冒号运算符" class="header-anchor">#</a> 双冒号运算符</h2> <p>箭头函数可以绑定<code>this</code>对象，大大减少了显式绑定<code>this</code>对象的写法（<code>call</code>、<code>apply</code>、<code>bind</code>）。但是，箭头函数并不适用于所有场合，所以现在有一个<a href="https://github.com/zenparsing/es-function-bind" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，提出了“函数绑定”（function bind）运算符，用来取代<code>call</code>、<code>apply</code>、<code>bind</code>调用。</p> <p>函数绑定运算符是并排的两个冒号（<code>::</code>），双冒号左边是一个对象，右边是一个函数。该运算符会自动将左边的对象，作为上下文环境（即<code>this</code>对象），绑定到右边的函数上面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token literal-property property">foo</span><span class="token operator">:</span><span class="token operator">:</span>bar<span class="token punctuation">;</span>
    214. 

  214. <span class="token comment">// 等同于</span>
    215. 

  215. <span class="token function">bar</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span>
    216. 

  216. 

  217. <span class="token literal-property property">foo</span><span class="token operator">:</span><span class="token operator">:</span><span class="token function">bar</span><span class="token punctuation">(</span><span class="token operator">...</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
    218. 

  218. <span class="token comment">// 等同于</span>
    219. 

  219. <span class="token function">bar</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>foo<span class="token punctuation">,</span> arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
    220. 

  220. 

  221. <span class="token keyword">const</span> hasOwnProperty <span class="token operator">=</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>hasOwnProperty<span class="token punctuation">;</span>
    222. 

  222. <span class="token keyword">function</span> <span class="token function">hasOwn</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> key</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    223. 

  223.   <span class="token keyword">return</span> <span class="token literal-property property">obj</span><span class="token operator">:</span><span class="token operator">:</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    224. 

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

  225. </code></pre> <div class="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>如果双冒号左边为空，右边是一个对象的方法，则等于将该方法绑定在该对象上面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> method <span class="token operator">=</span> obj<span class="token operator">:</span><span class="token operator">:</span>obj<span class="token punctuation">.</span>foo<span class="token punctuation">;</span>
    226. 

  226. <span class="token comment">// 等同于</span>
    227. 

  227. <span class="token keyword">var</span> method <span class="token operator">=</span> <span class="token operator">:</span><span class="token operator">:</span>obj<span class="token punctuation">.</span>foo<span class="token punctuation">;</span>
    228. 

  228. 

  229. <span class="token keyword">let</span> log <span class="token operator">=</span> <span class="token operator">:</span><span class="token operator">:</span>console<span class="token punctuation">.</span>log<span class="token punctuation">;</span>
    230. 

  230. <span class="token comment">// 等同于</span>
    231. 

  231. <span class="token keyword">var</span> log <span class="token operator">=</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span>console<span class="token punctuation">)</span><span class="token punctuation">;</span>
    232. 

  232. </code></pre> <div class="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>如果双冒号运算符的运算结果，还是一个对象，就可以采用链式写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> map<span class="token punctuation">,</span> takeWhile<span class="token punctuation">,</span> forEach <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&quot;iterlib&quot;</span><span class="token punctuation">;</span>
    233. 

  233. 

  234. <span class="token function">getPlayers</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    235. 

  235. <span class="token operator">:</span><span class="token operator">:</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">.</span><span class="token function">character</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    236. 

  236. <span class="token operator">:</span><span class="token operator">:</span><span class="token function">takeWhile</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">.</span>strength <span class="token operator">&gt;</span> <span class="token number">100</span><span class="token punctuation">)</span>
    237. 

  237. <span class="token operator">:</span><span class="token operator">:</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> 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><span class="token punctuation">;</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></div></div><h2 id="realm-api"><a href="#realm-api" class="header-anchor">#</a> Realm API</h2> <p><a href="https://github.com/tc39/proposal-realms" target="_blank" rel="noopener noreferrer">Realm API<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 提供沙箱功能（sandbox），允许隔离代码，防止那些被隔离的代码拿到全局对象。</p> <p>以前，经常使用<code>&lt;iframe&gt;</code>作为沙箱。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> globalOne <span class="token operator">=</span> window<span class="token punctuation">;</span>
    239. 

  239. <span class="token keyword">let</span> iframe <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">createElement</span><span class="token punctuation">(</span><span class="token string">'iframe'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    240. 

  240. document<span class="token punctuation">.</span>body<span class="token punctuation">.</span><span class="token function">appendChild</span><span class="token punctuation">(</span>iframe<span class="token punctuation">)</span><span class="token punctuation">;</span>
    241. 

  241. <span class="token keyword">const</span> globalTwo <span class="token operator">=</span> iframe<span class="token punctuation">.</span>contentWindow<span class="token punctuation">;</span>
    242. 

