The Proxy object enables you to create a proxy for another object, which can intercept and redefine fundamental operations for that object.

Description

A Proxy is created with two parameters:

target : the original object which you want to proxy

: the original object which you want to proxy handler : an object that defines which operations will be intercepted and how to redefine intercepted operations.

For example, this code defines a simple target with just two properties, and an even simpler handler with no properties:

const target = { message1: "hello", message2: "everyone" }; const handler1 = {}; const proxy1 = new Proxy(target, handler1);

Because the handler is empty, this proxy behaves just like the original target:

console.log(proxy1.message1); // hello console.log(proxy1.message2); // everyone

To customise the proxy, we define functions on the handler object:

const target = { message1: "hello", message2: "everyone" }; const handler2 = { get: function(target, prop, receiver) { return "world"; } }; const proxy2 = new Proxy(target, handler2);

Here we've provided an implementation of the get() handler, which intercepts attempts to access properties in the target.

Handler functions are sometimes called traps, presumably because they trap calls to the target object. The very simple trap in handler2 above redefines all property accessors:

console.log(proxy2.message1); // world console.log(proxy2.message2); // world

With the help of the Reflect class we can give some accessors the original behavior and redefine others:

const target = { message1: "hello", message2: "everyone" }; const handler3 = { get: function (target, prop, receiver) { if (prop === "message2") { return "world"; } return Reflect.get(...arguments); }, }; const proxy3 = new Proxy(target, handler3); console.log(proxy3.message1); // hello console.log(proxy3.message2); // world

Constructor

Proxy() Creates a new Proxy object.

Static methods

Proxy.revocable() Creates a revocable Proxy object.

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.

const handler = { get: function(obj, prop) { return prop in obj ? obj[prop] : 37; } }; const 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.

const target = {}; const 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.

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; } }; const 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.gender = 'M'; var Peter = new Boy('Peter', 13); console.log(Peter.gender); // "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'); //giving error here, i1 is null 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' Note: even if selected: !null, then giving oldval.setAttribute is not a function

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 Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js Proxy 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 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 Proxy() constructor 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 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.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 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 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 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 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.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 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 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 Full support 49 Edge Full support 79 Firefox Full support 49 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Firefox Android Full support 49 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 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 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 Full support 49 Edge Full support 12 Firefox Full support 31 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Firefox Android Full support 31 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 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 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 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 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 Full support 49 Edge Full support 12 Firefox Full support 49 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Firefox Android Full support 49 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 12 Firefox Full support 34 IE No support No Opera Full support 50 Safari Full support 10 WebView Android Full support 63 Chrome Android Full support 63 Firefox Android Full support 34 Opera Android Full support 46 Safari iOS Full support 10 Samsung Internet Android Full support 8.0 nodejs Full support 6.0.0 Legend Full support Full support No support No support See implementation notes. See implementation notes.

See also