Revision 1060006 of Proxy

<div>
<div>{{JSRef}}</div>
</div>

<p>The <strong>Proxy</strong> object is used to define custom behavior for fundamental operations (e.g. property lookup, assignment, enumeration, function invocation, etc).</p>

<h2 id="Terminology">Terminology</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="Syntax">Syntax</h2>

<pre class="syntaxbox">
var p = new Proxy(target, handler);
</pre>

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt><code>target</code></dt>
 <dd>A target object (can be any sort of objects, including a native array, a function or even another proxy) or function 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="Methods">Methods</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="Examples">Examples</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">
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">
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">
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;
  }
};

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">
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">
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;
  }
});

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">
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;
    }

    // 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;
  }
});

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/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">
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">
/*
  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="Specifications">Specifications</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('ES6', '#sec-proxy-objects', 'Proxy')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-proxy-objects', 'Proxy')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Edge</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatChrome(49.0)}}</td>
   <td>13 (10586)</td>
   <td>{{ CompatGeckoDesktop("18") }}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</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>{{CompatUnknown}}</td>
   <td>{{CompatChrome(49.0)}}</td>
   <td>{{ CompatGeckoDesktop("18") }}</td>
   <td>13 (10586)</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<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 ES6 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 ES6 Symbol.toStringTag is not yet implemented ({{bug(1114580)}}).</li>
</ul>

<h2 id="See_also">See also</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="https://www.slideshare.net/BrendanEich/metaprog-5303821">slides</a>)</li>
 <li><a class="external" href="https://wiki.ecmascript.org/doku.php?id=harmony:proxies">ECMAScript Harmony Proxy proposal page</a> and <a class="external" href="https://wiki.ecmascript.org/doku.php?id=harmony:proxies_semantics">ECMAScript Harmony proxy semantics page</a></li>
 <li><a class="external" href="https://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="https://wiki.ecmascript.org/doku.php">ECMAScript wiki</a> which content is licensed <a class="external" href="https://creativecommons.org/licenses/by-nc-sa/2.0/">CC 2.0 BY-NC-SA</a>.</p>