  242. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>&lt;iframe&gt;</code>的全局对象是独立的（<code>iframe.contentWindow</code>）。Realm API 可以取代这个功能。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> globalOne <span class="token operator">=</span> window<span class="token punctuation">;</span>
    243. 

  243. <span class="token keyword">const</span> globalTwo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Realm</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>global<span class="token punctuation">;</span>
    244. 

  244. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>Realm API</code>单独提供了一个全局对象<code>new Realm().global</code>。</p> <p>Realm API 提供一个<code>Realm()</code>构造函数，用来生成一个 Realm 对象。该对象的<code>global</code>属性指向一个新的顶层对象，这个顶层对象跟原始的顶层对象类似。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> globalOne <span class="token operator">=</span> window<span class="token punctuation">;</span>
    245. 

  245. <span class="token keyword">const</span> globalTwo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Realm</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>global<span class="token punctuation">;</span>
    246. 

  246. 

  247. globalOne<span class="token punctuation">.</span><span class="token function">evaluate</span><span class="token punctuation">(</span><span class="token string">'1 + 2'</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    248. 

  248. globalTwo<span class="token punctuation">.</span><span class="token function">evaluate</span><span class="token punctuation">(</span><span class="token string">'1 + 2'</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    249. 

  249. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，Realm 生成的顶层对象的<code>evaluate()</code>方法，可以运行代码。</p> <p>下面的代码可以证明，Realm 顶层对象与原始顶层对象是两个对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> a1 <span class="token operator">=</span> globalOne<span class="token punctuation">.</span><span class="token function">evaluate</span><span class="token punctuation">(</span><span class="token string">'[1,2,3]'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    250. 

  250. <span class="token keyword">let</span> a2 <span class="token operator">=</span> globalTwo<span class="token punctuation">.</span><span class="token function">evaluate</span><span class="token punctuation">(</span><span class="token string">'[1,2,3]'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    251. 

  251. a1<span class="token punctuation">.</span>prototype <span class="token operator">===</span> a2<span class="token punctuation">.</span>prototype<span class="token punctuation">;</span> <span class="token comment">// false</span>
    252. 

  252. a1 <span class="token keyword">instanceof</span> <span class="token class-name">globalTwo<span class="token punctuation">.</span>Array</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    253. 

  253. a2 <span class="token keyword">instanceof</span> <span class="token class-name">globalOne<span class="token punctuation">.</span>Array</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    254. 

  254. </code></pre> <div class="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，Realm 沙箱里面的数组的原型对象，跟原始环境里面的数组是不一样的。</p> <p>Realm 沙箱里面只能运行 ECMAScript 语法提供的 API，不能运行宿主环境提供的 API。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>globalTwo<span class="token punctuation">.</span><span class="token function">evaluate</span><span class="token punctuation">(</span><span class="token string">'console.log(1)'</span><span class="token punctuation">)</span>
    255. 

  255. <span class="token comment">// throw an error: console is undefined</span>
    256. 

  256. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，Realm 沙箱里面没有<code>console</code>对象，导致报错。因为<code>console</code>不是语法标准，是宿主环境提供的。</p> <p>如果要解决这个问题，可以使用下面的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>globalTwo<span class="token punctuation">.</span>console <span class="token operator">=</span> globalOne<span class="token punctuation">.</span>console<span class="token punctuation">;</span>
    257. 

  257. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>Realm()</code>构造函数可以接受一个参数对象，该参数对象的<code>intrinsics</code>属性可以指定 Realm 沙箱继承原始顶层对象的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> r1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Realm</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    258. 

  258. r1<span class="token punctuation">.</span>global <span class="token operator">===</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
    259. 

  259. r1<span class="token punctuation">.</span>global<span class="token punctuation">.</span><span class="token constant">JSON</span> <span class="token operator">===</span> <span class="token constant">JSON</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    260. 

  260. 

  261. <span class="token keyword">const</span> r2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Realm</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">intrinsics</span><span class="token operator">:</span> <span class="token string">'inherit'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    262. 

  262. r2<span class="token punctuation">.</span>global <span class="token operator">===</span> <span class="token keyword">this</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    263. 

  263. r2<span class="token punctuation">.</span>global<span class="token punctuation">.</span><span class="token constant">JSON</span> <span class="token operator">===</span> <span class="token constant">JSON</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    264. 

  264. </code></pre> <div class="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，正常情况下，沙箱的<code>JSON</code>方法不同于原始的<code>JSON</code>对象。但是，<code>Realm()</code>构造函数接受<code>{ intrinsics: 'inherit' }</code>作为参数以后，就会继承原始顶层对象的方法。</p> <p>用户可以自己定义<code>Realm</code>的子类，用来定制自己的沙箱。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">FakeWindow</span> <span class="token keyword">extends</span> <span class="token class-name">Realm</span> <span class="token punctuation">{</span>
    265. 

  265.   <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    266. 

  266.     <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    267. 

  267.     <span class="token keyword">let</span> global <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>global<span class="token punctuation">;</span>
    268. 

