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实例的方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#get" class="sidebar-link">get()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#set" class="sidebar-link">set()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#apply" class="sidebar-link">apply()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#has" class="sidebar-link">has()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#construct" class="sidebar-link">construct()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#deleteproperty" class="sidebar-link">deleteProperty()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f56ec2ab97d60483/#defineproperty" class="sidebar-link">defineProperty()</a></li><li class="sidebar-sub-header level3"><a 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href="/blog/pages/f56ec2ab97d60483/#this-问题" class="sidebar-link">this 问题</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/f56ec2ab97d60483/#实例-web-服务的客户端" class="sidebar-link">实例：Web 服务的客户端</a></li></ul></li><li><a href="/blog/pages/74de3e45e4491e95/" class="sidebar-link">Reflect</a></li><li><a href="/blog/pages/2810ae8985e9bd52/" class="sidebar-link">Promise 对象</a></li><li><a href="/blog/pages/48df907ad3570f3d/" class="sidebar-link">Iterator 和 for-of 循环</a></li><li><a href="/blog/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/blog/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/blog/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/blog/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/blog/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/blog/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/blog/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">Proxy<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="proxy"><a href="#proxy" class="header-anchor">#</a> Proxy</h1> <h2 id="概述"><a href="#概述" class="header-anchor">#</a> 概述</h2> <p>Proxy 用于修改某些操作的默认行为，等同于在语言层面做出修改，所以属于一种“<strong>元编程</strong>”（meta programming），即<strong>对编程语言进行编程</strong>。</p> <p>Proxy 可以理解成，在目标对象之前架设一层“拦截”，<strong>外界对该对象的访问，都必须先通过这层拦截</strong>，因此提供了一种机制，可以对外界的访问进行过滤和改写。Proxy 这个词的原意是代理，用在这里表示由它来“代理”某些操作，可以译为“代理器”。
    26. 

  26. </p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    27. 

  27.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    28. 

  28.     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">getting </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>propKey<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">!</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    29. 

  29.     <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span><span class="token punctuation">;</span>
    30. 

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

  31.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    32. 

  32.     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">setting </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>propKey<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">!</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    33. 

  33.     <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span><span class="token punctuation">;</span>
    34. 

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

  35. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    36. 

  36. </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>上面代码对一个空对象架设了一层拦截，重定义了属性的读取（<code>get</code>）和设置（<code>set</code>）行为。这里暂时先不解释具体的语法，只看运行结果。对设置了拦截行为的对象<code>obj</code>，去读写它的属性，就会得到下面的结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>obj<span class="token punctuation">.</span>count <span class="token operator">=</span> <span class="token number">1</span>
    37. 

  37. <span class="token comment">//  setting count!</span>
    38. 

  38. <span class="token operator">++</span>obj<span class="token punctuation">.</span>count
    39. 

  39. <span class="token comment">//  getting count!</span>
    40. 

  40. <span class="token comment">//  setting count!</span>
    41. 

  41. <span class="token comment">//  2</span>
    42. 

  42. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码说明，Proxy 实际上重载（overload）了点运算符，即用自己的定义覆盖了语言的原始定义。</p> <p>ES6 原生提供 Proxy 构造函数，用来生成 Proxy 实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<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></div></div><p>Proxy 对象的所有用法，都是上面这种形式，不同的只是<code>handler</code>参数的写法。其中，<code>new Proxy()</code>表示生成一个<code>Proxy</code>实例，<code>target</code>参数表示所要拦截的目标对象，<code>handler</code>参数也是一个对象，用来定制拦截行为。</p> <p>下面是另一个拦截读取属性行为的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    44. 

  44.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> propKey</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    45. 

  45.     <span class="token keyword">return</span> <span class="token number">35</span><span class="token punctuation">;</span>
    46. 

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

  47. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    48. 

  48. 

  49. proxy<span class="token punctuation">.</span>time <span class="token comment">// 35</span>
    50. 

  50. proxy<span class="token punctuation">.</span>name <span class="token comment">// 35</span>
    51. 

  51. proxy<span class="token punctuation">.</span>title <span class="token comment">// 35</span>
    52. 

  52. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，作为构造函数，<code>Proxy</code>接受两个参数。第一个参数是所要代理的目标对象（上例是一个空对象），即如果没有<code>Proxy</code>的介入，操作原来要访问的就是这个对象；第二个参数是一个配置对象，对于每一个被代理的操作，需要提供一个对应的处理函数，该函数将拦截对应的操作。比如，上面代码中，配置对象有一个<code>get</code>方法，用来拦截对目标对象属性的访问请求。<code>get</code>方法的两个参数分别是目标对象和所要访问的属性。可以看到，由于拦截函数总是返回<code>35</code>，所以访问任何属性都得到<code>35</code>。</p> <p>注意，要使得<code>Proxy</code>起作用，必须针对<code>Proxy</code>实例（上例是<code>proxy</code>对象）进行操作，而不是针对目标对象（上例是空对象）进行操作。</p> <p>如果<code>handler</code>没有设置任何拦截，那就等同于直接通向原对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> target <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    53. 

  53. <span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    54. 

  54. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    55. 

  55. proxy<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    56. 

  56. target<span class="token punctuation">.</span>a <span class="token comment">// &quot;b&quot;</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></div></div><p>上面代码中，<code>handler</code>是一个空对象，没有任何拦截效果，访问<code>proxy</code>就等同于访问<code>target</code>。</p> <p>一个技巧是将 Proxy 对象，设置到<code>object.proxy</code>属性，从而可以在<code>object</code>对象上调用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> object <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">proxy</span><span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    58. 

  58. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>Proxy 实例也可以作为其他对象的原型对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    59. 

  59.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> propKey</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    60. 

  60.     <span class="token keyword">return</span> <span class="token number">35</span><span class="token punctuation">;</span>
    61. 

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

  62. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    63. 

  63. 

  64. <span class="token keyword">let</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span><span class="token punctuation">;</span>
    65. 

  65. obj<span class="token punctuation">.</span>time <span class="token comment">// 35</span>
    66. 

  66. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>proxy</code>对象是<code>obj</code>对象的原型，<code>obj</code>对象本身并没有<code>time</code>属性，所以根据原型链，会在<code>proxy</code>对象上读取该属性，导致被拦截。</p> <p>同一个拦截器函数，可以设置拦截多个操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    67. 

  67.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    68. 

  68.     <span class="token keyword">if</span> <span class="token punctuation">(</span>name <span class="token operator">===</span> <span class="token string">'prototype'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    69. 

  69.       <span class="token keyword">return</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">;</span>
    70. 

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

  71.     <span class="token keyword">return</span> <span class="token string">'Hello, '</span> <span class="token operator">+</span> name<span class="token punctuation">;</span>
    72. 

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

  73. 

  74.   <span class="token function-variable function">apply</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> thisBinding<span class="token punctuation">,</span> args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    75. 

  75.     <span class="token keyword">return</span> args<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    76. 

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

  77. 

  78.   <span class="token function-variable function">construct</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    79. 

  79.     <span class="token keyword">return</span> <span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> args<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>
    80. 

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

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

  82. 

  83. <span class="token keyword">var</span> fproxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span><span class="token keyword">function</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>
    84. 

  84.   <span class="token keyword">return</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
    85. 

  85. <span class="token punctuation">}</span><span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    86. 

  86. 

  87. <span class="token function">fproxy</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 comment">// 1</span>
    88. 

  88. <span class="token keyword">new</span> <span class="token class-name">fproxy</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 comment">// {value: 2}</span>
    89. 

  89. fproxy<span class="token punctuation">.</span>prototype <span class="token operator">===</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
    90. 

  90. fproxy<span class="token punctuation">.</span>foo <span class="token operator">===</span> <span class="token string">&quot;Hello, foo&quot;</span> <span class="token comment">// true</span>
    91. 

