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+---
+title: Proxy
+slug: Web/JavaScript/Reference/Global_Objects/Proxy
+tags:
+  - Class
+  - ECMAScript 2015
+  - JavaScript
+  - Proxy
+translation_of: Web/JavaScript/Reference/Global_Objects/Proxy
+---
+<div>
+<div>{{JSRef}}</div>
+</div>
+
+<p><strong>Proxy</strong> 物件被使用於定義基本操作的自定行為（例如：尋找屬性、賦值、列舉、函式調用等等）。</p>
+
+<h2 id="術語">術語</h2>
+
+<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>
+</dl>
+
+<h2 id="語法">語法</h2>
+
+<pre class="syntaxbox notranslate">var p = new Proxy(target, handler);
+</pre>
+
+<h3 id="參數">參數</h3>
+
+<dl>
+ <dt><code>target</code></dt>
+ <dd>A target object (can be any sort of object, including a native array, a function or even another proxy) to wrap with <code>Proxy</code>.</dd>
+ <dt><code>handler</code></dt>
+ <dd>An object whose properties are functions which define the behavior of the proxy when an operation is performed on it.</dd>
+</dl>
+
+<h2 id="方法">方法</h2>
+
+<dl>
+ <dt>{{jsxref("Proxy.revocable()")}}</dt>
+ <dd>Creates a revocable <code>Proxy</code> object.</dd>
+</dl>
+
+<h2 id="Methods_of_the_handler_object">Methods of the handler object</h2>
+
+<p>The handler object is a placeholder object which contains traps for <code>Proxy</code>.</p>
+
+<div>{{page('/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler', 'Methods') }}</div>
+
+<h2 id="範例">範例</h2>
+
+<h3 id="Basic_example">Basic example</h3>
+
+<p>In this simple example the number <code>37</code> gets returned as the default value when the property name is not in the object. It is using the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/get"><code>get</code></a> handler.</p>
+
+<pre class="brush: js notranslate">var handler = {
+    get: function(target, name) {
+        return name in target ?
+            target[name] :
+            37;
+    }
+};
+
+var p = new Proxy({}, handler);
+p.a = 1;
+p.b = undefined;
+
+console.log(p.a, p.b); // 1, undefined
+console.log('c' in p, p.c); // false, 37
+</pre>
+
+<h3 id="No-op_forwarding_proxy">No-op forwarding proxy</h3>
+
+<p>In this example, we are using a native JavaScript object to which our proxy will forward all operations that are applied to it.</p>
+
+<pre class="brush: js notranslate">var target = {};
+var p = new Proxy(target, {});
+
+p.a = 37; // operation forwarded to the target
+
+console.log(target.a); // 37. The operation has been properly forwarded
+</pre>
+
+<h3 id="Validation">Validation</h3>
+
+<p>With a <code>Proxy</code>, you can easily validate the passed value for an object. This example uses the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/set"><code>set</code></a> handler.</p>
+
+<pre class="brush: js notranslate">let validator = {
+  set: function(obj, prop, value) {
+    if (prop === 'age') {
+      if (!Number.isInteger(value)) {
+        throw new TypeError('The age is not an integer');
+      }
+      if (value &gt; 200) {
+        throw new RangeError('The age seems invalid');
+      }
+    }
+
+    // The default behavior to store the value
+    obj[prop] = value;
+
+    // Indicate success
+    return true;
+  }
+};
+
+let person = new Proxy({}, validator);
+
+person.age = 100;
+console.log(person.age); // 100
+person.age = 'young'; // Throws an exception
+person.age = 300; // Throws an exception</pre>
+
+<h3 id="Extending_constructor">Extending constructor</h3>
+
+<p>A function proxy could easily extend a constructor with a new constructor. This example uses the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/construct"><code>construct</code></a> and <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/apply"><code>apply</code></a> handlers.</p>
+
+<pre class="brush: js notranslate">function extend(sup, base) {
+  var descriptor = Object.getOwnPropertyDescriptor(
+    base.prototype, 'constructor'
+  );
+  base.prototype = Object.create(sup.prototype);
+  var handler = {
+    construct: function(target, args) {
+      var obj = Object.create(base.prototype);
+      this.apply(target, obj, args);
+      return obj;
+    },
+    apply: function(target, that, args) {
+      sup.apply(that, args);
+      base.apply(that, args);
+    }
+  };
+  var proxy = new Proxy(base, handler);
+  descriptor.value = proxy;
+  Object.defineProperty(base.prototype, 'constructor', descriptor);
+  return proxy;
+}
+
+var Person = function(name) {
+  this.name = name;
+};
+
+var Boy = extend(Person, function(name, age) {
+  this.age = age;
+});
+
+Boy.prototype.sex = 'M';
+
+var Peter = new Boy('Peter', 13);
+console.log(Peter.sex);  // "M"
+console.log(Peter.name); // "Peter"
