Revision 976167 of Meta programming

<div>{{jsSidebar("JavaScript Guide")}} {{Previous("Web/JavaScript/Guide/Iterators_and_Generators")}}</div>

<p class="summary">从ECMAScript6开始，JavaScript添加了{{jsxref("Proxy")}}和 {{jsxref("Reflect")}}对象，允许你解释和定义客制化基础语言操作行为（比如，属性查找，赋值，枚举，函数调用，等等）。通过这两个对象，你可以在JavaScript元级别（meta level）编程。</p>

<h2 id="Proxies">代理（Proxies）</h2>

<p>从ECMAScript6开始引进，{{jsxref("Proxy")}}对象允许你解释特定操作和实现客制化行为。例如得到一个对象的属性：</p>

<pre class="brush: js">
var handler = {
  get: function(target, name){
    return name in target ? target[name] : 42;
}};
var p = new Proxy({}, handler);
p.a = 1;
console.log(p.a, p.b); // 1, 42
</pre>

<p>The <code>Proxy</code> object defines a <em>target</em> (an object here) and a <em>handler</em> object in which a <code>get</code> <em>trap</em> is implemented. Here, an object that is proxied will not return <code>undefined</code> when getting undefined properties, but will instead return the number 42.</p>

<div class="note">
<p>Additional examples are available on the {{jsxref("Proxy")}} reference page.</p>
</div>

<h3 id="Terminology">Terminology</h3>

<p>The following terms are used when talking about the functionality of proxies.</p>

<dl>
 <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler">handler</a></dt>
 <dd>Placeholder object which contains traps.</dd>
 <dt>traps</dt>
 <dd>The methods that provide property access. This is analogous to the concept of traps in operating systems.</dd>
 <dt>target</dt>
 <dd>Object which the proxy virtualizes. It is often used as storage backend for the proxy. Invariants (semantics that remain unchanged) regarding object non-extensibility or non-configurable properties are verified against the target.</dd>
 <dt>invariants</dt>
 <dd>Semantics that remain unchanged when implementing custom operations are called <em>invariants</em>. If you violate the invariants of a handler, a {{jsxref("TypeError")}} will be thrown.</dd>
</dl>

<h2 id="Handlers_and_traps">Handlers and traps</h2>

<p>The following table summarizes the available traps available to <code>Proxy</code> objects. See the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler">reference pages</a> for detailed explanations and examples.</p>

<table class="standard-table">
 <thead>
  <tr>
   <th>Handler / trap</th>
   <th>Interceptions</th>
   <th>Invariants</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/getPrototypeOf", "handler.getPrototypeOf()")}}</td>
   <td>{{jsxref("Object.getPrototypeOf()")}}<br />
    {{jsxref("Reflect.getPrototypeOf()")}}<br />
    {{jsxref("Object/proto", "__proto__")}}<br />
    {{jsxref("Object.prototype.isPrototypeOf()")}}<br />
    {{jsxref("Operators/instanceof", "instanceof")}}</td>
   <td><code>getPrototypeOf</code> method must return an object or <code>null</code>.<br />
    <br />
    If <code>target</code> is not extensible, <code>Object.getPrototypeOf(proxy)</code> method must return the same value as <code>Object.getPrototypeOf(target)</code>.</td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/setPrototypeOf", "handler.setPrototypeOf()")}}</td>
   <td>{{jsxref("Object.setPrototypeOf()")}}<br />
    {{jsxref("Reflect.setPrototypeOf()")}}</td>
   <td>
    <p>If <code>target</code> is not extensible, the <code>prototype</code> parameter must be the same value as <code>Object.getPrototypeOf(target)</code>.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/isExtensible", "handler.isExtensible()")}}</td>
   <td>
    <p>{{jsxref("Object.isExtensible()")}}</p>

    <p>{{jsxref("Reflect.isExtensible()")}}</p>
   </td>
   <td>
    <p><code>Object.isExtensible(proxy)</code> must return the same value as <code>Object.isExtensible(target)</code>.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/preventExtensions", "handler.preventExtensions()")}}</td>
   <td>
    <p>{{jsxref("Object.preventExtensions()")}}</p>

    <p>{{jsxref("Reflect.preventExtensions()")}}</p>
   </td>
   <td>
    <p><code>Object.preventExtensions(proxy)</code> only returns <code>true</code> if <code>Object.isExtensible(proxy)</code> is <code>false</code>.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/getOwnPropertyDescriptor", "handler.getOwnPropertyDescriptor()")}}</td>
   <td>
    <p>{{jsxref("Object.getOwnPropertyDescriptor()")}}</p>

    <p>{{jsxref("Reflect.getOwnPropertyDescriptor()")}}</p>
   </td>
   <td>
    <p><code>getOwnPropertyDescriptor</code> must return an object or <code>undefined</code>.</p>

    <p>A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.</p>

    <p>A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.</p>

    <p>A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.</p>

    <p>A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.</p>

    <p>The result of <code>Object.getOwnPropertyDescriptor(target)</code> can be applied to the target object using <code>Object.defineProperty</code> and will not throw an exception.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/defineProperty", "handler.defineProperty()")}}</td>
   <td>
    <p>{{jsxref("Object.defineProperty()")}}</p>

    <p>{{jsxref("Reflect.defineProperty()")}}</p>
   </td>
   <td>
    <p>A property cannot be added, if the target object is not extensible.</p>

    <p>A property cannot be added as or modified to be non-configurable, if it does not exists as a non-configurable own property of the target object.</p>

