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MobX Api Reference

Applies to MobX 4 and higher

Core API

These are the most important MobX API's.

Understanding observable, computed, reaction and action is enough to master MobX and use it in your applications!

Creating observables



  • observable(value)
  • @observable classProperty = value

Observable values can be JS primitives, references, plain objects, class instances, arrays and maps.

Note: observable(value) is a convenience API that will succeed only if it can be made into an observable data structure (Array, Map, or observable-object). For all other values, no conversion will be performed.

You can also directly create the desired observable type, see below.

The following conversion rules are applied, but can be fine-tuned by using decorators. See below.

  1. If value is an instance of an ES6 Map: a new Observable Map will be returned. Observable maps are very useful if you don't want to react just to the change of a specific entry, but also to the addition or removal of entries.
  2. If value is an array, a new Observable Array will be returned.
  3. If value is an object without prototype or its prototype is Object.prototype, the object will be cloned and all its current properties will be made observable. See Observable Object
  4. If value is an object with a prototype, a JavaScript primitive or function, there will be no change made to the value. If you do need a Boxed Observable, you can do one of the following:
    • Call observable.box(value) explicitly
    • Use @observable in the class definition
    • Call decorate()
    • Use extendObservable() to introduce properties on a class definition

MobX will not make objects with a prototype automatically observable; as that is the responsibility of its constructor function. Use extendObservable in the constructor, or @observable in its class definition instead.

These rules might seem complicated at first sight, but you will notice that in practice they are very intuitive to work with.

Some notes:

  • To create dynamically keyed objects, always use maps! Only initially existing properties on an object will be made observable, although new ones can be added using extendObservable.
  • To use the @observable decorator, make sure that decorators are enabled in your transpiler (babel or typescript).
  • By default making a data structure observable is infective; that means that observable is applied automatically to any value that is contained by the data structure, or will be contained by the data structure in the future. This behavior can be changed by using decorators.


@observable property = value

observable can also be used as property decorator. It requires decorators to be enabled and is syntactic sugar for extendObservable(this, { property: value }).


observable.box(value, options?)

Creates an observable box that stores an observable reference to a value. Use get() to get the current value of the box, and set() to update it. This is the foundation on which all other observables are built, but in practice you will use it rarely.

Normal boxes will automatically try to turn any new value into an observable if it isn't already. Use the {deep: false} option to disable this behavior.


observable.object(value, decorators?, options?)

Creates a clone of the provided object and makes all its properties observable. By default any values in those properties will be made observable as well, but when using the {deep: false} options, only the properties will be made into observable references, leaving the values untouched. (This holds also for any values assigned in the future).

The second argument in observable.object() can be used to fine tune the observability with decorators.


observable.array(value, options?)

Creates a new observable array based on the provided value.

Use the {deep: false} option if the values in the array should not be turned into observables.


observable.map(value, options?)

Creates a new observable map based on the provided value. Use the {deep: false} option if the values in the map should not be turned into observables.

Use map whenever you want to create a dynamically keyed collections and the addition / removal of keys needs to be observed. Since this uses the full-blown ES6 Map internally, you are free to use any type for the key and not limited to string keys.



Usage: extendObservable(target, properties, decorators?, options?).

For each key/value pair in each propertyMap a (new) observable property will be introduced on the target object. This can be used in constructor functions to introduce observable properties without using decorators. If a value of the propertyMap is a getter function, a computed property will be introduced.

Use extendObservable(target, props, decorators?, {deep: false}) if the new properties should not be infective (that is; newly assigned values should not be turned into observables automatically). Note that extendObservable enhances existing objects, unlike observable.object which creates a new object.



Use decorators to fine tune the observability of properties defined via observable, extendObservable and observable.object. They can also control the auto-conversion rules for specific properties.

