Watch for Object Changes with JavaScript

Watching for changes to an object’s property has always been a much sought after task; many shims have been used over the years to listen to object changes.  These days we have better methods for listening to object changes:  the Proxy API.  Sindre Sorhus has created on-change, a tiny JavaScript tool that allows you to listen for changes on JavaScript objects and arrays!

on-change JavaScript

Since the on-change code is so tiny, here it is in its full glory:

'use strict';

module.exports = (object, onChange) => {
	const handler = {
		get(target, property, receiver) {
			try {
				return new Proxy(target[property], handler);
			} catch (err) {
				return Reflect.get(target, property, receiver);
			}
		},
		defineProperty(target, property, descriptor) {
			onChange();
			return Reflect.defineProperty(target, property, descriptor);
		},
		deleteProperty(target, property) {
			onChange();
			return Reflect.deleteProperty(target, property);
		}
	};

	return new Proxy(object, handler);
};

The onChange function returns a proxy which you’ll use to make any object changes.

Using on-change

Pass onChange the object you’d like to spy on and a change handler function:

let j = {
    a: 1
};

// Upon change, save to server
let changeable = onChange(j, () => save(j));

Then use that Proxy to change properties and get notified of changes:

// Make a change that would trigger changes
changeable.a = 2;

// save() is triggered!

// j.a === 2

Note that the original object’s values also change — that’s the beauty of Proxy!  Also note that on-change doesn’t tell you which property changed; the use case, as Sindre describes, is saving an object (to server, etc.) when any part of an object changes.  You could also use ti for a small library with a render function, re-rendering the content whenever a property changed!

This on-change library is really nice if you don’t need to know which property changed, just that something changed.