  91. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><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>对于可以设置、但没有设置拦截的操作，则直接落在目标对象上，按照原先的方式产生结果。</p> <p>下面是 Proxy 支持的拦截操作一览，一共 13 种。</p> <ul><li><strong>get(target, propKey, receiver)</strong>：拦截对象属性的读取，比如<code>proxy.foo</code>和<code>proxy['foo']</code>。</li> <li><strong>set(target, propKey, value, receiver)</strong>：拦截对象属性的设置，比如<code>proxy.foo = v</code>或<code>proxy['foo'] = v</code>，返回一个布尔值。</li> <li><strong>has(target, propKey)</strong>：拦截<code>propKey in proxy</code>的操作，返回一个布尔值。</li> <li><strong>deleteProperty(target, propKey)</strong>：拦截<code>delete proxy[propKey]</code>的操作，返回一个布尔值。</li> <li><strong>ownKeys(target)</strong>：拦截<code>Object.getOwnPropertyNames(proxy)</code>、<code>Object.getOwnPropertySymbols(proxy)</code>、<code>Object.keys(proxy)</code>、<code>for...in</code>循环，返回一个数组。该方法返回目标对象所有自身的属性的属性名，而<code>Object.keys()</code>的返回结果仅包括目标对象自身的可遍历属性。</li> <li><strong>getOwnPropertyDescriptor(target, propKey)</strong>：拦截<code>Object.getOwnPropertyDescriptor(proxy, propKey)</code>，返回属性的描述对象。</li> <li><strong>defineProperty(target, propKey, propDesc)</strong>：拦截<code>Object.defineProperty(proxy, propKey, propDesc）</code>、<code>Object.defineProperties(proxy, propDescs)</code>，返回一个布尔值。</li> <li><strong>preventExtensions(target)</strong>：拦截<code>Object.preventExtensions(proxy)</code>，返回一个布尔值。</li> <li><strong>getPrototypeOf(target)</strong>：拦截<code>Object.getPrototypeOf(proxy)</code>，返回一个对象。</li> <li><strong>isExtensible(target)</strong>：拦截<code>Object.isExtensible(proxy)</code>，返回一个布尔值。</li> <li><strong>setPrototypeOf(target, proto)</strong>：拦截<code>Object.setPrototypeOf(proxy, proto)</code>，返回一个布尔值。如果目标对象是函数，那么还有两种额外操作可以拦截。</li> <li><strong>apply(target, object, args)</strong>：拦截 Proxy 实例作为函数调用的操作，比如<code>proxy(...args)</code>、<code>proxy.call(object, ...args)</code>、<code>proxy.apply(...)</code>。</li> <li><strong>construct(target, args)</strong>：拦截 Proxy 实例作为构造函数调用的操作，比如<code>new proxy(...args)</code>。</li></ul> <h2 id="proxy-实例的方法"><a href="#proxy-实例的方法" class="header-anchor">#</a> Proxy 实例的方法</h2> <p>下面是上面这些拦截方法的详细介绍。</p> <h3 id="get"><a href="#get" class="header-anchor">#</a> get()</h3> <p><code>get</code>方法用于拦截某个属性的读取操作，可以接受<strong>三个参数</strong>，依次为<strong>目标对象、属性名和 proxy 实例本身</strong>（严格地说，是操作行为所针对的对象），其中最后一个参数可选。</p> <p><code>get</code>方法的用法，上文已经有一个例子，下面是另一个拦截读取操作的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> person <span class="token operator">=</span> <span class="token punctuation">{</span>
    92. 

  92.   <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span>
    93. 

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

  94. 

  95. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>person<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    96. 

  96.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> propKey</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    97. 

  97.     <span class="token keyword">if</span> <span class="token punctuation">(</span>propKey <span class="token keyword">in</span> target<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    98. 

  98.       <span class="token keyword">return</span> target<span class="token punctuation">[</span>propKey<span class="token punctuation">]</span><span class="token punctuation">;</span>
    99. 

  99.     <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    100. 

  100.       <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">ReferenceError</span><span class="token punctuation">(</span><span class="token string">&quot;Prop name \&quot;&quot;</span> <span class="token operator">+</span> propKey <span class="token operator">+</span> <span class="token string">&quot;\&quot; does not exist.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    101. 

  101.     <span class="token punctuation">}</span>
    102. 

  102.   <span class="token punctuation">}</span>
    103. 

  103. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    104. 

  104. 

  105. proxy<span class="token punctuation">.</span>name <span class="token comment">// &quot;张三&quot;</span>
    106. 

  106. proxy<span class="token punctuation">.</span>age <span class="token comment">// 抛出一个错误</span>
    107. 

  107. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><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>上面代码表示，如果访问目标对象不存在的属性，会抛出一个错误。如果没有这个拦截函数，访问不存在的属性，只会返回<code>undefined</code>。</p> <p><strong><code>get</code>方法可以继承</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> proto <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    108. 

  108.   <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propertyKey<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    109. 

  109.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'GET '</span> <span class="token operator">+</span> propertyKey<span class="token punctuation">)</span><span class="token punctuation">;</span>
    110. 

  110.     <span class="token keyword">return</span> target<span class="token punctuation">[</span>propertyKey<span class="token punctuation">]</span><span class="token punctuation">;</span>
    111. 

  111.   <span class="token punctuation">}</span>
    112. 

  112. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    113. 

  113. 

  114. <span class="token keyword">let</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    115. 

  115. obj<span class="token punctuation">.</span>foo <span class="token comment">// &quot;GET foo&quot;</span>
    116. 

  116. </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，拦截操作定义在<code>Prototype</code>对象上面，所以如果读取<code>obj</code>对象继承的属性时，拦截会生效。</p> <p>下面的例子使用<code>get</code>拦截，实现数组读取负数的索引。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">createArray</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>elements</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    117. 

  117.   <span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    118. 

  118.     <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    119. 

  119.       <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token function">Number</span><span class="token punctuation">(</span>propKey<span class="token punctuation">)</span><span class="token punctuation">;</span>
    120. 

  120.       <span class="token keyword">if</span> <span class="token punctuation">(</span>index <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    121. 

  121.         propKey <span class="token operator">=</span> <span class="token function">String</span><span class="token punctuation">(</span>target<span class="token punctuation">.</span>length <span class="token operator">+</span> index<span class="token punctuation">)</span><span class="token punctuation">;</span>
    122. 

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

  123.       <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span><span class="token punctuation">;</span>
    124. 

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

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

  126. 

  127.   <span class="token keyword">let</span> target <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    128. 

  128.   target<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token operator">...</span>elements<span class="token punctuation">)</span><span class="token punctuation">;</span>
    129. 

  129.   <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    130. 

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

  131. 

  132. <span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token function">createArray</span><span class="token punctuation">(</span><span class="token string">'a'</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>
    133. 

  133. arr<span class="token punctuation">[</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">]</span> <span class="token comment">// c</span>
    134. 

  134. </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，数组的位置参数是<code>-1</code>，就会输出数组的倒数第一个成员。</p> <p>利用 Proxy，可以将读取属性的操作（<code>get</code>），转变为执行某个函数，从而实现属性的链式操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> pipe <span class="token operator">=</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>
    135. 

  135.   <span class="token keyword">return</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    136. 

  136.     <span class="token keyword">var</span> funcStack <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    137. 

  137.     <span class="token keyword">var</span> oproxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    138. 

  138.       <span class="token function-variable function">get</span> <span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">pipeObject<span class="token punctuation">,</span> fnName</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    139. 

  139.         <span class="token keyword">if</span> <span class="token punctuation">(</span>fnName <span class="token operator">===</span> <span class="token string">'get'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    140. 

  140.           <span class="token keyword">return</span> funcStack<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">val<span class="token punctuation">,</span> fn</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    141. 

  141.             <span class="token keyword">return</span> <span class="token function">fn</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span><span class="token punctuation">;</span>
    142. 

  142.           <span class="token punctuation">}</span><span class="token punctuation">,</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    143. 

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

  144.         funcStack<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>window<span class="token punctuation">[</span>fnName<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    145. 

