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    Simple, scalable state management

    Build Status Coverage Status Join the chat at Discuss MobX on Hashnode OpenCollective OpenCollective code style: prettier

    MobX is proudly sponsored by Mendix, Coinbase, Facebook Open Source, Canva, Algolia, Guilded and many individual sponsors

    Mendix Coinbase Facebook Open Source Canva Algolia Guilded


    Tip: Consider using the faster and smaller ES6 build if targetting a modern environment: lib/mobx.es6.js. For example by setting up a webpack alias: resolve: { alias: { mobx: __dirname + "/node_modules/mobx/lib/mobx.es6.js" }}

    Browser support

    MobX version Actively supported Supported browsers GitHub branch
    5.* Yes Any browser that supports ES6 Proxies (non polyfillable). NOT: IE 11 and lower, Node 5 and lower master
    4.* Yes (LTS) Any ES5 compliant browser mobx4-master
    1-3.* No Any ES5 compliant browser No active branch
    • MobX >=5 runs on any browser with ES6 proxy support. It will throw an error on startup on older environments such as IE11, Node.js <6 or React Native Android on old JavaScriptCore how-to-upgrade. Warning: since upgrading JSC is non-trivial, and decorators can be troublesome as well in React Native, for now it is recommended to stick to MobX 4.x for for React Native Android development.
    • MobX 4 runs on any ES5 browser and will be actively maintained. The MobX 4 and 5 api's are the same and semantically can achieve the same, but MobX 4 has some limitations.


    Getting started


    MobX is a battle tested, simple and scalable state management library transparently applying functional reactive programming (TFRP). The Mobx design principle is very simple:

    Anything that can be derived from the application state, should be derived. Automatically.

    This includes the UI, data serialization, server communication, etc.

    MobX unidirectional flow

    React and MobX together are a powerful combination. React renders the application state by providing mechanisms to translate it into a tree of renderable components. MobX provides the mechanism to store and update the application state that React then uses.

    Both React and MobX provide optimal and unique solutions to common problems in application development. React provides mechanisms to optimally render the UI by using a virtual DOM that reduces the number of costly DOM mutations. MobX provides mechanisms to optimally synchronize application state with React components by using a reactive virtual dependency state graph that is only updated when strictly needed and is never stale.

    Core concepts

    MobX has only a few core concepts. The following snippets can be tried online using codesandbox example.

    Observable state lesson 1: observable & observer

    MobX adds observable capabilities to existing data structures like objects, arrays and class instances. This can simply be done by annotating your class properties with the @observable decorator (ES.Next).

    import { observable } from "mobx"
    class Todo {
        id = Math.random();
        @observable title = "";
        @observable finished = false;

    Using observable is like turning a property of an object into a spreadsheet cell. But, unlike spreadsheets, these values can be not only primitive values, but also references, objects and arrays.

    If your environment doesn't support decorator syntax, don't worry. You can read here about how to set them up. Or you can skip them altoghether, as MobX can be used fine without decorator syntax, by leveraging the decorate utility. Many MobX users prefer the slightly more concise decorator syntax, but the following snippet achieves the same:

    import { decorate, observable } from "mobx"
    class Todo {
        id = Math.random();
        title = "";
        finished = false;
    decorate(Todo, {
        title: observable,
        finished: observable

    Computed values lesson 3: computed values

    With MobX you can define values that will be derived automatically when relevant data is modified. By using the @computed decorator or by using getter / setter functions when using (extend)Observable (Of course, you can use decorate here again as alternative to the @ syntax).

    class TodoList {
        @observable todos = [];
        @computed get unfinishedTodoCount() {
            return this.todos.filter(todo => !todo.finished).length;

    MobX will ensure that unfinishedTodoCount is updated automatically when a todo is added or when one of the finished properties is modified. Computations like these resemble formulas in spreadsheet programs like MS Excel. They update automatically and only when required.

    Reactions lesson 9: custom reactions

    Reactions are similar to a computed value, but instead of producing a new value, a reaction produces a side effect for things like printing to the console, making network requests, incrementally updating the React component tree to patch the DOM, etc. In short, reactions bridge reactive and imperative programming.