    <p>A property may not be non-configurable, if a corresponding configurable property of the target object exists.</p>

    <p>If a property has a corresponding target object property then <code>Object.defineProperty(target, prop, descriptor)</code> will not throw an exception.</p>

    <p>In strict mode, a <code>false</code> return value from the <code>defineProperty</code> handler will throw a {{jsxref("TypeError")}} exception.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/has", "handler.has()")}}</td>
   <td>
    <p>Property query: <code>foo in proxy</code></p>

    <p>Inherited property query: <code>foo in Object.create(proxy)</code></p>

    <p>{{jsxref("Reflect.has()")}}</p>
   </td>
   <td>
    <p>A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.</p>

    <p>A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/get", "handler.get()")}}</td>
   <td>
    <p>Property access: <code>proxy[foo]</code>and <code>proxy.bar</code></p>

    <p>Inherited property access: <code>Object.create(proxy)[foo]</code></p>

    <p>{{jsxref("Reflect.get()")}}</p>
   </td>
   <td>
    <p>The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable data property.</p>

    <p>The value reported for a property must be undefined if the corresponding target object property is non-configurable accessor property that has undefined as its [[Get]] attribute.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/set", "handler.set()")}}</td>
   <td>
    <p>Property assignment: <code>proxy[foo] = bar</code> and <code>proxy.foo = bar</code><br />
     <br />
     Inherited property assignment: <code>Object.create(proxy)[foo] = bar</code><br />
     <br />
     {{jsxref("Reflect.set()")}}</p>
   </td>
   <td>
    <p>Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable data property.</p>

    <p>Cannot set the value of a property if the corresponding target object property is a non-configurable accessor property that has <code>undefined</code> as its [[Set]] attribute.</p>

    <p>In strict mode, a <code>false</code> return value from the <code>set</code> handler will throw a {{jsxref("TypeError")}} exception.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/deleteProperty", "handler.deleteProperty()")}}</td>
   <td>
    <p>Property deletion: <code>delete proxy[foo]</code> and <code>delete proxy.foo</code><br />
     <br />
     {{jsxref("Reflect.deleteProperty()")}}</p>
   </td>
   <td>A property cannot be deleted, if it exists as a non-configurable own property of the target object.</td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/enumerate", "handler.enumerate()")}}</td>
   <td>
    <p>Property enumeration / for...in: <code>for (var name in proxy) {...}</code><br />
     <br />
     {{jsxref("Reflect.enumerate()")}}</p>
   </td>
   <td>The <code>enumerate</code> method must return an object.</td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/ownKeys", "handler.ownKeys()")}}</td>
   <td>
    <p>{{jsxref("Object.getOwnPropertyNames()")}}<br />
     {{jsxref("Object.getOwnPropertySymbols()")}}<br />
     {{jsxref("Object.keys()")}}<br />
     {{jsxref("Reflect.ownKeys()")}}</p>
   </td>
   <td>
    <p>The result of <code>ownKeys</code> is a List.<br />
     <br />
     The Type of each result List element is either {{jsxref("String")}} or {{jsxref("Symbol")}}.<br />
     <br />
     The result List must contain the keys of all non-configurable own properties of the target object.<br />
     <br />
     If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.</p>
   </td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/apply", "handler.apply()")}}</td>
   <td>
    <p><code>proxy(..args)</code><br />
     <br />
     {{jsxref("Function.prototype.apply()")}} and {{jsxref("Function.prototype.call()")}}<br />
     <br />
     {{jsxref("Reflect.apply()")}}</p>
   </td>
   <td>There are no invariants for the <code>handler.apply</code> method.</td>
  </tr>
  <tr>
   <td>{{jsxref("Global_Objects/Proxy/handler/construct", "handler.construct()")}}</td>
   <td>
    <p><code>new proxy(...args)</code><br />
     {{jsxref("Reflect.construct()")}}</p>
   </td>
   <td>
    <p>The result must be an <code>Object</code>.</p>
   </td>
  </tr>
 </tbody>
</table>

<h2 id="Revocable_Proxy">Revocable <code>Proxy</code></h2>

<p>The {{jsxref("Proxy.revocable()")}} method is used to create a revocable <code>Proxy</code> object. This means that the proxy can be revoked via the function <code>revoke</code> and switches the proxy off. Afterwards, any operation leads on the proxy leads to a {{jsxref("TypeError")}}.</p>

<pre class="brush: js">
var revocable = Proxy.revocable({}, {
  get: function(target, name) {
    return "[[" + name + "]]";
  }
});
var proxy = revocable.proxy;
console.log(proxy.foo); // "[[foo]]"

revocable.revoke();

console.log(proxy.foo); // TypeError is thrown
proxy.foo = 1           // TypeError again
delete proxy.foo;       // still TypeError
typeof proxy            // "object", typeof doesn't trigger any trap</pre>

<h2 id="Reflection">Reflection</h2>

<p>{{jsxref("Reflect")}} is a built-in object that provides methods for interceptable JavaScript operations. The methods are the same as those of the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler">proxy handlers</a>. <code>Reflect</code> It is not a function object.</p>

<p><code>Reflect</code> helps with forwarding default operations from the handler to the target. Note that <code>Reflect</code> is not implemented in Firefox yet.</p>

<p>With {{jsxref("Reflect.has()")}} for example, you get the <a href="/en-US/docs/Web/JavaScript/Reference/Operators/in"><code>in</code> operator</a> as a function:</p>

<pre class="brush: js">
Reflect.has(Object, "assign"); // true
</pre>

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