Proxy

Introduction

Proxies are objects for which the programmer has to define the semantics in JavaScript. The default object semantics are implemented in the JavaScript engine, often written in lower-level languages like C++. Proxies let the programmer define most of the behavior of an object in JavaScript. They are said to provide a meta-programming API.

Note: The SpiderMonkey Proxy implementation is a prototype and the Proxy API and semantics specifications are unstable. The SpiderMonkey implementation may not reflect the latest specification draft. It is subject to change anytime. It is provided as an experimental feature. Do not rely on it for production code.

This page describes the new API (called 'direct_proxies') which is part of Firefox 18. For the previous API (Firefox 17 and below), visit the old proxy API page

Terminology

catch-all mechanism (or "intercession API")

The technical term for this feature.

proxy

The object whose accesses are being intercepted.

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 regarding object non-extensibility or non-configurable properties are verified against the target.

Proxy API

Proxies are new objects; it's not possible to "proxyfy" an existing object. Here is how to create a proxy

var p = new Proxy(target, handler);

Where:

• target is an object (can be any sort of objects, including a native array, a function or even another proxy).
• handler is an object whose properties are functions which define the behavior of the proxy when an operation is performed on it.

Handler API

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

Invariants

Even though proxies provide a lot of power to users, some operations are not trapped in order to keep the language consistent:

• The double and triple equal (==, ===) operator is not trapped. p1 === p2 if and only if p1 and p2 refer to the same proxy.
• At present, Object.getPrototypeOf(proxy) unconditionally returns Object.getPrototypeOf(target), because the ES6 getPrototypeOf trap is not yet implemented.
• typeof proxy unconditionally returns typeof target. In particular, proxy may be used as a function only if target is already a function.
• Array.isArray(proxy) unconditionally returns Array.isArray(target).
• Object.prototype.toString.call(proxy) unconditionally returns Object.prototype.toString.call(target), because ES6 Symbol.toStringTag is not yet implemented.

Examples

Very simple example

An object with 37 as its default value when the property name is not in the object

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

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 proxy

console.log(target.a); // 37. The operation has been properly forwarded

Validation

With a Proxy, you can easily validate the passed value for an object.

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.

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:

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') {
      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'] <- 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();
  },
  "iterate": function (oTarget, sKey) {
    return oTarget.keys();
  },
  "ownKeys": function (oTarget, sKey) {
    return oTarget.keys();
  },
  "has": function (oTarget, sKey) {
    return sKey in oTarget || oTarget.hasItem(sKey);
  },
  "hasOwn": function (oTarget, sKey) {
    return oTarget.hasItem(sKey);
  },
  "defineProperty": function (oTarget, sKey, oDesc) {
    if (oDesc && "value" in oDesc) { oTarget.setItem(sKey, oDesc.value); }
    return oTarget;
  },
  "getPropertyNames": function (oTarget) {
    return Object.getPropertyNames(oTarget).concat(oTarget.keys());
  },
  "getOwnPropertyNames": function (oTarget) {
    return Object.getOwnPropertyNames(oTarget).concat(oTarget.keys());
  },
  "getPropertyDescriptor": function (oTarget, sKey) {
    var vValue = oTarget[sKey] || oTarget.getItem(sKey)
    return vValue ? {
      "value": vValue,
      "writable": true,
      "enumerable": true,
      "configurable": false
    } : undefined;
  },
  "getOwnPropertyDescriptor": function (oTarget, sKey) {
    var vValue = oTarget.getItem(sKey);
    return vValue ? {
      "value": vValue,
      "writable": true,
      "enumerable": true,
      "configurable": false
    } : undefined;
  },
  "fix":  function (oTarget) {
    return "not implemented yet!";
  },
});

/* Cookies test */

alert(docCookies.my_cookie1 = "First value");
alert(docCookies.getItem("my_cookie1"));

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

See also

• "Proxies are awesome" Brendan Eich presentation at JSConf (slides)
• ECMAScript Harmony Proxy proposal page and ECMAScript Harmony proxy semantics page
• Tutorial on proxies
• Old Proxy API page
• Object.watch is a non-standard feature but has been supported in Gecko for a long time.

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