    React components lesson 1: observable & observer

    If you are using React, you can turn your (stateless function) components into reactive components by simply adding the observer function / decorator from the mobx-react package onto them.

    import React, {Component} from 'react';
    import ReactDOM from 'react-dom';
    import {observer} from 'mobx-react';
    class TodoListView extends Component {
        render() {
            return <div>
                    { =>
                        <TodoView todo={todo} key={} />
                Tasks left: {this.props.todoList.unfinishedTodoCount}
    const TodoView = observer(({todo}) =>
                onClick={() => todo.finished = !todo.finished}
    const store = new TodoList();
    ReactDOM.render(<TodoListView todoList={store} />, document.getElementById('mount'));

    observer turns React (function) components into derivations of the data they render. When using MobX there are no smart or dumb components. All components render smartly but are defined in a dumb manner. MobX will simply make sure the components are always re-rendered whenever needed, but also no more than that. So the onClick handler in the above example will force the proper TodoView to render, and it will cause the TodoListView to render if the number of unfinished tasks has changed. However, if you would remove the Tasks left line (or put it into a separate component), the TodoListView will no longer re-render when ticking a box. You can verify this yourself by changing the JSFiddle.

    Custom reactions

    Custom reactions can simply be created using the autorun, reaction or when functions to fit your specific situations.

    For example the following autorun prints a log message each time the amount of unfinishedTodoCount changes:

    autorun(() => {
        console.log("Tasks left: " + todos.unfinishedTodoCount)

    What will MobX react to?

    Why does a new message get printed each time the unfinishedTodoCount is changed? The answer is this rule of thumb:

    MobX reacts to any existing observable property that is read during the execution of a tracked function.

    For an in-depth explanation about how MobX determines to which observables needs to be reacted, check understanding what MobX reacts to.

    Actions lesson 5: actions

    Unlike many flux frameworks, MobX is unopinionated about how user events should be handled.

    • This can be done in a Flux like manner.
    • Or by processing events using RxJS.
    • Or by simply handling events in the most straightforward way possible, as demonstrated in the above onClick handler.

    In the end it all boils down to: somehow the state should be updated.

    After updating the state MobX will take care of the rest in an efficient, glitch-free manner. So, simple statements, like the ones below, are enough to automatically update the user interface.

    There is no technical need for firing events, calling a dispatcher, etc. A React component in the end is nothing more than a fancy representation of your state, i.e. a derivation that will be managed by MobX.

        new Todo("Get Coffee"),
        new Todo("Write simpler code")
    store.todos[0].finished = true;

    Nonetheless, MobX has an optional built-in concept of actions. Read this section as well if you want to know more about writing asynchronous actions. It's easy! Use them to your advantage; they will help you to structure your code better and make wise decisions about when and where state should be modified.

    MobX: Simple and scalable

    MobX is a simple, very scaleable and unobtrusive state management library.

    Using classes and real references

    With MobX you don't need to normalize your data. This makes the library very suitable for very complex domain models. (At Mendix, for example, there are ~500 different domain classes in a single application.)

    Referential integrity is guaranteed

    Since data doesn't need to be normalized and MobX automatically tracks the relations between state and derivations, you get referential integrity for free.

    Rendering something that is accessed through three levels of indirection? No problem. MobX will track them and re-render whenever one of the references changes. As a result, staleness bugs are eliminated. As a programmer, you might forget that changing some data might influence a seemingly unrelated component, but MobX won't forget.

    Simpler actions are easier to maintain

    As demonstrated above, modifying state when using MobX is very straightforward. You simply write down your intentions. MobX will take care of the rest.

    Fine grained observability is efficient

    MobX builds a graph of all the derivations in your application to find the least number of re-computations that are needed to prevent staleness. "Derive everything" might sound expensive, but MobX builds a virtual derivation graph to minimize the number of recomputations needed to keep derivations in sync with the state.