  145.         <span class="token keyword">return</span> oproxy<span class="token punctuation">;</span>
    146. 

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

  147.     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    148. 

  148. 

  149.     <span class="token keyword">return</span> oproxy<span class="token punctuation">;</span>
    150. 

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

  151. <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>
    152. 

  152. 

  153. <span class="token keyword">var</span> <span class="token function-variable function">double</span> <span class="token operator">=</span> <span class="token parameter">n</span> <span class="token operator">=&gt;</span> n <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">;</span>
    154. 

  154. <span class="token keyword">var</span> <span class="token function-variable function">pow</span>    <span class="token operator">=</span> <span class="token parameter">n</span> <span class="token operator">=&gt;</span> n <span class="token operator">*</span> n<span class="token punctuation">;</span>
    155. 

  155. <span class="token keyword">var</span> <span class="token function-variable function">reverseInt</span> <span class="token operator">=</span> <span class="token parameter">n</span> <span class="token operator">=&gt;</span> n<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">&quot;&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">&quot;&quot;</span><span class="token punctuation">)</span> <span class="token operator">|</span> <span class="token number">0</span><span class="token punctuation">;</span>
    156. 

  156. 

  157. <span class="token function">pipe</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span>double<span class="token punctuation">.</span>pow<span class="token punctuation">.</span>reverseInt<span class="token punctuation">.</span>get<span class="token punctuation">;</span> <span class="token comment">// 63</span>
    158. 

  158. </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><p>上面代码设置 Proxy 以后，达到了将函数名链式使用的效果。</p> <p>下面的例子则是利用<code>get</code>拦截，实现一个生成各种 DOM 节点的通用函数<code>dom</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> dom <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    159. 

  159.   <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> property<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    160. 

  160.     <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">attrs <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token operator">...</span>children</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    161. 

  161.       <span class="token keyword">const</span> el <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">createElement</span><span class="token punctuation">(</span>property<span class="token punctuation">)</span><span class="token punctuation">;</span>
    162. 

  162.       <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> prop <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>attrs<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    163. 

  163.         el<span class="token punctuation">.</span><span class="token function">setAttribute</span><span class="token punctuation">(</span>prop<span class="token punctuation">,</span> attrs<span class="token punctuation">[</span>prop<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    164. 

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

  165.       <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> child <span class="token keyword">of</span> children<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    166. 

  166.         <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> child <span class="token operator">===</span> <span class="token string">'string'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    167. 

  167.           child <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">createTextNode</span><span class="token punctuation">(</span>child<span class="token punctuation">)</span><span class="token punctuation">;</span>
    168. 

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

  169.         el<span class="token punctuation">.</span><span class="token function">appendChild</span><span class="token punctuation">(</span>child<span class="token punctuation">)</span><span class="token punctuation">;</span>
    170. 

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

  171.       <span class="token keyword">return</span> el<span class="token punctuation">;</span>
    172. 

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

  173.   <span class="token punctuation">}</span>
    174. 

  174. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    175. 

  175. 

  176. <span class="token keyword">const</span> el <span class="token operator">=</span> dom<span class="token punctuation">.</span><span class="token function">div</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
    177. 

  177.   <span class="token string">'Hello, my name is '</span><span class="token punctuation">,</span>
    178. 

  178.   dom<span class="token punctuation">.</span><span class="token function">a</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">href</span><span class="token operator">:</span> <span class="token string">'//example.com'</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'Mark'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    179. 

  179.   <span class="token string">'. I like:'</span><span class="token punctuation">,</span>
    180. 

  180.   dom<span class="token punctuation">.</span><span class="token function">ul</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
    181. 

  181.     dom<span class="token punctuation">.</span><span class="token function">li</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">'The web'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    182. 

  182.     dom<span class="token punctuation">.</span><span class="token function">li</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">'Food'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    183. 

  183.     dom<span class="token punctuation">.</span><span class="token function">li</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">'…actually that\'s it'</span><span class="token punctuation">)</span>
    184. 

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

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

  186. 

  187. document<span class="token punctuation">.</span>body<span class="token punctuation">.</span><span class="token function">appendChild</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>
    188. 

  188. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><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>下面是一个<code>get</code>方法的第三个参数的例子，它总是指向原始的读操作所在的那个对象，一般情况下就是 Proxy 实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    189. 

  189.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    190. 

  190.     <span class="token keyword">return</span> receiver<span class="token punctuation">;</span>
    191. 

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

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

  193. proxy<span class="token punctuation">.</span>getReceiver <span class="token operator">===</span> proxy <span class="token comment">// true</span>
    194. 

  194. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>proxy</code>对象的<code>getReceiver</code>属性是由<code>proxy</code>对象提供的，所以<code>receiver</code>指向<code>proxy</code>对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    195. 

  195.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    196. 

  196.     <span class="token keyword">return</span> receiver<span class="token punctuation">;</span>
    197. 

  197.   <span class="token punctuation">}</span>
    198. 

  198. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    199. 

  199. 

  200. <span class="token keyword">const</span> d <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span><span class="token punctuation">;</span>
    201. 

  201. d<span class="token punctuation">.</span>a <span class="token operator">===</span> d <span class="token comment">// true</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><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>d</code>对象本身没有<code>a</code>属性，所以读取<code>d.a</code>的时候，会去<code>d</code>的原型<code>proxy</code>对象找。这时，<code>receiver</code>就指向<code>d</code>，代表原始的读操作所在的那个对象。</p> <p>如果一个属性不可配置（configurable）且不可写（writable），则 Proxy 不能修改该属性，否则通过 Proxy 对象访问该属性会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperties</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>
    203. 

  203.   <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    204. 

  204.     <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
    205. 

  205.     <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    206. 

  206.     <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    207. 

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

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

  209. 

  210. <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    211. 

  211.   <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propKey<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    212. 

  212.     <span class="token keyword">return</span> <span class="token string">'abc'</span><span class="token punctuation">;</span>
    213. 

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

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

  215. 

  216. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    217. 

  217. 

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

  219. <span class="token comment">// TypeError: Invariant check failed</span>
    220. 

  220. </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><h3 id="set"><a href="#set" class="header-anchor">#</a> set()</h3> <p><code>set</code>方法用来拦截某个属性的赋值操作，可以接受四个参数，依次为目标对象、属性名、属性值和 Proxy 实例本身，其中最后一个参数可选。</p> <p>假定<code>Person</code>对象有一个<code>age</code>属性，该属性应该是一个不大于 200 的整数，那么可以使用<code>Proxy</code>保证<code>age</code>的属性值符合要求。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> validator <span class="token operator">=</span> <span class="token punctuation">{</span>
    221. 

  221.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    222. 

  222.     <span class="token keyword">if</span> <span class="token punctuation">(</span>prop <span class="token operator">===</span> <span class="token string">'age'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    223. 

  223.       <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    224. 

  224.         <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">'The age is not an integer'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    225. 

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

  226.       <span class="token keyword">if</span> <span class="token punctuation">(</span>value <span class="token operator">&gt;</span> <span class="token number">200</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    227. 

  227.         <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RangeError</span><span class="token punctuation">(</span><span class="token string">'The age seems invalid'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    228. 

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

  229.     <span class="token punctuation">}</span>
    230. 

  230. 

  231.     <span class="token comment">// 对于满足条件的 age 属性以及其他属性，直接保存</span>
    232. 

  232.     obj<span class="token punctuation">[</span>prop<span class="token punctuation">]</span> <span class="token operator">=</span> value<span class="token punctuation">;</span>
    233. 

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

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

  235. 

  236. <span class="token keyword">let</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> validator<span class="token punctuation">)</span><span class="token punctuation">;</span>
    237. 

  237. 

  238. person<span class="token punctuation">.</span>age <span class="token operator">=</span> <span class="token number">100</span><span class="token punctuation">;</span>
    239. 

  239. 

  240. person<span class="token punctuation">.</span>age <span class="token comment">// 100</span>
    241. 