+console.log(Peter.age);  // 13</pre>
+
+<h3 id="Manipulating_DOM_nodes">Manipulating DOM nodes</h3>
+
+<p>Sometimes you want to toggle the attribute or class name of two different elements. Here's how using the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/set"><code>set</code></a> handler.</p>
+
+<pre class="brush: js notranslate">let view = new Proxy({
+  selected: null
+},
+{
+  set: function(obj, prop, newval) {
+    let oldval = obj[prop];
+
+    if (prop === 'selected') {
+      if (oldval) {
+        oldval.setAttribute('aria-selected', 'false');
+      }
+      if (newval) {
+        newval.setAttribute('aria-selected', 'true');
+      }
+    }
+
+    // The default behavior to store the value
+    obj[prop] = newval;
+
+    // Indicate success
+    return true;
+  }
+});
+
+let i1 = view.selected = document.getElementById('item-1');
+console.log(i1.getAttribute('aria-selected')); // 'true'
+
+let i2 = view.selected = document.getElementById('item-2');
+console.log(i1.getAttribute('aria-selected')); // 'false'
+console.log(i2.getAttribute('aria-selected')); // 'true'</pre>
+
+<h3 id="Value_correction_and_an_extra_property">Value correction and an extra property</h3>
+
+<p>The <code>products</code> proxy object evaluates the passed value and convert it to an array if needed. The object also supports an extra property called <code>latestBrowser</code> both as a getter and a setter.</p>
+
+<pre class="brush: js notranslate">let products = new Proxy({
+  browsers: ['Internet Explorer', 'Netscape']
+},
+{
+  get: function(obj, prop) {
+    // An extra property
+    if (prop === 'latestBrowser') {
+      return obj.browsers[obj.browsers.length - 1];
+    }
+
+    // The default behavior to return the value
+    return obj[prop];
+  },
+  set: function(obj, prop, value) {
+    // An extra property
+    if (prop === 'latestBrowser') {
+      obj.browsers.push(value);
+      return true;
+    }
+
+    // Convert the value if it is not an array
+    if (typeof value === 'string') {
+      value = [value];
+    }
+
+    // The default behavior to store the value
+    obj[prop] = value;
+
+    // Indicate success
+    return true;
+  }
+});
+
+console.log(products.browsers); // ['Internet Explorer', 'Netscape']
+products.browsers = 'Firefox'; // pass a string (by mistake)
+console.log(products.browsers); // ['Firefox'] &lt;- no problem, the value is an array
+
+products.latestBrowser = 'Chrome';
+console.log(products.browsers); // ['Firefox', 'Chrome']
+console.log(products.latestBrowser); // 'Chrome'</pre>
+
+<h3 id="Finding_an_array_item_object_by_its_property">Finding an array item object by its property</h3>
+
+<p>This proxy extends an array with some utility features. As you see, you can flexibly "define" properties without using <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties"><code>Object.defineProperties</code></a>. This example can be adapted to find a table row by its cell. In that case, the target will be <a href="/en-US/docs/DOM/table.rows"><code>table.rows</code></a>.</p>
+
+<pre class="brush: js notranslate">let products = new Proxy([
+  { name: 'Firefox', type: 'browser' },
+  { name: 'SeaMonkey', type: 'browser' },
+  { name: 'Thunderbird', type: 'mailer' }
+],
+{
+  get: function(obj, prop) {
+    // The default behavior to return the value; prop is usually an integer
+    if (prop in obj) {
+      return obj[prop];
+    }
+
+    // Get the number of products; an alias of products.length
+    if (prop === 'number') {
+      return obj.length;
+    }
+
+    let result, types = {};
+
+    for (let product of obj) {
+      if (product.name === prop) {
+        result = product;
+      }
+      if (types[product.type]) {
+        types[product.type].push(product);
+      } else {
+        types[product.type] = [product];
+      }
+    }
+
+    // Get a product by name
+    if (result) {
+      return result;
+    }
+
+    // Get products by type
+    if (prop in types) {
+      return types[prop];
+    }
+
+    // Get product types
+    if (prop === 'types') {
+      return Object.keys(types);
+    }
+
+    return undefined;
+  }
+});
+
+console.log(products[0]); // { name: 'Firefox', type: 'browser' }
+console.log(products['Firefox']); // { name: 'Firefox', type: 'browser' }
+console.log(products['Chrome']); // undefined
+console.log(products.browser); // [{ name: 'Firefox', type: 'browser' }, { name: 'SeaMonkey', type: 'browser' }]
+console.log(products.types); // ['browser', 'mailer']
+console.log(products.number); // 3
+</pre>
+
+<h3 id="A_complete_traps_list_example">A complete <code>traps</code> list example</h3>
