Proxy object polyfill
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README.md

This is a polyfill for the Proxy object, part of ES6. See the MDN docs or Introducing ES2015 Proxies for more information on Proxy itself. Unlike other polyfills, this does not require Object.observe, which is deprecated.

The polyfill supports just a limited number of proxy 'traps'. It also works by calling seal on the object passed to Proxy. This means that the properties you want to proxy must be known at creation time.

Additionally, your objects' prototypes will be snapshotted at the time a proxy is created. The properties of your objects can still change - you're just unable to define new ones. For example, proxying unrestricted dictionaries is not a good use-case for this polyfill.

Currently, the following traps are supported-

  • get
  • set
  • apply
  • construct

The Proxy.revocable method is also supported, but only for calls to the above traps.

This has no external dependencies. Skip down to usage to get started.

Example

The most compelling use case for Proxy is to provide change notifications.

function observe(o, callback) {
  return new Proxy(o, {
    set(target, property, value) {
      callback(property, value);
      target[property] = value;
    },
  });
}

const x = {'name': 'BB-8'};
const p = observe(x, (property, value) => console.info(property, value));
p.name = 'BB-9';
// name BB-9

You can extend this to generate change notifications for anywhere in an object tree-

function observe(o, callback) {
  function buildProxy(prefix, o) {
    return new Proxy(o, {
      set(target, property, value) {
        // same as above, but add prefix
        callback(prefix + property, value);
        target[property] = value;
      },
      get(target, property) {
        // return a new proxy if possible, add to prefix
        const out = target[property];
        if (out instanceof Object) {
          return buildProxy(prefix + property + '.', out);
        }
        return out;  // primitive, ignore
      },
    });
  }

  return buildProxy('', o);
}

const x = {'model': {name: 'Falcon'}};
const p = observe(x, (property, value) => console.info(property, value));
p.model.name = 'Commodore';
// model.name Commodore

Adding new properties

The following line will fail (with a TypeError in strict mode) with the polyfill, as it's unable to intercept new properties-

p.model.year = 2016;  // error in polyfill

However, you can replace the entire object at once - once you access it again, your code will see the proxied version.

p.model = {name: 'Falcon', year: 2016};
// model Object {name: "Falcon", year: 2016}

For a similar reason, this polyfill can't proxy Array objects very well - but you can replace them all at once.

Usage

To assign Proxy to the global object

Include the JavaScript at the start of your page, as an ES6 module (although browsers that support ES6 modules support Proxy natively) or include it as a dependency to your build steps. The source is in ES6, but the included, minified version is ES5.

To consume Proxy as a function

Require from your app the file ./src/proxy.js, which exports a proxy polyfill functionvian commonJS.

// commonJS require
const ProxyPolyfill = require('proxy-polyfill/src/proxy');

// Your environment may also support transparent rewriting of commonJS to ES6:
import ProxyPolyfill from 'proxy-polyfill/src/proxy';

// Then use...
const myProxy = new ProxyPolyfill(...);

Installation

Available via NPM or Bower-

$ npm install proxy-polyfill
$ bower install proxy-polyfill

If this is imported as a Node module, it will polyfill the global namespace rather than returning the Proxy object. If you'd like to just get the polyfill'ed version, use the require statement as above.

Supports

The polyfill supports browsers that implement the full ES5 spec, such as IE9+ and Safari 6+. It may work in other non-browser environments too.

Note that Firefox, Chrome, Safari 10+ and Edge support Proxy natively.

Release

Run

$ npm run build