    In fact, when testing MobX at Mendix we found out that using this library to track the relations in our code is often a lot more efficient than pushing changes through our application by using handwritten events or "smart" selector based container components.

    The simple reason is that MobX will establish far more fine grained 'listeners' on your data than you would do as a programmer.

    Secondly, MobX sees the causality between derivations, so it can order them in such a way that no derivation has to run twice or introduce a glitch.

    How that works? See this in-depth explanation of MobX.

    Easy interoperability

    MobX works with plain JavaScript structures. Due to its unobtrusiveness, it works with most JavaScript libraries out of the box without needing MobX specific library add-ons.

    So, you can simply keep using your existing router, data fetching, and utility libraries like react-router, director, superagent, lodash, etc.

    For the same reason, you can use it with both server and client side, isomorphic and react-native applications.

    The result of this is that you often need to learn fewer new concepts when using MobX in comparison to other state management solutions.


    MobX is inspired by reactive programming principles found in spreadsheets. It is inspired by MVVM frameworks such as MeteorJS tracker, Knockout and Vue.js. But, MobX brings Transparent Functional Reactive Programming to the next level and provides a stand alone implementation. It implements TFRP in a glitch-free, synchronous, predictable and efficient manner.

    A ton of credit goes to Mendix for providing the flexibility and support to maintain MobX and the chance to prove the philosophy of MobX in real, complex, performance critical applications.

    And finally, kudos to all the people that believed in, tried, validated and even sponsored MobX.

    Further resources and documentation

    What others are saying...

    Guise, #mobx isn't pubsub, or your grandpa's observer pattern. Nay, it is a carefully orchestrated observable dimensional portal fueled by the power cosmic. It doesn't do change detection, it's actually a level 20 psionic with soul knife, slashing your viewmodel into submission.

    After using #mobx for lone projects for a few weeks, it feels awesome to introduce it to the team. Time: 1/2, Fun: 2X

    Working with #mobx is basically a continuous loop of me going “this is way too simple, it definitely won’t work” only to be proven wrong

    Try react-mobx with es6 and you will love it so much that you will hug someone.

    I have built big apps with MobX already and comparing to the one before that which was using Redux, it is simpler to read and much easier to reason about.

    The #mobx is the way I always want things to be! It's really surprising simple and fast! Totally awesome! Don't miss it!

    I've been using MobX for over 2 years now, and it still feels like cheating! 😎


    • Feel free to send small pull requests. Please discuss new features or big changes in a GitHub issue first.
    • Use npm test to run the basic test suite, npm run coverage for the test suite with coverage and npm run test:performance for the performance tests.
    • Please note that if you want to backport a feature / fix to MobX 4 a second PR needs to be opened to the mobx4-master branch.

    MobX 4 vs MobX 5

    The difference between MobX 4 and MobX 5 is that the latter uses Proxies to do property tracking. As a consequence, MobX 5 runs only on Proxy supporting browsers, in contrast to MobX 4 that runs on any ES 5 environment.

    The most noteable limitations of MobX 4:

    • Observable arrays are not real arrays, so they won't pass the Array.isArray() check. The practical consequence is that you often need to .slice() the array first (to get a real array shallow copy) before passing to third party libraries.
    • Adding properties to existing observable objects after creation is not automatically picked up. Instead, either use observable maps or use the the built-in utility functions to read / write / iterate objects that you want to dynamically add properties to.

    For more details see the caveats page.

    Flow support

    MobX ships with flow typings. Flow will automatically include them when you import MobX modules. Although you do not need to import the types explicitly, you can still do it like this: import type { ... } from 'mobx'.

    To use the flow typings shipped with MobX:

    • In .flowconfig, you cannot ignore node_modules.
    • In .flowconfig, you cannot import it explicitly in the [libs] section.
    • You do not need to install library definition using flow-typed.


    Was MobX key in making your project a success? Join our open collective or use the donate button!


    Support us with a monthly donation and help us continue our activities. [Become a backer]

    One time donations through paypal are welcome as well and are recorded in the sponsors list.



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