uTools-Manuals/docs/javascript/Reference/Global_Objects/Proxy/handler/get.html

<article id="wikiArticle">
<div></div>
<p><strong><code>handler.get()</code></strong> 方法用于拦截对象的读取属性操作。</p>
<h2 id="语法">语法</h2>
<pre><code  class="language-javascript">var p = new Proxy(target, {
  get: function(target, property, receiver) {
  }
});
</code></pre>
<h3 id="参数">参数</h3>
<p>以下是传递给get方法的参数，<code>this上下文绑定在</code>handler对象上.</p>
<dl>
<dt><code>target</code></dt>
<dd>目标对象。</dd>
<dt><code>property</code></dt>
<dd>被获取的属性名。</dd>
<dt><code>receiver</code></dt>
<dd>Proxy或者继承Proxy的对象</dd>
</dl>
<h3 id="返回值">返回值</h3>
<p>get方法可以返回任何值。</p>
<h2 id="描述">描述</h2>
<p><strong><code>handler.get</code></strong> 方法用于拦截对象的读取属性操作。</p>
<h3 id="拦截">拦截</h3>
<p>该方法会拦截目标对象的以下操作:</p>
<ul>
<li>访问属性: <code>proxy[foo]和</code> <code>proxy.bar</code></li>
<li>访问原型链上的属性: <code>Object.create(proxy)[foo]</code></li>
<li><a href="Reference/Global_Objects/Reflect/get" title="Reflect.get() 方法的工作方式，就像从 object (target[propertyKey]) 中获取属性，但它是作为一个函数执行的。"><code>Reflect.get()</code></a></li>
</ul>
<h3 id="约束">约束</h3>
<p>如果违背了以下的约束，proxy会抛出 <a href="Reference/Global_Objects/TypeError" title="TypeError（类型错误） 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a>:</p>
<ul>
<li>如果要访问的目标属性是不可写以及不可配置的，则返回的值必须与该目标属性的值相同。</li>
<li>如果要访问的目标属性没有配置访问方法，即get方法是undefined的，则返回值必须为undefined。</li>
</ul>
<h2 id="示例">示例</h2>
<p>以下代码演示如何拦截属性值的读取操作。</p>
<pre><code  class="language-javascript">var p = new Proxy({}, {
  get: function(target, prop, receiver) {
    console.log("called: " + prop);
    return 10;
  }
});

console.log(p.a); // "called: a"
                  // 10
</code></pre>
<p>以下代码演示违反约束的情况。</p>
<pre><code  class="language-javascript">var obj = {};
Object.defineProperty(obj, "a", { 
  configurable: false, 
  enumerable: false, 
  value: 10, 
  writable: false 
});

var p = new Proxy(obj, {
  get: function(target, prop) {
    return 20;
  }
});

p.a; //会抛出TypeError
</code></pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-proxy-object-internal-methods-and-internal-slots-get-p-receiver" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">[[Get]]</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.</td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-proxy-object-internal-methods-and-internal-slots-get-p-receiver" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">[[Get]]</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<div><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div></div>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><a href="/en-US/Firefox/Releases/18" title="Released on 2013-01-08.">18</a> (18)</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td>18.0 (18)</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="另见">另见</h2>
<ul>
<li><a href="Reference/Global_Objects/Proxy" title="Proxy 对象用于定义基本操作的自定义行为（如属性查找，赋值，枚举，函数调用等）。"><code>Proxy</code></a></li>
<li><a href="Reference/Global_Objects/Proxy/handler" title="处理器对象用来自定义代理对象的各种可代理操作。"><code>handler</code></a></li>
<li><a href="Reference/Global_Objects/Reflect/get" title="Reflect.get() 方法的工作方式，就像从 object (target[propertyKey]) 中获取属性，但它是作为一个函数执行的。"><code>Reflect.get()</code></a></li>
</ul>
</article>