+
+<p>Now in order to create a complete sample <code>traps</code> list, for didactic purposes, we will try to proxify a <em>non native</em> object that is particularly suited to this type of operation: the <code>docCookies</code> global object created by <a href="https://developer.mozilla.org/en-US/docs/Web/API/Document/cookie/Simple_document.cookie_framework" title="https://developer.mozilla.org/en-US/docs/DOM/document.cookie#A_little_framework.3A_a_complete_cookies_reader.2Fwriter_with_full_unicode_support">the "little framework" published on the <code>document.cookie</code> page</a>.</p>
+
+<pre class="brush: js notranslate">/*
+  var docCookies = ... get the "docCookies" object here:
+  https://developer.mozilla.org/en-US/docs/DOM/document.cookie#A_little_framework.3A_a_complete_cookies_reader.2Fwriter_with_full_unicode_support
+*/
+
+var docCookies = new Proxy(docCookies, {
+  get: function (oTarget, sKey) {
+    return oTarget[sKey] || oTarget.getItem(sKey) || undefined;
+  },
+  set: function (oTarget, sKey, vValue) {
+    if (sKey in oTarget) { return false; }
+    return oTarget.setItem(sKey, vValue);
+  },
+  deleteProperty: function (oTarget, sKey) {
+    if (sKey in oTarget) { return false; }
+    return oTarget.removeItem(sKey);
+  },
+  enumerate: function (oTarget, sKey) {
+    return oTarget.keys();
+  },
+  ownKeys: function (oTarget, sKey) {
+    return oTarget.keys();
+  },
+  has: function (oTarget, sKey) {
+    return sKey in oTarget || oTarget.hasItem(sKey);
+  },
+  defineProperty: function (oTarget, sKey, oDesc) {
+    if (oDesc &amp;&amp; 'value' in oDesc) { oTarget.setItem(sKey, oDesc.value); }
+    return oTarget;
+  },
+  getOwnPropertyDescriptor: function (oTarget, sKey) {
+    var vValue = oTarget.getItem(sKey);
+    return vValue ? {
+      value: vValue,
+      writable: true,
+      enumerable: true,
+      configurable: false
+    } : undefined;
+  },
+});
+
+/* Cookies test */
+
+console.log(docCookies.my_cookie1 = 'First value');
+console.log(docCookies.getItem('my_cookie1'));
+
+docCookies.setItem('my_cookie1', 'Changed value');
+console.log(docCookies.my_cookie1);</pre>
+
+<h2 id="規範">規範</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>{{SpecName('ES2015', '#sec-proxy-objects', 'Proxy')}}</td>
+   <td>{{Spec2('ES2015')}}</td>
+   <td>Initial definition.</td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ESDraft', '#sec-proxy-objects', 'Proxy')}}</td>
+   <td>{{Spec2('ESDraft')}}</td>
+   <td></td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="瀏覽器相容性">瀏覽器相容性</h2>
+
+<p>{{Compat("javascript.builtins.Proxy", 2)}}</p>
+
+<h2 id="Gecko_specific_notes">Gecko specific notes</h2>
+
+<ul>
+ <li>At present, <code>Object.getPrototypeOf(proxy)</code> unconditionally returns <code>Object.getPrototypeOf(target)</code>, because the ES2015 getPrototypeOf trap is not yet implemented ({{bug(795904)}}, {{bug(888969)}}).</li>
+ <li><code>Array.isArray(proxy)</code> unconditionally returns <code>Array.isArray(target)</code> ({{bug(1096753)}}, {{bug(1111785)}}).</li>
+ <li><code>Object.prototype.toString.call(proxy)</code> unconditionally returns <code>Object.prototype.toString.call(target)</code>, because ES2015 Symbol.toStringTag is not yet implemented ({{bug(1114580)}}).</li>
+</ul>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><a class="external" href="https://www.youtube.com/watch?v=sClk6aB_CPk">"Proxies are awesome" Brendan Eich presentation at JSConf</a> (<a class="external" href="http://www.slideshare.net/BrendanEich/metaprog-5303821">slides</a>)</li>
+ <li><a class="external" href="http://wiki.ecmascript.org/doku.php?id=harmony:proxies">ECMAScript Harmony Proxy proposal page</a> and <a class="external" href="http://wiki.ecmascript.org/doku.php?id=harmony:proxies_semantics">ECMAScript Harmony proxy semantics page</a></li>
+ <li><a class="external" href="http://soft.vub.ac.be/~tvcutsem/proxies/">Tutorial on proxies</a></li>
+ <li><a href="/en-US/docs/JavaScript/Old_Proxy_API" title="/en-US/docs/JavaScript/Old_Proxy_API">SpiderMonkey specific Old Proxy API</a></li>
+ <li>{{jsxref("Object.watch()")}} is a non-standard feature but has been supported in Gecko for a long time.</li>
+</ul>
+
+<h2 id="Licensing_note">Licensing note</h2>
+
+<p>Some content (text, examples) in this page has been copied or adapted from the <a class="external" href="http://wiki.ecmascript.org/doku.php">ECMAScript wiki</a> which content is licensed <a class="external" href="http://creativecommons.org/licenses/by-nc-sa/2.0/">CC 2.0 BY-NC-SA</a>.</p>