isabella232 / proxy-polyfill

Proxy object polyfill

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 no longer supported anywhere.

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

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

Install via your favourite package manager as proxy-polyfill.

To polyfill Proxy everywhere

You should include proxy-polyfill into your build system (just require it directly, it doesn't export anything), or import the proxy.min.js file directly. This is the recommended approach and works on the web, in Node, or React Native.

To consume the polyfill as a function

Requires ./src/proxy.js, which exports a proxy polyfill builder function in commonJS.

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

// Your environment may also support transparent rewriting of commonJS to ES6:
import ProxyPolyfillBuilder from 'proxy-polyfill/src/proxy';
const proxyPolyfill = ProxyPolyfillBuilder();

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

Support

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. You don't need this if you're only targeting these modern browsers.