The following decorators are available:

  • observable.deep: This is the default decorator, used by any observable. It converts any assigned, non-primitive value into an observable if it isn't one yet.
  • observable.ref: Disables automatic observable conversion, just creates an observable reference instead.
  • observable.shallow: Can only be used in combination with collections. Turns any assigned collection into an collection, which is shallowly observable (instead of deep). In other words; the values inside the collection won't become observables automatically.
  • computed: Creates a derived property, see computed
  • action: Creates an action, see action
  • action.bound: Creates a bound action, see action

You can apply these decorators using the @decorator syntax:

import {observable, action} from 'mobx';

class TaskStore {
    @observable.shallow tasks = []
    @action addTask(task) { /* ... */ }

Or by passing in property decorators via observable.object / observable.extendObservable or decorate(). Note that decorators always 'stick' to the property. So they will remain in effect even if a new value is assigned.

import {observable, action} from 'mobx';

const taskStore = observable({
    tasks: [],
    addTask(task) { /* ... */ }
}, {
    tasks: observable.shallow,
    addTask: action



Usage: decorate(object, decorators) This is a convenience method to apply observability decorators to the properties of a plain object or class instance. The second argument is an object with properties set to certain decorators.

Use this if you cannot use the @decorator syntax or need more control over setting observability.

class TodoList {
    todos = {}
    get unfinishedTodoCount() {
        return values(this.todos).filter(todo => !todo.finished).length
    addTodo() {
        const t = new Todo()
        t.title = 'Test_' + Math.random()
        set(this.todos, t.id, t)

decorate(TodoList, {
    todos: observable,
    unfinishedTodoCount: computed,
    addTodo: action.bound

Computed values


  • computed(() => expression)
  • computed(() => expression, (newValue) => void)
  • computed(() => expression, options)
  • @computed get classProperty() { return expression; }
  • @computed({equals: compareFn}) get classProperty() { return expression; }
  • @computed.struct get classProperty() { return expression; }

Creates a computed property. The expression should not have side effects but return a value. The expression will automatically be re-evaluated if any observables it uses changes, but only if it is in use by some reaction.

There are various options that can be used to control the behavior of computed. These include:

  • equals: (value, value) => boolean Comparison method can be used to override the default detection on when something is changed. Built-in comparers are: comparer.identity, comparer.default, comparer.structural.
  • name: string Provide a debug name to this computed property
  • requiresReaction: boolean Wait for a change in value of the tracked observables, before recomputing the derived property
  • get: () => value) Override the getter for the computed property.
  • set: (value) => void Override the setter for the computed property



Any application has actions. Actions are anything that modify the state.

With MobX you can make it explicit in your code where your actions live by marking them. Actions helps you to structure your code better. It is advised to use them on any function that modifies observables or has side effects. action also provides useful debugging information in combination with the devtools. Note: using action is mandatory when strict mode is enabled, see enforceActions. «details»


  • action(fn)
  • action(name, fn)
  • @action classMethod
  • @action(name) classMethod
  • @action boundClassMethod = (args) => { body }
  • @action.bound boundClassMethod(args) { body }

For one-time-actions runInAction(name?, fn) can be used, which is sugar for action(name, fn)().


Usage: flow(function* (args) { })

flow() takes a generator function as its only input

When dealing with async actions, the code that executes in the callback is not wrapped by action. This means the observable state that you are mutating, will fail the enforceActions check. An easy way to retain the action semantics is by wrapping the async function with flow. This will ensure to wrap all your callbacks in action().

Note that the async function must be a generator and you must only yield to promises inside. flow gives you back a promise that you can cancel() if you want.

import { configure } from 'mobx';

// don't allow state modifications outside actions
configure({enforceActions: true});

class Store {
    @observable githubProjects = [];
    @observable state = "pending"; // "pending" / "done" / "error"

    fetchProjects = flow(function* fetchProjects() { // <- note the star, this a generator function!
        this.githubProjects = [];
        this.state = "pending";
        try {
            const projects = yield fetchGithubProjectsSomehow(); // yield instead of await
            const filteredProjects = somePreprocessing(projects);

            // the asynchronous blocks will automatically be wrapped actions
            this.state = "done";
            this.githubProjects = filteredProjects;
        } catch (error) {
            this.state = "error";

Tip: it is recommended to give the generator function a name, this is the name that will show up in dev tools and such

Flows can be cancelled

Flows are canceallable, that means that you can call cancel() on the returned promise. This will stop the generator immediately, but any finally clause will still be processed. The returned promise itself will reject with FLOW_CANCELLED

Flows support async iterators

Flows support async iterators, that means you can use async generators:

async function* someNumbers() {
    yield Promise.resolve(1)
    yield Promise.resolve(2)
    yield Promise.resolve(3)

const count = mobx.flow(async function*() {
    // use for await to loop async iterators
    for await (const number of someNumbers()) {
        total += number
    return total

const res = await count() // 6

Reactions & Derivations

Computed values are values that react automatically to state changes. Reactions are side effects that react automatically to state changes. Reactions can be used to ensure that a certain side effect (mainly I/O) is automatically executed when relevant state changes, like logging, network requests etc. The most commonly used reaction is the observer decorator for React components (see below).


