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El objeto Proxy se usa para definir un comportamiento personalizado para operaciones fundamentales (por ejemplo, para observar propiedades, cuando se asignan, enumeración, invocación de funciones, etc).


Objeto que gestiona las intercepciones a las propiedades del objeto proxy.
Son los métodos interceptores que proveen acceso a las propiedades. Es análogo al concepto de traps en los sistemas operativos.
El objeto que virtualiza este objeto. Suele usarse como backend de almacenamiento del proxy. Invariantes (semántica que no acepta cambios) respecto a la no extensibilidad del objeto o propiedades no configurables se verifican contra este target.


var p = new Proxy(target, handler);


Un objeto target (puede ser cualquier órden de objetos, incluyendo un array nativa, funcion o incluso otro proxy) o función que contenga el Proxy
Un objeto cuyas propiedades son funciones que definen el comportamiento del proxy cuando una operación es realizada en él.


Crea un objeto Proxy revocable

Métodos del objeto handler

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

All traps are optional. If a trap has not been defined, the default behavior is to forward the operation to the target.

A trap for Object.getPrototypeOf.
A trap for Object.setPrototypeOf.
A trap for Object.isExtensible.
A trap for Object.preventExtensions.
A trap for Object.getOwnPropertyDescriptor.
A trap for Object.defineProperty.
A trap for the in operator.
A trap for getting property values.
A trap for setting property values.
A trap for the delete operator.
A trap for Object.getOwnPropertyNames and Object.getOwnPropertySymbols.
A trap for a function call.
A trap for the new operator.

Some non-standard traps are obsolete and have been removed.


Ejemplo básico

En este simple ejemplo el número 37 se devuelve como valor predeterminado cuando la propiedad name no se encuentra en el objeto. Se utilizando el manejador get.

var handler = {
    get: function(target, name){
        return name in target?
            target[name] :

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


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;

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 = Object.create(sup.prototype);
  var handler = {
    construct: function(target, args) {
      var obj = Object.create(base.prototype);
      return obj;
    apply: function(target, that, args) {
  var proxy = new Proxy(base,handler);
  descriptor.value = proxy;
  Object.defineProperty(base.prototype, "constructor", descriptor);
  return proxy;

var Person = function(name){ = name;

var Boy = extend(Person, function(name, age) {
  this.age = age;
}); = "M";

var Peter = new Boy("Peter", 13);
console.log(;  // "M"
console.log(; // "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;

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 convert 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') {

    // 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'] <- 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 ( === prop) {
        result = product;
      if (types[product.type]) {
      } 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:

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");

docCookies.setItem("my_cookie1", "Changed value");


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
La definición de 'Proxy' en esta especificación.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
La definición de 'Proxy' en esta especificación.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 49.0 13 (10586) 18 (18) Sin soporte ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? 49.0 18 (18) 13 (10586) ? ?

Gecko specific notes

  • At present, Object.getPrototypeOf(proxy) unconditionally returns Object.getPrototypeOf(target), because the ES6 getPrototypeOf trap is not yet implemented (error 888969, error 888969).
  • Array.isArray(proxy) unconditionally returns Array.isArray(target) (error 1111785, error 1111785).
  • unconditionally returns, because ES6 Symbol.toStringTag is not yet implemented (error 1114580).

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.