  241. person<span class="token punctuation">.</span>age <span class="token operator">=</span> <span class="token string">'young'</span> <span class="token comment">// 报错</span>
    242. 

  242. person<span class="token punctuation">.</span>age <span class="token operator">=</span> <span class="token number">300</span> <span class="token comment">// 报错</span>
    243. 

  243. </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></div></div><p>上面代码中，由于设置了存值函数<code>set</code>，任何不符合要求的<code>age</code>属性赋值，都会抛出一个错误，这是数据验证的一种实现方法。利用<code>set</code>方法，还可以数据绑定，即每当对象发生变化时，会自动更新 DOM。</p> <p>有时，我们会在对象上面设置内部属性，属性名的第一个字符使用下划线开头，表示这些属性不应该被外部使用。结合<code>get</code>和<code>set</code>方法，就可以做到防止这些内部属性被外部读写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    244. 

  244.   <span class="token function">get</span> <span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    245. 

  245.     <span class="token function">invariant</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> <span class="token string">'get'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    246. 

  246.     <span class="token keyword">return</span> target<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">;</span>
    247. 

  247.   <span class="token punctuation">}</span><span class="token punctuation">,</span>
    248. 

  248.   <span class="token function">set</span> <span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    249. 

  249.     <span class="token function">invariant</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> <span class="token string">'set'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    250. 

  250.     target<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> value<span class="token punctuation">;</span>
    251. 

  251.     <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    252. 

  252.   <span class="token punctuation">}</span>
    253. 

  253. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    254. 

  254. <span class="token keyword">function</span> <span class="token function">invariant</span> <span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> action</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    255. 

  255.   <span class="token keyword">if</span> <span class="token punctuation">(</span>key<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token string">'_'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    256. 

  256.     <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 template-string"><span class="token template-punctuation string">`</span><span class="token string">Invalid attempt to </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>action<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> private &quot;</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>key<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">&quot; property</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    257. 

  257.   <span class="token punctuation">}</span>
    258. 

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

  259. <span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    260. 

  260. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    261. 

  261. proxy<span class="token punctuation">.</span>_prop
    262. 

  262. <span class="token comment">// Error: Invalid attempt to get private &quot;_prop&quot; property</span>
    263. 

  263. proxy<span class="token punctuation">.</span>_prop <span class="token operator">=</span> <span class="token string">'c'</span>
    264. 

  264. <span class="token comment">// Error: Invalid attempt to set private &quot;_prop&quot; property</span>
    265. 

  265. </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></div></div><p>上面代码中，只要读写的属性名的第一个字符是下划线，一律抛错，从而达到禁止读写内部属性的目的。</p> <p>下面是<code>set</code>方法第四个参数的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    266. 

  266.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    267. 

  267.     obj<span class="token punctuation">[</span>prop<span class="token punctuation">]</span> <span class="token operator">=</span> receiver<span class="token punctuation">;</span>
    268. 

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

  269. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    270. 

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

  271. proxy<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    272. 

  272. proxy<span class="token punctuation">.</span>foo <span class="token operator">===</span> proxy <span class="token comment">// true</span>
    273. 

  273. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>set</code>方法的第四个参数<code>receiver</code>，指的是原始的操作行为所在的那个对象，一般情况下是<code>proxy</code>实例本身，请看下面的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    274. 

  274.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    275. 

  275.     obj<span class="token punctuation">[</span>prop<span class="token punctuation">]</span> <span class="token operator">=</span> receiver<span class="token punctuation">;</span>
    276. 

  276.   <span class="token punctuation">}</span>
    277. 

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

  278. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    279. 

  279. <span class="token keyword">const</span> myObj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    280. 

  280. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>myObj<span class="token punctuation">,</span> proxy<span class="token punctuation">)</span><span class="token punctuation">;</span>
    281. 

  281. 

  282. myObj<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    283. 

  283. myObj<span class="token punctuation">.</span>foo <span class="token operator">===</span> myObj <span class="token comment">// true</span>
    284. 

  284. </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，设置<code>myObj.foo</code>属性的值时，<code>myObj</code>并没有<code>foo</code>属性，因此引擎会到<code>myObj</code>的原型链去找<code>foo</code>属性。<code>myObj</code>的原型对象<code>proxy</code>是一个 Proxy 实例，设置它的<code>foo</code>属性会触发<code>set</code>方法。这时，第四个参数<code>receiver</code>就指向原始赋值行为所在的对象<code>myObj</code>。</p> <p>注意，如果目标对象自身的某个属性，不可写且不可配置，那么<code>set</code>方法将不起作用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    285. 

  285. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    286. 

  286.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">,</span>
    287. 

  287.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    288. 

  288. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    289. 

  289. 

  290. <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    291. 

  291.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    292. 

  292.     obj<span class="token punctuation">[</span>prop<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token string">'baz'</span><span class="token punctuation">;</span>
    293. 

  293.   <span class="token punctuation">}</span>
    294. 

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

  295. 

  296. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    297. 

  297. proxy<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'baz'</span><span class="token punctuation">;</span>
    298. 

  298. proxy<span class="token punctuation">.</span>foo <span class="token comment">// &quot;bar&quot;</span>
    299. 

  299. </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>obj.foo</code>属性不可写，Proxy 对这个属性的<code>set</code>代理将不会生效。</p> <p>注意，严格模式下，<code>set</code>代理如果没有返回<code>true</code>，就会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'use strict'</span><span class="token punctuation">;</span>
    300. 

  300. <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    301. 

  301.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    302. 

  302.     obj<span class="token punctuation">[</span>prop<span class="token punctuation">]</span> <span class="token operator">=</span> receiver<span class="token punctuation">;</span>
    303. 

  303.     <span class="token comment">// 无论有没有下面这一行，都会报错</span>
    304. 

  304.     <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    305. 

  305.   <span class="token punctuation">}</span>
    306. 

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

  307. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    308. 

  308. proxy<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    309. 

  309. <span class="token comment">// TypeError: 'set' on proxy: trap returned falsish for property 'foo'</span>
    310. 

  310. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><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，严格模式下，<code>set</code>代理返回<code>false</code>或者<code>undefined</code>，都会报错。</p> <h3 id="apply"><a href="#apply" class="header-anchor">#</a> apply()</h3> <p><code>apply</code>方法拦截函数的调用、<code>call</code>和<code>apply</code>操作。</p> <p><code>apply</code>方法可以接受三个参数，分别是目标对象、目标对象的上下文对象（<code>this</code>）和目标对象的参数数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    311. 

  311.   <span class="token function">apply</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> ctx<span class="token punctuation">,</span> args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    312. 

  312.     <span class="token keyword">return</span> <span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token operator">...</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
    313. 

  313.   <span class="token punctuation">}</span>
    314. 

  314. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    315. 

  315. </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>下面是一个例子。</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">target</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token string">'I am the target'</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    316. 

  316. <span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    317. 

  317.   <span class="token function-variable function">apply</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>
    318. 

  318.     <span class="token keyword">return</span> <span class="token string">'I am the proxy'</span><span class="token punctuation">;</span>
    319. 

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

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

  321. 

  322. <span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    323. 

  323. 

  324. <span class="token function">p</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    325. 

  325. <span class="token comment">// &quot;I am the proxy&quot;</span>
    326. 

  326. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，变量<code>p</code>是 Proxy 的实例，当它作为函数调用时（<code>p()</code>），就会被<code>apply</code>方法拦截，返回一个字符串。</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> twice <span class="token operator">=</span> <span class="token punctuation">{</span>
    327. 

  327.   <span class="token function">apply</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> ctx<span class="token punctuation">,</span> args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    328. 

  328.     <span class="token keyword">return</span> <span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token operator">...</span>arguments<span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">;</span>
    329. 

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

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

  331. <span class="token keyword">function</span> <span class="token function">sum</span> <span class="token punctuation">(</span><span class="token parameter">left<span class="token punctuation">,</span> right</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    332. 

  332.   <span class="token keyword">return</span> left <span class="token operator">+</span> right<span class="token punctuation">;</span>
    333. 