Can be used as higher order component around a React component. The component will then automatically re-render if any of the observables used in the render function of the component has changed. Note that observer is provided by the "mobx-react" package and not by "mobx" itself.



  • observer(React.createClass({ ... }))
  • observer((props, context) => ReactElement)
  • observer(class MyComponent extends React.Component { ... })
  • @observer class MyComponent extends React.Component { ... }


Usage: autorun(() => { sideEffect }, options). Autorun runs the provided sideEffect and tracks which observable state is accessed while running the side effect. Whenever one of the used observables is changed in the future, the same sideEffect will be run again. Returns a disposer function to cancel the side effect.



  • name?: string: A name for easier identification and debugging
  • delay?: number: the sideEffect will be delayed and debounced with the given delay. Defaults to 0.
  • onError?: (error) => void: error handler that will be triggered if the autorun function throws an exception
  • scheduler?: (callback) => void: Set a custom scheduler to determine how re-running the autorun function should be scheduled


Usage: when(() => condition, () => { sideEffect }, options). The condition expression will react automatically to any observables it uses. As soon as the expression returns true the sideEffect function will be invoked, but only once.

Note: the effect-function (second argument) is actually optional. If no effect-function is provided, it will return a cancelable promise (i.e. having a cancel() method on the promise)

when returns a disposer to prematurely cancel the whole thing.



  • name?: string: A name for easier identification and debugging
  • onError?: (error) => void: error handler that will be triggered if the predicate-function or the effect-function throws an exception
  • timeout: number a timeout in milliseconds, after which the onError handler will be triggered to signal the condition not being met within a certain time


Usage: reaction(() => data, data => { sideEffect }, options). A variation on autorun that gives more fine-grained control on which observables that will be tracked. It takes two function, the first one is tracked and returns data that is used as input for the second one, the side effect. Unlike autorun the side effect won't be run initially, and any observables that are accessed while executing the side effect will not be tracked. The side effect can be debounced, just like autorunAsync.



  • fireImmediately?: boolean: Wait for a change before firing the effect function. Defaults to false.
  • delay?: number: the sideEffect will be delayed and debounced with the given delay. Defaults to 0.
  • equals: Custom equality function to determine whether the expr function differed from it's previous result, and hence should fire effect. Accepts the same options as the equals option of computed.
  • Also accepts all of the options from autorun


Usage: onReactionError(handler: (error: any, derivation) => void)

This method attaches a global error listener, which is invoked for every error that is thrown from a reaction. This can be used for monitoring or test purposes.


Here are some utilities that might make working with observable objects or computed values more convenient. More, less trivial utilities can be found in the * mobx-utils package.

Provider (mobx-react package)

Can be used to pass stores to child components using React's context mechanism. See the mobx-react docs.

inject (mobx-react package)

Higher order component and counterpart of Provider. Can be used to pick stores from React's context and pass it as props to the target component. Usage:

  • inject("store1", "store2")(observer(MyComponent))
  • @inject("store1", "store2") @observer MyComponent
  • @inject((stores, props, context) => props) @observer MyComponent
  • @observer(["store1", "store2"]) MyComponent is a shorthand for the the @inject() @observer combo.


Usage: toJS(observableDataStructure, options?). Converts observable data structures back to plain javascript objects, ignoring computed values.

The options include:

  • detectCycles: boolean: Checks for cyclical references in the observable data-structure. Defaults to true.
  • exportMapsAsObjects: boolean: Treats ES6 Maps as regular objects for export. Defaults to true


isObservable and isObservableProp

Usage: isObservable(thing) or isObservableProp(thing, property?). Returns true if the given thing, or the property of the given thing is observable. Works for all observables, computed values and disposer functions of reactions.


isObservableObject|Array|Map and isBoxedObservable

Usage: isObservableObject(thing), isObservableArray(thing), isObservableMap(thing), isBoxedObservable(thing). Returns true if.., well, do the math.