  268. 

  269.     global<span class="token punctuation">.</span>document <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FakeDocument</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    270. 

  270.     global<span class="token punctuation">.</span>alert <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>fakeAlert<span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    271. 

  271.     <span class="token comment">// ...</span>
    272. 

  272.   <span class="token punctuation">}</span>
    273. 

  273. <span class="token punctuation">}</span>
    274. 

  274. </code></pre> <div class="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>FakeWindow</code>模拟了一个假的顶层对象<code>window</code>。</p> <h2 id="命令"><a href="#命令" class="header-anchor">#</a> <code>#!</code>命令</h2> <p>Unix 的命令行脚本都支持<code>#!</code>命令，又称为 Shebang 或 Hashbang。这个命令放在脚本的第一行，用来指定脚本的执行器。</p> <p>比如 Bash 脚本的第一行。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code><span class="token shebang important">#!/bin/sh</span>
    275. 

  275. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>Python 脚本的第一行。</p> <div class="language-python line-numbers-mode"><pre class="language-python"><code><span class="token comment">#!/usr/bin/env python</span>
    276. 

  276. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>现在有一个<a href="https://github.com/tc39/proposal-hashbang" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，为 JavaScript 脚本引入了<code>#!</code>命令，写在脚本文件或者模块文件的第一行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 写在脚本文件第一行</span>
    277. 

  277. #<span class="token operator">!</span><span class="token operator">/</span>usr<span class="token operator">/</span>bin<span class="token operator">/</span>env node
    278. 

  278. <span class="token string">'use strict'</span><span class="token punctuation">;</span>
    279. 

  279. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    280. 

  280. 

  281. <span class="token comment">// 写在模块文件第一行</span>
    282. 

  282. #<span class="token operator">!</span><span class="token operator">/</span>usr<span class="token operator">/</span>bin<span class="token operator">/</span>env node
    283. 

  283. <span class="token keyword">export</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    284. 

  284. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    285. 

  285. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><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>有了这一行以后，Unix 命令行就可以直接执行脚本。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code><span class="token comment"># 以前执行脚本的方式</span>
    286. 

  286. $ <span class="token function">node</span> hello.js
    287. 

  287. 

  288. <span class="token comment"># hashbang 的方式</span>
    289. 

  289. $ hello.js
    290. 

  290. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>对于 JavaScript 引擎来说，会把<code>#!</code>理解成注释，忽略掉这一行。</p> <h2 id="import-meta"><a href="#import-meta" class="header-anchor">#</a> import.meta</h2> <p>开发者使用一个模块时，有时需要知道模板本身的一些信息（比如模块的路径）。现在有一个<a href="https://github.com/tc39/proposal-import-meta" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，为 import 命令添加了一个元属性<code>import.beta</code>，返回当前模块的元信息。</p> <p><code>import.meta</code>只能在模块内部使用，如果在模块外部使用会报错。</p> <p>这个属性返回一个对象，该对象的各种属性就是当前运行的脚本的元信息。具体包含哪些属性，标准没有规定，由各个运行环境自行决定。一般来说，<code>import.meta</code>至少会有下面两个属性。</p> <p><strong>（1）import.meta.url</strong></p> <p><code>import.meta.url</code>返回当前模块的 URL 路径。举例来说，当前模块主文件的路径是<code>https://foo.com/main.js</code>，<code>import.meta.url</code>就返回这个路径。如果模块里面还有一个数据文件<code>data.txt</code>，那么就可以用下面的代码，获取这个数据文件的路径。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">new</span> <span class="token class-name">URL</span><span class="token punctuation">(</span><span class="token string">'data.txt'</span><span class="token punctuation">,</span> <span class="token keyword">import</span><span class="token punctuation">.</span>meta<span class="token punctuation">.</span>url<span class="token punctuation">)</span>
    291. 

  291. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>注意，Node.js 环境中，<code>import.meta.url</code>返回的总是本地路径，即是<code>file:URL</code>协议的字符串，比如<code>file:///home/user/foo.js</code>。</p> <p><strong>（2）import.meta.scriptElement</strong></p> <p><code>import.meta.scriptElement</code>是浏览器特有的元属性，返回加载模块的那个<code>&lt;script&gt;</code>元素，相当于<code>document.currentScript</code>属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// HTML 代码为</span>
    292. 

  292. <span class="token comment">// &lt;script type=&quot;module&quot; src=&quot;my-module.js&quot; data-foo=&quot;abc&quot;&gt;&lt;/script&gt;</span>
    293. 

  293. 

  294. <span class="token comment">// my-module.js 内部执行下面的代码</span>
    295. 

  295. <span class="token keyword">import</span><span class="token punctuation">.</span>meta<span class="token punctuation">.</span>scriptElement<span class="token punctuation">.</span>dataset<span class="token punctuation">.</span>foo
    296. 

  296. <span class="token comment">// &quot;abc&quot;</span>
    297. 

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