on-change

on-change

Watch an object or array for changes

It works recursively, so it will even detect if you modify a deep property like obj.a.b[0].c = true.

Uses the Proxy API.

Install

npm install on-change

Usage

import onChange from 'on-change';

const object = {
	foo: false,
	a: {
		b: [
			{
				c: false
			}
		]
	}
};

let index = 0;
const watchedObject = onChange(object, function (path, value, previousValue, applyData) {
	console.log('Object changed:', ++index);
	console.log('this:', this);
	console.log('path:', path);
	console.log('value:', value);
	console.log('previousValue:', previousValue);
	console.log('applyData:', applyData);
});

watchedObject.foo = true;
//=> 'Object changed: 1'
//=> 'this: {
//   	foo: true,
//   	a: {
//   		b: [
//   			{
//   				c: false
//   			}
//   		]
//   	}
//   }'
//=> 'path: "foo"'
//=> 'value: true'
//=> 'previousValue: false'
//=> 'applyData: undefined'

watchedObject.a.b[0].c = true;
//=> 'Object changed: 2'
//=> 'this: {
//   	foo: true,
//   	a: {
//   		b: [
//   			{
//   				c: true
//   			}
//   		]
//   	}
//   }'
//=> 'path: "a.b.0.c"'
//=> 'value: true'
//=> 'previousValue: false'
//=> 'applyData: undefined'

watchedObject.a.b.push(3);
//=> 'Object changed: 3'
//=> 'this: {
//   	foo: true,
//   	a: {
//   		b: [
//   			{
//   				c: true
//   			},
//   			3
//   		]
//   	}
//   }'
//=> 'path: "a.b"'
//=> 'value: [{c: true}, 3]'
//=> 'previousValue: [{c: true}]'
//=> 'applyData: {
//       name: "push",
//       args: [3],
//       result: 2,
//   }'

// Access the original object
onChange.target(watchedObject).foo = false;
// Callback isn't called

// Unsubscribe
onChange.unsubscribe(watchedObject);
watchedObject.foo = 'bar';
// Callback isn't called

API

onChange(object, onChange, options?)

Returns a version of object that is watched. It's the exact same object, just with some Proxy traps.

object

Type: object

Object to watch for changes.

onChange

Type: Function

Function that gets called anytime the object changes.

The function receives four arguments:

• A path to the value that was changed. A change to c in the above example would return a.b.0.c.
• The new value at the path.
• The previous value at the path. Changes in WeakSets and WeakMaps will return undefined.
• An object with the name of the method that produced the change, the args passed to the method, and the result of the method.

The context (this) is set to the original object passed to onChange (with Proxy).

options

Type: object

Options for altering the behavior of onChange.

isShallow

Type: boolean
Default: false

Deep changes will not trigger the callback. Only changes to the immediate properties of the original object.

equals

Type: Function
Default: Object.is

The function receives two arguments to be compared for equality. Should return true if the two values are determined to be equal. Useful if you only need a more loose form of equality.

ignoreSymbols

Type: boolean
Default: false

Setting properties as Symbol won't trigger the callback.

ignoreKeys

Type: Array<string | symbol>
Default: undefined

Setting properties in this array won't trigger the callback.

ignoreUnderscores

Type: boolean
Default: false

Setting properties with an underscore as the first character won't trigger the callback.

pathAsArray

Type: boolean
Default: false

The path will be provided as an array of keys instead of a delimited string. Recommended when working with Sets, Maps, or property keys that are Symbols.

ignoreDetached

Type: boolean
Default: false

Ignore changes to objects that become detached from the watched object.

details

Type: boolean|string[]
Default: false

Trigger callbacks for each change within specified method calls or all method calls. When enabled, mutating array and collection methods will emit per-item paths (for example selectedFeatures.0) instead of the collection path. Use details: ['forEach'] to expand only specific methods.

onValidate

Type: Function

The function receives the same arguments and context as the onChange callback. The function is called whenever a change is attempted. Returning true will allow the change to be made and the onChange callback to execute, returning anything else will prevent the change from being made and the onChange callback will not trigger.

onChange.target(object)

Returns the original unwatched object.

object

Type: object

Object that is already being watched for changes.

onChange.unsubscribe(object)

Cancels all future callbacks on a watched object and returns the original unwatched object.

object

Type: object

Object that is already being watched for changes.

Use-case

I had some code that was like:

const foo = {
	a: 0,
	b: 0
};

// …

foo.a = 3;
save(foo);

// …

foo.b = 7;
save(foo);


// …

foo.a = 10;
save(foo);

Now it can be simplified to:

const foo = onChange({
	a: 0,
	b: 0
}, () => save(foo));

// …

foo.a = 3;

// …

foo.b = 7;

// …

foo.a = 10;

Related

• known - Allow only access to known object properties (Uses Proxy too)
• negative-array - Negative array index support array[-1] (Uses Proxy too)
• atama - State manager (Uses Proxy too)
• introspected - Never-ending Proxy with multiple observers (Uses Proxy too)

Maintainers

• Sindre Sorhus
• Darren Wright

Similar packages

Other packages similar to on-change

proxy-compare

The proxy-compare package provides utilities for creating proxies that can detect changes in objects and arrays. It is similar to on-change in that it uses JavaScript proxies to track changes, but it focuses more on efficient comparison and change detection for use in state management libraries.

immer

Immer is a package that allows you to work with immutable state by using a proxy-based approach to detect changes and produce new state objects. While on-change focuses on detecting changes and triggering callbacks, Immer is more about immutability and producing new state from changes.

mobx

MobX is a state management library that uses observable objects to automatically track changes and update the UI. It provides more comprehensive state management features compared to on-change, which is more lightweight and focused on change detection.