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

  334. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>sum<span class="token punctuation">,</span> twice<span class="token punctuation">)</span><span class="token punctuation">;</span>
    335. 

  335. <span class="token function">proxy</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 comment">// 6</span>
    336. 

  336. <span class="token function">proxy</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">null</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 comment">// 22</span>
    337. 

  337. <span class="token function">proxy</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// 30</span>
    338. 

  338. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，每当执行<code>proxy</code>函数（直接调用或<code>call</code>和<code>apply</code>调用），就会被<code>apply</code>方法拦截。</p> <p>另外，直接调用<code>Reflect.apply</code>方法，也会被拦截。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>proxy<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">9</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// 38</span>
    339. 

  339. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="has"><a href="#has" class="header-anchor">#</a> has()</h3> <p><code>has</code>方法用来拦截<code>HasProperty</code>操作，即判断对象是否具有某个属性时，这个方法会生效。典型的操作就是<code>in</code>运算符。</p> <p><code>has</code>方法可以接受两个参数，分别是目标对象、需查询的属性名。</p> <p>下面的例子使用<code>has</code>方法隐藏某些属性，不被<code>in</code>运算符发现。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    340. 

  340.   <span class="token function">has</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    341. 

  341.     <span class="token keyword">if</span> <span class="token punctuation">(</span>key<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token string">'_'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    342. 

  342.       <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    343. 

  343.     <span class="token punctuation">}</span>
    344. 

  344.     <span class="token keyword">return</span> key <span class="token keyword">in</span> target<span class="token punctuation">;</span>
    345. 

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

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

  347. <span class="token keyword">var</span> target <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">_prop</span><span class="token operator">:</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token literal-property property">prop</span><span class="token operator">:</span> <span class="token string">'foo'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    348. 

  348. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    349. 

  349. <span class="token string">'_prop'</span> <span class="token keyword">in</span> proxy <span class="token comment">// false</span>
    350. 

  350. </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，如果原对象的属性名的第一个字符是下划线，<code>proxy.has</code>就会返回<code>false</code>，从而不会被<code>in</code>运算符发现。</p> <p>如果原对象不可配置或者禁止扩展，这时<code>has</code>拦截会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">10</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    351. 

  351. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    352. 

  352. 

  353. <span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    354. 

  354.   <span class="token function-variable function">has</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> prop</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    355. 

  355.     <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    356. 

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

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

  358. 

  359. <span class="token string">'a'</span> <span class="token keyword">in</span> p <span class="token comment">// TypeError is thrown</span>
    360. 

  360. </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>obj</code>对象禁止扩展，结果使用<code>has</code>拦截就会报错。也就是说，如果某个属性不可配置（或者目标对象不可扩展），则<code>has</code>方法就不得“隐藏”（即返回<code>false</code>）目标对象的该属性。</p> <p>值得注意的是，<code>has</code>方法拦截的是<code>HasProperty</code>操作，而不是<code>HasOwnProperty</code>操作，即<code>has</code>方法不判断一个属性是对象自身的属性，还是继承的属性。</p> <p>另外，虽然<code>for...in</code>循环也用到了<code>in</code>运算符，但是<code>has</code>拦截对<code>for...in</code>循环不生效。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> stu1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'张三'</span><span class="token punctuation">,</span> <span class="token literal-property property">score</span><span class="token operator">:</span> <span class="token number">59</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    361. 

  361. <span class="token keyword">let</span> stu2 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'李四'</span><span class="token punctuation">,</span> <span class="token literal-property property">score</span><span class="token operator">:</span> <span class="token number">99</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    362. 

  362. 

  363. <span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    364. 

  364.   <span class="token function">has</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> prop</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    365. 

  365.     <span class="token keyword">if</span> <span class="token punctuation">(</span>prop <span class="token operator">===</span> <span class="token string">'score'</span> <span class="token operator">&amp;&amp;</span> target<span class="token punctuation">[</span>prop<span class="token punctuation">]</span> <span class="token operator">&lt;</span> <span class="token number">60</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    366. 

  366.       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>target<span class="token punctuation">.</span>name<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> 不及格</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    367. 

  367.       <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    368. 

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

  369.     <span class="token keyword">return</span> prop <span class="token keyword">in</span> target<span class="token punctuation">;</span>
    370. 

  370.   <span class="token punctuation">}</span>
    371. 

  371. <span class="token punctuation">}</span>
    372. 

  372. 

  373. <span class="token keyword">let</span> oproxy1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>stu1<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    374. 

  374. <span class="token keyword">let</span> oproxy2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>stu2<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    375. 

  375. 

  376. <span class="token string">'score'</span> <span class="token keyword">in</span> oproxy1
    377. 

  377. <span class="token comment">// 张三 不及格</span>
    378. 

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

  379. 

  380. <span class="token string">'score'</span> <span class="token keyword">in</span> oproxy2
    381. 

  381. <span class="token comment">// true</span>
    382. 

  382. 

  383. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> a <span class="token keyword">in</span> oproxy1<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    384. 

  384.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>oproxy1<span class="token punctuation">[</span>a<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    385. 

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

  386. <span class="token comment">// 张三</span>
    387. 

  387. <span class="token comment">// 59</span>
    388. 

  388. 

  389. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> b <span class="token keyword">in</span> oproxy2<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    390. 

  390.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>oproxy2<span class="token punctuation">[</span>b<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    391. 

  391. <span class="token punctuation">}</span>
    392. 

  392. <span class="token comment">// 李四</span>
    393. 

  393. <span class="token comment">// 99</span>
    394. 

  394. </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></div></div><p>上面代码中，<code>has</code>拦截只对<code>in</code>运算符生效，对<code>for...in</code>循环不生效，导致不符合要求的属性没有被<code>for...in</code>循环所排除。</p> <h3 id="construct"><a href="#construct" class="header-anchor">#</a> construct()</h3> <p><code>construct</code>方法用于拦截<code>new</code>命令，下面是拦截对象的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    395. 

  395.   <span class="token function">construct</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> args<span class="token punctuation">,</span> newTarget</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    396. 

  396.     <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">target</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
    397. 

  397.   <span class="token punctuation">}</span>
    398. 

  398. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    399. 

  399. </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>construct</code>方法可以接受两个参数。</p> <ul><li><code>target</code>：目标对象</li> <li><code>args</code>：构造函数的参数对象</li> <li><code>newTarget</code>：创造实例对象时，<code>new</code>命令作用的构造函数（下面例子的<code>p</code>）</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    400. 

  400.   <span class="token function-variable function">construct</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    401. 

  401.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'called: '</span> <span class="token operator">+</span> args<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">', '</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    402. 

  402.     <span class="token keyword">return</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> args<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">*</span> <span class="token number">10</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    403. 

  403.   <span class="token punctuation">}</span>
    404. 

  404. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    405. 

  405. 

  406. <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">p</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>value
    407. 

  407. <span class="token comment">// &quot;called: 1&quot;</span>
    408. 

  408. <span class="token comment">// 10</span>
    409. 

  409. </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><code>construct</code>方法返回的必须是一个对象，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    410. 

  410.   <span class="token function-variable function">construct</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> argumentsList</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    411. 

  411.     <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
    412. 

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

  413. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    414. 

  414. 

  415. <span class="token keyword">new</span> <span class="token class-name">p</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
    416. 

  416. <span class="token comment">// Uncaught TypeError: 'construct' on proxy: trap returned non-object ('1')</span>
    417. 

  417. </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="deleteproperty"><a href="#deleteproperty" class="header-anchor">#</a> deleteProperty()</h3> <p><code>deleteProperty</code>方法用于拦截<code>delete</code>操作，如果这个方法抛出错误或者返回<code>false</code>，当前属性就无法被<code>delete</code>命令删除。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    418. 

  418.   <span class="token function">deleteProperty</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    419. 

  419.     <span class="token function">invariant</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> <span class="token string">'delete'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    420. 

  420.     <span class="token keyword">delete</span> target<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">;</span>
    421. 