Usage: isArrayLike(thing). Returns true if the given thing is either a true JS-array or an observable (MobX-)array. This is intended as convenience/shorthand. Note that observable arrays can be .slice()d to turn them into true JS-arrays.


Usage: isAction(func). Returns true if the given function is wrapped / decorated with action.

isComputed and isComputedProp

Usage: isComputed(thing) or isComputedProp(thing, property?). Returns true if the given thing is a boxed computed value, or if the designated property is a computed value.


Usage: intercept(object, property?, interceptor). Api that can be used to intercept changes before they are applied to an observable api. Useful for validation, normalization or cancellation.



Usage: observe(object, property?, listener, fireImmediately = false) Low-level api that can be used to observe a single observable value.


onBecomeObserved and onBecomeUnobserved

Usage: onBecomeObserved(observable, property?, listener: () => void): (() => void) and onBecomeUnobserved(observable, property?, listener: () => void): (() => void)

These functions are hooks into the observability system of MobX and get notified when observables start / stop becoming observed. It can be used to execute some lazy operations or perform network fetches.

The return value is a diposer-function that will detach the listener.

export class City {
    @observable location
    @observable temperature

    constructor(location) {
        this.location = location
        // only start data fetching if temperature is actually used!
        onBecomeObserved(this, 'temperature', this.resume)
        onBecomeUnobserved(this, 'temperature', this.suspend)

    resume = () => {
        log(`Resuming ${this.location}`)
        this.interval = setInterval(() => this.fetchTemperature(), 5000)

    suspend = () => {
        log(`Suspending ${this.location}`)
        this.temperature = undefined

    @flow fetchTemperature = function*() {
        // data fetching logic


Usage: configure(options). Sets global behavior settings on the active MobX instance. Use this to change how MobX behaves as a whole.

import { configure } from "mobx";

    // ...

arrayBuffer: number

Increases the default created size of observable arrays to arrayBuffer, if the maximum size isn't yet there.

Observable arrays lazily create getters on members of ObservableArray.prototype starting at 0. This will create the members from 0 to arrayBuffer if they don't yet exist. Use arrayBuffer if you know you'll have a common minimum array size and don't want to risk first creating those getters in hot code paths. See also observable.

computedRequiresReaction: boolean

Forbids the access of any unobserved computed value. Use this if you want to check whether you are using computed properties without a reactive context.

configure({ computedRequiresReaction: true });

disableErrorBoundaries: boolean

By default, MobX will catch and rethrow exceptions happening in your code to make sure that a reaction in one exception does not prevent the scheduled execution of other, possibly unrelated, reactions. This means exceptions are not propagated back to the original causing code and therefore you won't be able to catch them using try/catch.

There may be times when you want to catch those errors, for example when unit testing your reactions. You can disable this behaviour using disableErrorBoundaries.

configure({ disableErrorBoundaries: true });

Please note that MobX won't recover from errors when using this configuration. For that reason, you may need to use _resetGlobalState after each exception. Example:

configure({ disableErrorBoundaries: true })

test('Throw if age is negative', () => {
  expect(() => {
    const age = observable.box(10)
    autorun(() => { if (age.get() < 0) throw new Error('Age should not be negative') })
  _resetGlobalState() // Needed after each exception

Prior to MobX 4, _resetGlobalState was extras.resetGlobalState.

enforceActions: boolean

Also known as "strict mode". In strict mode, it is not allowed to change any state outside of an action. This is recommended when working in larger applications with complex layers of state computations. See also allowStateChanges.

Prior to MobX 4, this behavior was enabled by useStrict(): void.

configure({ enforceActions: true });

Since MobX 4.2, it is also possible to specify

configure({ enforceActions: "strict" });

In sloppy (true) mode, MobX will only throw if an observed observable is modified, in "strict" mode it will even throw for non-observed observables

isolateGlobalState: boolean

Isolates the global state of MobX, when there are multiple instances of MobX in the same environment. This is useful when you have an encapsulated library that is using MobX, living in the same page as the app that is using MobX. The reactivity inside the library will remain self-contained when you call configure({isolateGlobalState: true}) inside the library. Additionally, MobX won't throw an error that there are multiple instances in the global scope.

configure({ isolatedGlobalState: true });

reactionScheduler: (f: () => void) => void

Sets a new function that executes all MobX reactions. By default reactionScheduler just runs the f reaction without any other behavior. This can be useful for basic debugging, or slowing down reactions to visualize application updates.

    reactionScheduler: (f): void => {
        console.log("Running an event after a delay:", f);
        setTimeout(f, 100);

Direct Observable manipulation

There is now an utility API that enables manipulating observable maps, objects and arrays with the same API. These api's are fully reactive, which means that even new property declarations can be detected by mobx if set is used to add them, and values or keys to iterate them.

