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    0.6.1-rc.2 • Public • Published

    The Observer Utility

    Transparent reactivity with 100% language coverage. Made with ❤️ and ES6 Proxies.

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    Table of Contents


    Popular frontend frameworks - like Angular, React and Vue - use a reactivity system to automatically update the view when the state changes. This is necessary for creating modern web apps and staying sane at the same time.

    The Observer Utililty is a similar reactivity system, with a modern twist. It uses ES6 Proxies to achieve true transparency and a 100% language coverage. Ideally you would like to manage your state with plain JS code and expect the view to update where needed. In practice some reactivity systems require extra syntax - like React's setState. Others have limits on the language features, which they can react on - like dynamic properties or the delete keyword. These are small nuisances, but they lead to long hours lost among special docs and related issues.

    The Observer Utility aims to eradicate these edge cases. It comes with a tiny learning curve and with a promise that you won't have to dig up hidden docs and issues later. Give it a try, things will just work.


    This is a framework independent library, which powers the reactivity system behind other state management solutions. These are the currently available bindings.

    • React Easy State is a state management solution for React with a minimal learning curve.
    • preact-ns-observer provides a simple @observable decorator that makes Preact components reactive.


    $ npm install @rsjs/observer-util


    The two building blocks of reactivity are observables and reactions. Observable objects represent the state and reactions are functions, that react to state changes. In case of transparent reactivity, these reactions are called automatically on relevant state changes.


    Observables are transparent Proxies, which can be created with the observable function. From the outside they behave exactly like plain JS objects.

    import { observable } from '@rsjs/observer-util';
    const counter = observable({ num: 0 });
    // observables behave like plain JS objects
    counter.num = 12;


    Reactions are functions, which use observables. They can be created with the observe function and they are automatically executed whenever the observables - used by them - change.

    Vanilla JavaScript

    import { observable, observe } from '@rsjs/observer-util';
    const counter = observable({ num: 0 });
    const countLogger = observe(() => console.log(counter.num));
    // this calls countLogger and logs 1

    React Component

    import { store, view } from 'react-easy-state';
    // this is an observable store
    const counter = store({
      num: 0,
      up() {
    // this is a reactive component, which re-renders whenever counter.num changes
    const UserComp = view(() => <div onClick={counter.up}>{counter.num}</div>);

    Preact Component

    import { observer } from "preact-nx-observer";
    let store = observable({ title: "This is foo's data"});
    @observer // Component will now re-render whenever store.title changes.
    class Foo extends Component {
      render() {
        return <h1>{store.title}</h1>

    More examples

    Dynamic properties
    import { observable, observe } from '@rsjs/observer-util';
    const profile = observable();
    observe(() => console.log(;
    // logs 'Bob' = 'Bob';
    Nested properties
    import { observable, observe } from '@rsjs/observer-util';
    const person = observable({
      name: {
        first: 'John',
        last: 'Smith'
      age: 22
    observe(() => console.log(`${} ${}`));
    // logs 'Bob Smith' = 'Bob';
    Getter properties
    import { observable, observe } from '@rsjs/observer-util';
    const person = observable({
      firstName: 'Bob',
      lastName: 'Smith',
      get name() {
        return `${this.firstName} ${this.lastName}`;
    observe(() => console.log(;
    // logs 'Ann Smith'
    person.firstName = 'Ann';
    import { observable, observe } from '@rsjs/observer-util';
    const person = observable({
      gender: 'male',
      name: 'Potato'
    observe(() => {
      if (person.gender === 'male') {
        console.log(`Mr. ${}`);
      } else {
        console.log(`Ms. ${}`);
    // logs 'Ms. Potato'
    person.gender = 'female';
    import { observable, observe } from '@rsjs/observer-util';
    const users = observable([]);
    observe(() => console.log(users.join(', ')));
    // logs 'Bob'
    // logs 'Bob, John'
    // logs 'Bob'
    ES6 collections
    import { observable, observe } from '@rsjs/observer-util';
    const people = observable(new Map());
    observe(() => {
      for (let [name, age] of people) {
        console.log(`${name}, ${age}`);
    // logs 'Bob, 22'
    people.set('Bob', 22);
    // logs 'Bob, 22' and 'John, 35'
    people.set('John', 35);
    Inherited properties
    import { observable, observe } from '@rsjs/observer-util';
    const defaultUser = observable({
      name: 'Unknown',
      job: 'developer'
    const user = observable(Object.create(defaultUser));
    // logs 'Unknown is a developer'
    observe(() => console.log(`${} is a ${user.job}`));
    // logs 'Bob is a developer' = 'Bob';
    // logs 'Bob is a stylist'
    user.job = 'stylist';
    // logs 'Unknown is a stylist'

    Reaction scheduling

    Reactions are scheduled to run whenever the relevant observable state changes. The default scheduler runs the reactions synchronously, but custom schedulers can be passed to change this behavior. Schedulers are usually functions which receive the scheduled reaction as argument.

    import { observable, observe } from '@rsjs/observer-util';
    // this scheduler delays reactions by 1 second
    const scheduler = reaction => setTimeout(reaction, 1000);
    const person = observable({ name: 'Josh' });
    observe(() => console.log(, { scheduler });
    // this logs 'Barbie' after a one second delay = 'Barbie';

    Alternatively schedulers can be objects with an add and delete method. Check out the below examples for more.

