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

Terminology

handler Placeholder object which contains traps. traps The methods that provide property access. This is analogous to the concept of traps in operating systems. target 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.

Syntax

var p = new Proxy(target, handler);

Parameters

target A target object to wrap with Proxy . It can be any sort of object, including a native array, a function or even another proxy. handler An object whose properties are functions which define the behavior of the proxy when an operation is performed on it.

Methods

Proxy.revocable() Creates a revocable Proxy object.

Methods of the handler object

The handler object is a placeholder object which contains traps for Proxy .

Examples

Basic example

In this simple example the number 37 gets returned as the default value when the property name is not in the object. It is using the get handler.

var handler = { get: function(obj, prop) { return prop in obj ? obj[prop] : 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

No-op forwarding proxy

In this example, we are using a native JavaScript object to which our proxy will forward all operations that are applied to it.

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

Note that while this "no-op" works for JavaScript objects it does not work for native browser objects like DOM Elements. See this for one solution.

Validation

With a Proxy , you can easily validate the passed value for an object. This example uses the set handler.

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 > 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

Extending constructor

A function proxy could easily extend a constructor with a new constructor. This example uses the construct and apply handlers.

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

Manipulating DOM nodes

Sometimes you want to toggle the attribute or class name of two different elements. Here's how using the set handler.

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'

Value correction and an extra property

The products proxy object evaluates the passed value and converts it to an array if needed. The object also supports an extra property called latestBrowser both as a getter and a setter.

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'] <- no problem, the value is an array products.latestBrowser = 'Chrome'; console.log(products.browsers); // ['Firefox', 'Chrome'] console.log(products.latestBrowser); // 'Chrome'

Finding an array item object by its property

This proxy extends an array with some utility features. As you see, you can flexibly "define" properties without using Object.defineProperties . This example can be adapted to find a table row by its cell. In that case, the target will be table.rows .

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

A complete traps list example

Now in order to create a complete sample traps list, for didactic purposes, we will try to proxify a non native object that is particularly suited to this type of operation: the docCookies global object created by the "little framework" published on the document.cookie page.

/* 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 && '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);

Specifications

Browser compatibility

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub Desktop Mobile Server Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js Basic support Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 revocable Chrome Full support 63 Edge Full support Yes Firefox Full support 34 IE No support No Opera Full support Yes Safari Full support 10 WebView Android Full support 63 Chrome Android Full support 63 Edge Mobile Full support Yes Firefox Android Full support 34 Opera Android Full support Yes Safari iOS Full support 10 Samsung Internet Android Full support Yes nodejs Full support 6.0.0 handler.apply Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.construct Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.defineProperty Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.deleteProperty Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.enumerate Deprecated Non-standard Chrome No support No Edge No support No Firefox No support 37 — 47 IE No support No Opera No support No Safari No support No WebView Android No support No Chrome Android No support No Edge Mobile No support No Firefox Android No support 37 — 47 Opera Android No support No Safari iOS No support No Samsung Internet Android No support No nodejs No support No handler.get Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.getOwnPropertyDescriptor Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.getPrototypeOf Chrome No support No Edge No support No Firefox Full support 49 IE No support No Opera No support No Safari No support No WebView Android No support No Chrome Android No support No Edge Mobile No support No Firefox Android Full support 49 Opera Android No support No Safari iOS No support No Samsung Internet Android No support No nodejs Full support 6.0.0 handler.has Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.isExtensible Chrome ? Edge ? Firefox Full support 31 IE No support No Opera ? Safari ? WebView Android ? Chrome Android ? Edge Mobile ? Firefox Android Full support 31 Opera Android ? Safari iOS ? Samsung Internet Android ? nodejs Full support 6.0.0 handler.ownKeys Chrome Full support 49 Edge Full support 12 Firefox Full support 18 Notes Full support 18 Notes Notes In Firefox 42, the implementation got updated to reflect the final ES2015 specification: The result is now checked if it is an array and if the array elements are either of type string or of type symbol. Enumerating duplicate own property names is not a failure anymore. IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Notes Full support 18 Notes Notes In Firefox 42, the implementation got updated to reflect the final ES2015 specification: The result is now checked if it is an array and if the array elements are either of type string or of type symbol. Enumerating duplicate own property names is not a failure anymore. Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.preventExtensions Chrome Full support 49 Edge Full support 12 Firefox Full support 22 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 22 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.set Chrome Full support 49 Edge Full support 12 Firefox Full support 18 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Edge Mobile Full support Yes Firefox Android Full support 18 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0 handler.setPrototypeOf Chrome ? Edge ? Firefox Full support 49 IE No support No Opera ? Safari ? WebView Android ? Chrome Android ? Edge Mobile ? Firefox Android Full support 49 Opera Android ? Safari iOS ? Samsung Internet Android ? nodejs Full support 6.0.0 Legend Full support Full support No support No support Compatibility unknown Compatibility unknown Non-standard. Expect poor cross-browser support. Non-standard. Expect poor cross-browser support. Deprecated. Not for use in new websites. Deprecated. Not for use in new websites. See implementation notes. See implementation notes.

See also

Licensing note

Some content (text, examples) in this page has been copied or adapted from the ECMAScript wiki which content is licensed CC 2.0 BY-NC-SA.