A simple set of functions for intercepting and observing JavaScript objects and arrays.

Usage no npm install needed!

<script type="module">
  import webqitObserver from 'https://cdn.skypack.dev/@webqit/observer';


The Observer API

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Take a one-minute rundown of the Observer API.


Observe operations on any object or array...

let obj = {};

...using the Observer.observe() method.

Observer.observe(obj, mutations => {
    mutations.forEach(mutation => {
        console.log(mutation.type, mutation.name, mutation.path, mutation.value, mutation.oldValue);

Now changes will be delivered synchronously - as they happen.


Programmatically make reactive changes using the Reflect-like set of operators...

// A single set operation
Observer.set(obj, 'prop1', 'value1');
// A batch set operation
Observer.set(obj, {
    prop2: 'value2',
    prop3: 'value3',

...or switch to using object accessors - using the Observer.accessorize() method...

// Accessorize all (existing) properties
// Accessorize specific properties (existing or new)
Observer.accessorize(obj, ['prop1', 'prop5', 'prop9']);
// Make reactive operations
obj.prop1 = 'value1';
obj.prop5 = 'value5';
obj.prop9 = 'value9';

...or even go with a reactive Proxy of your object, and imply properties on the fly.

// Obtain a reactive Proxy
let _obj = Observer.proxy(obj);
// Make reactive operations
_obj.prop1 = 'value1';
_obj.prop4 = 'value4';
_obj.prop8 = 'value8';

And no problem if you inadvertently cascade the approaches. No bizzare behaviours.

// Accessorized properties are already reactive
Observer.accessorize(obj, ['prop1', 'prop6', 'prop10']);

// But no problem if you inadvertently proxy an accessorized object
let _obj = Observer.proxy(obj);

// And yet no problem if you inadvertently made a programmatic call over an already reactive Proxy
Observer.set(_obj, 'prop1', 'value1');


How about some level of indirection - the ability to hook into operators like Observer.set() and Observer.deleteProperty() to repurpose their operation? That's all possible using the Observer.intercept() method!

Below, we catch any attempt to set an HTTP URL and force it to an HTTPS URL.

Observer.intercept(obj, 'set', (action, previous, next) => {
    if (action.name === 'url' && action.value.startsWith('http:')) {
        return next(action.value.replace('http:', 'https:'));
    return next();

Now, only the first of the following will fly as-is.

Observer.set(obj, 'url', 'https://webqit.io');
Observer.set(obj, 'url', 'http://webqit.io');

Pass Some Detail to Observers

Operators, like Observer.set(), can pass arbitrary value to observers via a params.detail property.

// A set operation with detail
Observer.set(obj, {
    prop2: 'value2',
    prop3: 'value3',
}, { detail: 'Certain detail' });

Observers will recieve this value in a mutation.detail property.

// An observer with detail
Observer.observe(obj, 'prop1', mutation => {
    console.log('An operation has been made with detail:' + mutation.detail);

Negotiate with Observers

Observers can access and act on a special object called the Response Object.

// An observer and the response object
Observer.observe(obj, 'prop1', (mutation, event) => {
    if (1) {
        event.preventDefault(); // Or return false
    } else if (2) {
        event.stopPropagation(); // Or return false
    } else if (3) {
        event.waitUntil(new Promise); // Or return new Promise

Operators can access and honour the event's state.

// A set operation that returns the eventTypeReturn
let event = Observer.set(obj, {
    prop2: 'value2',
    prop3: 'value3',
}, { eventTypeReturn: true });
if (event.defaultPrevented) {
    // event.preventDefault() was called
} else if (event.propagationStopped) {
    // event.stopPropagation() was called
} else if (event.promises) {
    // event.waitUntil() was called
    event.promises.then(() => {


Learn more about negotiation here

Clean Up Anytime

Need to undo certain bindings? There are the methods for that!

The End?

Certainly not! But this rundown should be a good start. Next:


To report bugs or request features, please submit an issue.