  421.     <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    422. 

  422.   <span class="token punctuation">}</span>
    423. 

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

  424. <span class="token keyword">function</span> <span class="token function">invariant</span> <span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> action</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    425. 

  425.   <span class="token keyword">if</span> <span class="token punctuation">(</span>key<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token string">'_'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    426. 

  426.     <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 template-string"><span class="token template-punctuation string">`</span><span class="token string">Invalid attempt to </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>action<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> private &quot;</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>key<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">&quot; property</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    427. 

  427.   <span class="token punctuation">}</span>
    428. 

  428. <span class="token punctuation">}</span>
    429. 

  429. 

  430. <span class="token keyword">var</span> target <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">_prop</span><span class="token operator">:</span> <span class="token string">'foo'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    431. 

  431. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    432. 

  432. <span class="token keyword">delete</span> proxy<span class="token punctuation">.</span>_prop
    433. 

  433. <span class="token comment">// Error: Invalid attempt to delete private &quot;_prop&quot; property</span>
    434. 

  434. </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，<code>deleteProperty</code>方法拦截了<code>delete</code>操作符，删除第一个字符为下划线的属性会报错。</p> <p>注意，目标对象自身的不可配置（configurable）的属性，不能被<code>deleteProperty</code>方法删除，否则报错。</p> <h3 id="defineproperty"><a href="#defineproperty" class="header-anchor">#</a> defineProperty()</h3> <p><code>defineProperty</code>方法拦截了<code>Object.defineProperty</code>操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    435. 

  435.   <span class="token function">defineProperty</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> descriptor</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    436. 

  436.     <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    437. 

  437.   <span class="token punctuation">}</span>
    438. 

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

  439. <span class="token keyword">var</span> target <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    440. 

  440. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    441. 

  441. proxy<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'bar'</span> <span class="token comment">// 不会生效</span>
    442. 

  442. </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>defineProperty</code>方法返回<code>false</code>，导致添加新属性总是无效。</p> <p>注意，如果目标对象不可扩展（non-extensible），则<code>defineProperty</code>不能增加目标对象上不存在的属性，否则会报错。另外，如果目标对象的某个属性不可写（writable）或不可配置（configurable），则<code>defineProperty</code>方法不得改变这两个设置。</p> <h3 id="getownpropertydescriptor"><a href="#getownpropertydescriptor" class="header-anchor">#</a> getOwnPropertyDescriptor()</h3> <p><code>getOwnPropertyDescriptor</code>方法拦截<code>Object.getOwnPropertyDescriptor()</code>，返回一个属性描述对象或者<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    443. 

  443.   <span class="token function">getOwnPropertyDescriptor</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    444. 

  444.     <span class="token keyword">if</span> <span class="token punctuation">(</span>key<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token string">'_'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    445. 

  445.       <span class="token keyword">return</span><span class="token punctuation">;</span>
    446. 

  446.     <span class="token punctuation">}</span>
    447. 

  447.     <span class="token keyword">return</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    448. 

  448.   <span class="token punctuation">}</span>
    449. 

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

  450. <span class="token keyword">var</span> target <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 string">'bar'</span><span class="token punctuation">,</span> <span class="token literal-property property">baz</span><span class="token operator">:</span> <span class="token string">'tar'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    451. 

  451. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    452. 

  452. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>proxy<span class="token punctuation">,</span> <span class="token string">'wat'</span><span class="token punctuation">)</span>
    453. 

  453. <span class="token comment">// undefined</span>
    454. 

  454. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>proxy<span class="token punctuation">,</span> <span class="token string">'_foo'</span><span class="token punctuation">)</span>
    455. 

  455. <span class="token comment">// undefined</span>
    456. 

  456. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>proxy<span class="token punctuation">,</span> <span class="token string">'baz'</span><span class="token punctuation">)</span>
    457. 

  457. <span class="token comment">// { value: 'tar', writable: true, enumerable: true, configurable: true }</span>
    458. 

  458. </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>上面代码中，<code>handler.getOwnPropertyDescriptor</code>方法对于第一个字符为下划线的属性名会返回<code>undefined</code>。</p> <h3 id="getprototypeof"><a href="#getprototypeof" class="header-anchor">#</a> getPrototypeOf()</h3> <p><code>getPrototypeOf</code>方法主要用来拦截获取对象原型。具体来说，拦截下面这些操作。</p> <ul><li><code>Object.prototype.__proto__</code></li> <li><code>Object.prototype.isPrototypeOf()</code></li> <li><code>Object.getPrototypeOf()</code></li> <li><code>Reflect.getPrototypeOf()</code></li> <li><code>instanceof</code></li></ul> <p>下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> proto <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    459. 

  459. <span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    460. 

  460.   <span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    461. 

  461.     <span class="token keyword">return</span> proto<span class="token punctuation">;</span>
    462. 

  462.   <span class="token punctuation">}</span>
    463. 

  463. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    464. 

  464. Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span> <span class="token operator">===</span> proto <span class="token comment">// true</span>
    465. 

  465. </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>getPrototypeOf</code>方法拦截<code>Object.getPrototypeOf()</code>，返回<code>proto</code>对象。</p> <p>注意，<code>getPrototypeOf</code>方法的返回值必须是对象或者<code>null</code>，否则报错。另外，如果目标对象不可扩展（non-extensible）， <code>getPrototypeOf</code>方法必须返回目标对象的原型对象。</p> <h3 id="isextensible"><a href="#isextensible" class="header-anchor">#</a> isExtensible()</h3> <p><code>isExtensible</code>方法拦截<code>Object.isExtensible</code>操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    466. 

  466.   <span class="token function-variable function">isExtensible</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    467. 

  467.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">&quot;called&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    468. 

  468.     <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    469. 

  469.   <span class="token punctuation">}</span>
    470. 

  470. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    471. 

  471. 

  472. Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span>
    473. 

  473. <span class="token comment">// &quot;called&quot;</span>
    474. 

  474. <span class="token comment">// true</span>
    475. 

  475. </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>上面代码设置了<code>isExtensible</code>方法，在调用<code>Object.isExtensible</code>时会输出<code>called</code>。</p> <p>注意，该方法只能返回布尔值，否则返回值会被自动转为布尔值。</p> <p>这个方法有一个强限制，它的返回值必须与目标对象的<code>isExtensible</code>属性保持一致，否则就会抛出错误。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span> <span class="token operator">===</span> Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>target<span class="token punctuation">)</span>
    476. 

  476. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    477. 

  477.   <span class="token function-variable function">isExtensible</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    478. 

  478.     <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    479. 

  479.   <span class="token punctuation">}</span>
    480. 

  480. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    481. 

  481. 

  482. Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span>
    483. 

  483. <span class="token comment">// Uncaught TypeError: 'isExtensible' on proxy: trap result does not reflect extensibility of proxy target (which is 'true')</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></div></div><h3 id="ownkeys"><a href="#ownkeys" class="header-anchor">#</a> ownKeys()</h3> <p><code>ownKeys</code>方法用来拦截对象自身属性的读取操作。具体来说，拦截以下操作。</p> <ul><li><code>Object.getOwnPropertyNames()</code></li> <li><code>Object.getOwnPropertySymbols()</code></li> <li><code>Object.keys()</code></li> <li><code>for...in</code>循环</li></ul> <p>下面是拦截<code>Object.keys()</code>的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> target <span class="token operator">=</span> <span class="token punctuation">{</span>
    485. 

  485.   <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    486. 

  486.   <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
    487. 

  487.   <span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token number">3</span>
    488. 

  488. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    489. 

  489. 

  490. <span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    491. 

  491.   <span class="token function">ownKeys</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    492. 

  492.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    493. 

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

  494. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    495. 

  495. 

  496. <span class="token keyword">let</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    497. 

  497. 

  498. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span>
    499. 

  499. <span class="token comment">// [ 'a' ]</span>
    500. 

