Please note that this polyfill is now in maintenance mode, as of Jan, 2023. We are not planning to add more features or enhancements.
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.
Proxy
natively.
You don't need this if you're only targeting these modern browsers.
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 either:
- 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.
Do not import ./src/proxy.js
in old browsers directly, as it uses modern JavaScript features.
It needs to be compiled, which is why we have the proxy.min.js
file.
To consume the polyfill as a function
This is an advanced pattern.
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.