    More examples

    React Scheduler

    The React scheduler simply calls setState on relevant observable changes. This delegates the render scheduling to React Fiber. It works roughly like this.

    import { observe } from '@rsjs/observer-util';
    class ReactiveComp extends BaseComp {
      constructor() {
        // ...
        this.render = observe(this.render, {
          scheduler: () => this.setState()
    Batched updates with ES6 Sets

    Schedulers can be objects with an add and delete method, which schedule and unschedule reactions. ES6 Sets can be used as a scheduler, that automatically removes duplicate reactions.

    import { observable, observe } from '@rsjs/observer-util';
    const reactions = new Set();
    const person = observable({ name: 'Josh' });
    observe(() => console.log(person), { scheduler: reactions });
    // this throttles reactions to run with a minimal 1 second interval
    setInterval(() => {
      reactions.forEach(reaction => reaction());
    }, 1000);
    // these will cause { name: 'Barbie', age: 30 } to be logged once = 'Barbie';
    person.age = 87;
    Batched updates with queues

    Queues from the Queue Util can be used to implement complex scheduling patterns by combining automatic priority based and manual execution.

    import { observable, observe } from '@rsjs/observer-util';
    import { Queue, priorities } from '@nx-js/queue-util';
    const scheduler = new Queue(priorities.LOW);
    const person = observable({ name: 'Josh' });
    observe(() => console.log(person), { scheduler });
    // these will cause { name: 'Barbie', age: 30 } to be logged once
    // when everything is idle and there is free time to do it = 'Barbie';
    person.age = 87;

    Queues are automatically scheduling reactions - based on their priority - but they can also be stopped, started and cleared manually at any time. Learn more about them here.


    Proxy = observable(object)

    Creates and returns a proxied observable object, which behaves just like the originally passed object. The original object is not modified.

    • If no argument is provided, it returns an empty observable object.
    • If an object is passed as argument, it wraps the passed object in an observable.
    • If an observable object is passed, it returns the passed observable object.

    boolean = isObservable(object)

    Returns true if the passed object is an observable, returns false otherwise.

    reaction = observe(function, config)

    Wraps the passed function with a reaction, which behaves just like the original function. The reaction is automatically scheduled to run whenever an observable - used by it - changes. The original function is not modified.

    observe also accepts an optional config object with the following options.

    • lazy: A boolean, which controls if the reaction is executed when it is created or not. If it is true, the reaction has to be called once manually to trigger the reactivity process. Defaults to false.

    • scheduler: A function, which is called with the reaction when it is scheduled to run. It can also be an object with an add and delete method - which schedule and unschedule reactions. The default scheduler runs the reaction synchronously on observable mutations. You can learn more about reaction scheduling in the related docs section.

    • debugger: An optional function. It is called with contextual metadata object on basic operations - like set, get, delete, etc. The metadata object can be used to determine why the operation wired or scheduled the reaction and it always has enough data to reverse the operation. The debugger is always called before the scheduler.


    Unobserves the passed reaction. Unobserved reactions won't be automatically run anymore.

    import { observable, observe, unobserve } from '@rsjs/observer-util';
    const counter = observable({ num: 0 });
    const logger = observe(() => console.log(counter.num));
    // after this the logger won't be automatically called on counter.num changes

    obj = raw(observable)

    Original objects are never modified, but transparently wrapped by observable proxies. raw can access the original non-reactive object. Modifying and accessing properties on the raw object doesn't trigger reactions.

    Using raw at property access

    import { observable, observe, raw } from '@rsjs/observer-util';
    const person = observable();
    const logger = observe(() => console.log(;
    // this logs 'Bob' = 'Bob';
    // `name` is used from the raw non-reactive object, this won't log anything
    raw(person).name = 'John';

    Using raw at property mutation

    import { observable, observe, raw } from '@rsjs/observer-util';
    const person = observable({ age: 20 });
    observe(() => console.log(`${}: ${raw(person).age}`));
    // this logs 'Bob: 20' = 'Bob';
    // `age` is used from the raw non-reactive object, this won't log anything
    person.age = 33;

    Platform support

    • Node: 6.5 and above
    • Chrome: 49 and above
    • Firefox: 38 and above
    • Safari: 10 and above
    • Edge: 12 and above
    • Opera: 36 and above
    • IE is not supported


    Contributions are always welcomed! Just send a PR for fixes and doc updates and open issues for new features beforehand. Make sure that the tests and the linter pass and that the coverage remains high. Thanks!


    npm i @rsjs/observer-util

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