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

const onChange = require('on-change');

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

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

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

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'

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 three arguments:

1. A path to the value that was changed. A change to c in the above example would return a.b.0.c.
2. The new value at the path.
3. The previous value at the path.

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

options

Type: object

isShallow

Type: boolean
Default: false

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

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)
• statux - State manager (Uses Proxy too)
• introspected - Never-ending Proxy with multiple observers (Uses Proxy too)

Maintainers

• Sindre Sorhus
• Darren Wright