  500. </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>上面代码拦截了对于<code>target</code>对象的<code>Object.keys()</code>操作，只返回<code>a</code>、<code>b</code>、<code>c</code>三个属性之中的<code>a</code>属性。</p> <p>下面的例子是拦截第一个字符为下划线的属性名。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> target <span class="token operator">=</span> <span class="token punctuation">{</span>
    501. 

  501.   <span class="token literal-property property">_bar</span><span class="token operator">:</span> <span class="token string">'foo'</span><span class="token punctuation">,</span>
    502. 

  502.   <span class="token literal-property property">_prop</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">,</span>
    503. 

  503.   <span class="token literal-property property">prop</span><span class="token operator">:</span> <span class="token string">'baz'</span>
    504. 

  504. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    505. 

  505. 

  506. <span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    507. 

  507.   <span class="token function">ownKeys</span> <span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    508. 

  508.     <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>target<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token parameter">key</span> <span class="token operator">=&gt;</span> key<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">!==</span> <span class="token string">'_'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    509. 

  509.   <span class="token punctuation">}</span>
    510. 

  510. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    511. 

  511. 

  512. <span class="token keyword">let</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    513. 

  513. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    514. 

  514.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>target<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    515. 

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

  516. <span class="token comment">// &quot;baz&quot;</span>
    517. 

  517. </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>注意，使用<code>Object.keys</code>方法时，有三类属性会被<code>ownKeys</code>方法自动过滤，不会返回。</p> <ul><li>目标对象上不存在的属性</li> <li>属性名为 Symbol 值</li> <li>不可遍历（<code>enumerable</code>）的属性</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> target <span class="token operator">=</span> <span class="token punctuation">{</span>
    518. 

  518.   <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    519. 

  519.   <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
    520. 

  520.   <span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span>
    521. 

  521.   <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span><span class="token function">for</span><span class="token punctuation">(</span><span class="token string">'secret'</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token string">'4'</span><span class="token punctuation">,</span>
    522. 

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

  523. 

  524. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> <span class="token string">'key'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    525. 

  525.   <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    526. 

  526.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    527. 

  527.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    528. 

  528.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'static'</span>
    529. 

  529. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    530. 

  530. 

  531. <span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    532. 

  532.   <span class="token function">ownKeys</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    533. 

  533.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'d'</span><span class="token punctuation">,</span> Symbol<span class="token punctuation">.</span><span class="token function">for</span><span class="token punctuation">(</span><span class="token string">'secret'</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token string">'key'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    534. 

  534.   <span class="token punctuation">}</span>
    535. 

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

  536. 

  537. <span class="token keyword">let</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    538. 

  538. 

  539. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span>
    540. 

  540. <span class="token comment">// ['a']</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></div></div><p>上面代码中，<code>ownKeys</code>方法之中，显式返回不存在的属性（<code>d</code>）、Symbol 值（<code>Symbol.for('secret')</code>）、不可遍历的属性（<code>key</code>），结果都被自动过滤掉。</p> <p><code>ownKeys</code>方法还可以拦截<code>Object.getOwnPropertyNames()</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    542. 

  542.   <span class="token function-variable function">ownKeys</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    543. 

  543.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token string">'a'</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>
    544. 

  544.   <span class="token punctuation">}</span>
    545. 

  545. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    546. 

  546. 

  547. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span>
    548. 

  548. <span class="token comment">// [ 'a', 'b', 'c' ]</span>
    549. 

  549. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p><code>for...in</code>循环也受到<code>ownKeys</code>方法的拦截。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">hello</span><span class="token operator">:</span> <span class="token string">'world'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    550. 

  550. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    551. 

  551.   <span class="token function-variable function">ownKeys</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>
    552. 

  552.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    553. 

  553.   <span class="token punctuation">}</span>
    554. 

  554. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    555. 

  555. 

  556. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">in</span> proxy<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    557. 

  557.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 没有任何输出</span>
    558. 

  558. <span class="token punctuation">}</span>
    559. 

  559. </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>ownkeys</code>指定只返回<code>a</code>和<code>b</code>属性，由于<code>obj</code>没有这两个属性，因此<code>for...in</code>循环不会有任何输出。</p> <p><code>ownKeys</code>方法返回的数组成员，只能是字符串或 Symbol 值。如果有其他类型的值，或者返回的根本不是数组，就会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><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>
    560. 

  560. 

  561. <span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    562. 

  562.   <span class="token function-variable function">ownKeys</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    563. 

  563.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token number">123</span><span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">null</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><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    564. 

  564.   <span class="token punctuation">}</span>
    565. 

  565. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    566. 

  566. 

  567. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span>
    568. 

  568. <span class="token comment">// Uncaught TypeError: 123 is not a valid property name</span>
    569. 

  569. </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>ownKeys</code>方法虽然返回一个数组，但是每一个数组成员都不是字符串或 Symbol 值，因此就报错了。</p> <p>如果目标对象自身包含不可配置的属性，则该属性必须被<code>ownKeys</code>方法返回，否则报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><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>
    570. 

  570. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    571. 

  571.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    572. 

  572.   <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    573. 

  573.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">10</span> <span class="token punctuation">}</span>
    574. 

  574. <span class="token punctuation">)</span><span class="token punctuation">;</span>
    575. 

  575. 

  576. <span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    577. 

  577.   <span class="token function-variable function">ownKeys</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    578. 

  578.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    579. 

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

  580. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    581. 

  581. 

  582. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span>
    583. 

  583. <span class="token comment">// Uncaught TypeError: 'ownKeys' on proxy: trap result did not include 'a'</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></div></div><p>上面代码中，<code>obj</code>对象的<code>a</code>属性是不可配置的，这时<code>ownKeys</code>方法返回的数组之中，必须包含<code>a</code>，否则会报错。</p> <p>另外，如果目标对象是不可扩展的（non-extensible），这时<code>ownKeys</code>方法返回的数组之中，必须包含原对象的所有属性，且不能包含多余的属性，否则报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    585. 

  585.   <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span>
    586. 

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

  587. 

  588. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    589. 

  589. 

  590. <span class="token keyword">var</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    591. 

  591.   <span class="token function-variable function">ownKeys</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    592. 

  592.     <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    593. 

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

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

  595. 

  596. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span>
    597. 

  597. <span class="token comment">// Uncaught TypeError: 'ownKeys' on proxy: trap returned extra keys but proxy target is non-extensible</span>
    598. 

  598. </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>obj</code>对象是不可扩展的，这时<code>ownKeys</code>方法返回的数组之中，包含了<code>obj</code>对象的多余属性<code>b</code>，所以导致了报错。</p> <h3 id="preventextensions"><a href="#preventextensions" class="header-anchor">#</a> preventExtensions()</h3> <p><code>preventExtensions</code>方法拦截<code>Object.preventExtensions()</code>。该方法必须返回一个布尔值，否则会被自动转为布尔值。</p> <p>这个方法有一个限制，只有目标对象不可扩展时（即<code>Object.isExtensible(proxy)</code>为<code>false</code>），<code>proxy.preventExtensions</code>才能返回<code>true</code>，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    599. 

  599.   <span class="token function-variable function">preventExtensions</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    600. 

  600.     <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    601. 

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

  602. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    603. 

  603. 

  604. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span>
    605. 

  605. <span class="token comment">// Uncaught TypeError: 'preventExtensions' on proxy: trap returned truish but the proxy target is extensible</span>
    606. 

  606. </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>proxy.preventExtensions</code>方法返回<code>true</code>，但这时<code>Object.isExtensible(proxy)</code>会返回<code>true</code>，因此报错。</p> <p>为了防止出现这个问题，通常要在<code>proxy.preventExtensions</code>方法里面，调用一次<code>Object.preventExtensions</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    607. 

  607.   <span class="token function-variable function">preventExtensions</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    608. 

  608.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'called'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    609. 

  609.     Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>target<span class="token punctuation">)</span><span class="token punctuation">;</span>
    610. 

  610.     <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    611. 

  611.   <span class="token punctuation">}</span>
    612. 

  612. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    613. 

  613. 

  614. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>proxy<span class="token punctuation">)</span>
    615. 