  • values(thing) returns all values in the collection as array
  • keys(thing) returns all keys in the collection as array
  • set(thing, key, value) or set(thing, { key: value }) Updates the given collection with the provided key / value pair(s).
  • remove(thing, key) removes the specified child from the collection. For arrays splicing is used.
  • has(thing, key) returns true if the collection has the specified observable property.
  • get(thing, key) returns the chlid under the specified key.

Development utilities

The following api's might come in handy if you want to build cool tools on top of MobX or if you want to inspect the internal state of MobX

"mobx-react-devtools" package

The mobx-react-devtools is a powerful package that helps you to investigate the performance and dependencies of your react components. Also has a powerful logger utility based on spy.




  • trace(enterDebugger?)
  • trace(Reaction object / ComputedValue object / disposer function, enterDebugger?)
  • trace(object, computedValuePropertyName, enterDebugger?)

trace is a small utility that you can use inside a computed value or reaction. If it is enabled, it will start logging when the value is being invalidated, and why. If enterDebugger is set to true, and developer tools are enabled, the javascript engine will break on the point where it is triggered.



Usage: spy(listener). Registers a global spy listener that listens to all events that happen in MobX. It is similar to attaching an observe listener to all observables at once, but also notifies about running (trans/re)actions and computations. Used for example by the mobx-react-devtools.



Usage: getAtom(thing, property?). Returns the backing Atom of a given observable object, property, reaction etc.


Usage: getDebugName(thing, property?) Returns a (generated) friendly debug name of an observable object, property, reaction etc. Used by for example the mobx-react-devtools.


Usage: getDependencyTree(thing, property?). Returns a tree structure with all observables the given reaction / computation currently depends upon.


Usage: getObserverTree(thing, property?). Returns a tree structure with all reactions / computations that are observing the given observable.

"mobx-react" development hooks

The mobx-react package exposes the following additional api's that are used by the mobx-react-devtools:

  • trackComponents(): enables the tracking of observer based React components
  • renderReporter.on(callback): callback will be invoked on each rendering of an observer enabled React component, with timing information etc
  • componentByNodeRegistery: ES6 WeakMap that maps from DOMNode to a observer based React component instance

Internal functions

The following methods are all used internally by MobX, and might come in handy in rare cases. But usually MobX offers more declarative alternatives to tackle the same problem. They might come in handy though if you try to extend MobX


Transaction is a low-level api, it is recommended to use actions instead

transaction(worker: () => void) can be used to batch a bunch of updates without notifying any observers until the end of the transaction. transaction takes a single, parameterless worker function as argument and runs it. No observers are notified until this function has completed. transaction returns any value that was returned by the worker function. Note that transaction runs completely synchronously. Transactions can be nested. Only after completing the outermost transaction pending reactions will be run.

import {observable, transaction, autorun} from "mobx";

const numbers = observable([]);

autorun(() => console.log(numbers.length, "numbers!"));
// Prints: '0 numbers!'

transaction(() => {
    transaction(() => {
// Prints: '3 numbers!'


Untracked allows you to run a piece of code without establishing observers. Like transaction, untracked is automatically applied by (@)action, so usually it makes more sense to use actions than to use untracked directly. Example:

const person = observable({
    firstName: "Michel",
    lastName: "Weststrate"

autorun(() => {
        // this untracked block will return the person's firstName without establishing a dependency
        untracked(() => person.firstName)
// prints: Weststrate, Michel

person.firstName = "G.K.";
// doesn't print!

person.lastName = "Chesterton";
// prints: Chesterton, G.K.


Utility function that can be used to create your own observable data structures and hook them up to MobX. Used internally by all observable data types.