  615. <span class="token comment">// &quot;called&quot;</span>
    616. 

  616. <span class="token comment">// Proxy {}</span>
    617. 

  617. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h3 id="setprototypeof"><a href="#setprototypeof" class="header-anchor">#</a> setPrototypeOf()</h3> <p><code>setPrototypeOf</code>方法主要用来拦截<code>Object.setPrototypeOf</code>方法。</p> <p>下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    618. 

  618.   <span class="token function">setPrototypeOf</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> proto</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    619. 

  619.     <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">'Changing the prototype is forbidden'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    620. 

  620.   <span class="token punctuation">}</span>
    621. 

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

  622. <span class="token keyword">var</span> proto <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    623. 

  623. <span class="token keyword">var</span> <span class="token function-variable function">target</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    624. 

  624. <span class="token keyword">var</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    625. 

  625. Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>proxy<span class="token punctuation">,</span> proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    626. 

  626. <span class="token comment">// Error: Changing the prototype is forbidden</span>
    627. 

  627. </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，只要修改<code>target</code>的原型对象，就会报错。</p> <p>注意，该方法只能返回布尔值，否则会被自动转为布尔值。另外，如果目标对象不可扩展（non-extensible），<code>setPrototypeOf</code>方法不得改变目标对象的原型。</p> <h2 id="proxy-revocable"><a href="#proxy-revocable" class="header-anchor">#</a> Proxy.revocable()</h2> <p><code>Proxy.revocable</code>方法返回一个可取消的 Proxy 实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> target <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    628. 

  628. <span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    629. 

  629. 

  630. <span class="token keyword">let</span> <span class="token punctuation">{</span>proxy<span class="token punctuation">,</span> revoke<span class="token punctuation">}</span> <span class="token operator">=</span> Proxy<span class="token punctuation">.</span><span class="token function">revocable</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    631. 

  631. 

  632. proxy<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    633. 

  633. proxy<span class="token punctuation">.</span>foo <span class="token comment">// 123</span>
    634. 

  634. 

  635. <span class="token function">revoke</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    636. 

  636. proxy<span class="token punctuation">.</span>foo <span class="token comment">// TypeError: Revoked</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></div></div><p><code>Proxy.revocable</code>方法返回一个对象，该对象的<code>proxy</code>属性是<code>Proxy</code>实例，<code>revoke</code>属性是一个函数，可以取消<code>Proxy</code>实例。上面代码中，当执行<code>revoke</code>函数之后，再访问<code>Proxy</code>实例，就会抛出一个错误。</p> <p><code>Proxy.revocable</code>的一个使用场景是，目标对象不允许直接访问，必须通过代理访问，一旦访问结束，就收回代理权，不允许再次访问。</p> <h2 id="this-问题"><a href="#this-问题" class="header-anchor">#</a> this 问题</h2> <p>虽然 Proxy 可以代理针对目标对象的访问，但它不是目标对象的透明代理，即不做任何拦截的情况下，也无法保证与目标对象的行为一致。主要原因就是在 Proxy 代理的情况下，目标对象内部的<code>this</code>关键字会指向 Proxy 代理。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token punctuation">{</span>
    638. 

  638.   <span class="token function-variable function">m</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>
    639. 

  639.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">===</span> proxy<span class="token punctuation">)</span><span class="token punctuation">;</span>
    640. 

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

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

  642. <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    643. 

  643. 

  644. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    645. 

  645. 

  646. target<span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    647. 

  647. proxy<span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// true</span>
    648. 

  648. </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，一旦<code>proxy</code>代理<code>target.m</code>，后者内部的<code>this</code>就是指向<code>proxy</code>，而不是<code>target</code>。</p> <p>下面是一个例子，由于<code>this</code>指向的变化，导致 Proxy 无法代理目标对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> _name <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">WeakMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    649. 

  649. 

  650. <span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
    651. 

  651.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    652. 

  652.     _name<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
    653. 

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

  654.   <span class="token keyword">get</span> <span class="token function">name</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    655. 

  655.     <span class="token keyword">return</span> _name<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    656. 

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

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

  658. 

  659. <span class="token keyword">const</span> jane <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">'Jane'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    660. 

  660. jane<span class="token punctuation">.</span>name <span class="token comment">// 'Jane'</span>
    661. 

  661. 

  662. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>jane<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>
    663. 

  663. proxy<span class="token punctuation">.</span>name <span class="token comment">// undefined</span>
    664. 

  664. </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>上面代码中，目标对象<code>jane</code>的<code>name</code>属性，实际保存在外部<code>WeakMap</code>对象<code>_name</code>上面，通过<code>this</code>键区分。由于通过<code>proxy.name</code>访问时，<code>this</code>指向<code>proxy</code>，导致无法取到值，所以返回<code>undefined</code>。</p> <p>此外，有些原生对象的内部属性，只有通过正确的<code>this</code>才能拿到，所以 Proxy 也无法代理这些原生对象的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    665. 

  665. <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    666. 

  666. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    667. 

  667. 

  668. proxy<span class="token punctuation">.</span><span class="token function">getDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    669. 

  669. <span class="token comment">// TypeError: this is not a Date object.</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></div></div><p>上面代码中，<code>getDate</code>方法只能在<code>Date</code>对象实例上面拿到，如果<code>this</code>不是<code>Date</code>对象实例就会报错。这时，<code>this</code>绑定原始对象，就可以解决这个问题。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'2015-01-01'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    671. 

  671. <span class="token keyword">const</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
    672. 

  672.   <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> prop<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    673. 

  673.     <span class="token keyword">if</span> <span class="token punctuation">(</span>prop <span class="token operator">===</span> <span class="token string">'getDate'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    674. 

  674.       <span class="token keyword">return</span> target<span class="token punctuation">.</span><span class="token function">getDate</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span>target<span class="token punctuation">)</span><span class="token punctuation">;</span>
    675. 

  675.     <span class="token punctuation">}</span>
    676. 

  676.     <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> prop<span class="token punctuation">)</span><span class="token punctuation">;</span>
    677. 

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

  678. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    679. 

  679. <span class="token keyword">const</span> proxy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    680. 

  680. 

  681. proxy<span class="token punctuation">.</span><span class="token function">getDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    682. 

  682. </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><h2 id="实例-web-服务的客户端"><a href="#实例-web-服务的客户端" class="header-anchor">#</a> 实例：Web 服务的客户端</h2> <p>Proxy 对象可以拦截目标对象的任意属性，这使得它很合适用来写 Web 服务的客户端。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> service <span class="token operator">=</span> <span class="token function">createWebService</span><span class="token punctuation">(</span><span class="token string">'http://example.com/data'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    683. 

  683. 

  684. service<span class="token punctuation">.</span><span class="token function">employees</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">json</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    685. 

  685.   <span class="token keyword">const</span> employees <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>json<span class="token punctuation">)</span><span class="token punctuation">;</span>
    686. 

  686.   <span class="token comment">// ···</span>
    687. 

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

  688. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码新建了一个 Web 服务的接口，这个接口返回各种数据。Proxy 可以拦截这个对象的任意属性，所以不用为每一种数据写一个适配方法，只要写一个 Proxy 拦截就可以了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">createWebService</span><span class="token punctuation">(</span><span class="token parameter">baseUrl</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    689. 

  689.   <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</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>
    690. 

  690.     <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> propKey<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    691. 

  691.       <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">httpGet</span><span class="token punctuation">(</span>baseUrl <span class="token operator">+</span> <span class="token string">'/'</span> <span class="token operator">+</span> propKey<span class="token punctuation">)</span><span class="token punctuation">;</span>
    692. 

  692.     <span class="token punctuation">}</span>
    693. 

  693.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    694. 

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

  695. </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>同理，Proxy 也可以用来实现数据库的 ORM 层。</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/14.Proxy.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/0c21dae358fca16b/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Set 和 Map 数据结构</div></a> <a href="/blog/pages/74de3e45e4491e95/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Reflect</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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