Utility functions and common patterns for MobX
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Utility functions and common patterns for MobX

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This package provides utility functions and common MobX patterns build on top of MobX. It is encouraged to take a peek under the hood and read the sources of these utilities. Feel free to open a PR with your own utilities. For large new features, please open an issue first.

Installation & Usage

NPM: npm install mobx-utils --save

CDN: https://unpkg.com/mobx-utils/mobx-utils.umd.js

import {function_name} from 'mobx-utils'



fromPromise takes a Promise and returns a new Promise wrapping the original one. The returned Promise is also extended with 2 observable properties that track the status of the promise. The returned object has the following observable properties:

  • value: either the initial value, the value the Promise resolved to, or the value the Promise was rejected with. use .state if you need to be able to tell the difference.
  • state: one of "pending", "fulfilled" or "rejected"

And the following methods:

  • case({fulfilled, rejected, pending}): maps over the result using the provided handlers, or returns undefined if a handler isn't available for the current promise state.
  • then((value: TValue) => TResult1 | PromiseLike<TResult1>, [(rejectReason: any) => any]): chains additional handlers to the provided promise.

The returned object implements PromiseLike<TValue>, so you can chain additional Promise handlers using then. You may also use it with await in async functions.

Note that the status strings are available as constants: mobxUtils.PENDING, mobxUtils.REJECTED, mobxUtil.FULFILLED

Observable promises can be created immediately in a certain state using fromPromise.reject(reason) or fromPromise.resolve(value?). The main advantage of fromPromise.resolve(value) over fromPromise(Promise.resolve(value)) is that the first synchronously starts in the desired state.

It is possible to directly create a promise using a resolve, reject function: fromPromise((resolve, reject) => setTimeout(() => resolve(true), 1000))


  • promise IThenable<T> The promise which will be observed


const fetchResult = fromPromise(fetch("http://someurl"))

// combine with when..
  () => fetchResult.state !== "pending"
  () => {
    console.log("Got ", fetchResult.value)

// or a mobx-react component..
const myComponent = observer(({ fetchResult }) => {
  switch(fetchResult.state) {
     case "pending": return <div>Loading...</div>
     case "rejected": return <div>Ooops... {fetchResult.value}</div>
     case "fulfilled": return <div>Gotcha: {fetchResult.value}</div>

// or using the case method instead of switch:

const myComponent = observer(({ fetchResult }) =>
    pending:   () => <div>Loading...</div>,
    rejected:  error => <div>Ooops.. {error}</div>,
    fulfilled: value => <div>Gotcha: {value}</div>,

// chain additional handler(s) to the resolve/reject:

  (result) =>  doSomeTransformation(result),
  (rejectReason) => console.error('fetchResult was rejected, reason: ' + rejectReason)
  (transformedResult) => console.log('transformed fetchResult: ' + transformedResult)

Returns IPromiseBasedObservable<T>


Returns true if the provided value is a promise-based observable.


  • value any

Returns boolean


Moves an item from one position to another, checking that the indexes given are within bounds.



const source = observable([1, 2, 3])
moveItem(source, 0, 1)
console.log(source.map(x => x)) // [2, 1, 3]

Returns ObservableArray<T>


lazyObservable creates an observable around a fetch method that will not be invoked until the observable is needed the first time. The fetch method receives a sink callback which can be used to replace the current value of the lazyObservable. It is allowed to call sink multiple times to keep the lazyObservable up to date with some external resource.

Note that it is the current() call itself which is being tracked by MobX, so make sure that you don't dereference to early.


  • fetch
  • initialValue T optional initialValue that will be returned from current as long as the sink has not been called at least once (optional, default undefined)


const userProfile = lazyObservable(
  sink => fetch("/myprofile").then(profile => sink(profile))

// use the userProfile in a React component:
const Profile = observer(({ userProfile }) =>
  userProfile.current() === undefined
  ? <div>Loading user profile...</div>
  : <div>{userProfile.current().displayName}</div>

// triggers refresh the userProfile


fromResource creates an observable whose current state can be inspected using .current(), and which can be kept in sync with some external datasource that can be subscribed to.

The created observable will only subscribe to the datasource if it is in use somewhere, (un)subscribing when needed. To enable fromResource to do that two callbacks need to be provided, one to subscribe, and one to unsubscribe. The subscribe callback itself will receive a sink callback, which can be used to update the current state of the observable, allowing observes to react.

Whatever is passed to sink will be returned by current(). The values passed to the sink will not be converted to observables automatically, but feel free to do so. It is the current() call itself which is being tracked, so make sure that you don't dereference to early.

For inspiration, an example integration with the apollo-client on github, or the implementation of mobxUtils.now

The following example code creates an observable that connects to a dbUserRecord, which comes from an imaginary database and notifies when it has changed.


  • subscriber
  • unsubscriber IDisposer (optional, default NOOP)
  • initialValue T the data that will be returned by get() until the sink has emitted its first data (optional, default undefined)


function createObservableUser(dbUserRecord) {
  let currentSubscription;
  return fromResource(
    (sink) => {
      // sink the current state
      // subscribe to the record, invoke the sink callback whenever new data arrives
      currentSubscription = dbUserRecord.onUpdated(() => {
    () => {
      // the user observable is not in use at the moment, unsubscribe (for now)

// usage:
const myUserObservable = createObservableUser(myDatabaseConnector.query("name = 'Michel'"))

// use the observable in autorun
autorun(() => {
  // printed everytime the database updates its records

// ... or a component
const userComponent = observer(({ user }) =>


Converts an expression to an observable stream (a.k.a. TC 39 Observable / RxJS observable). The provided expression is tracked by mobx as long as there are subscribers, automatically emitting when new values become available. The expressions respect (trans)actions.


  • expression
  • fireImmediately boolean (by default false)


const user = observable({
  firstName: "C.S",
  lastName: "Lewis"

  .from(mobxUtils.toStream(() => user.firstname + user.lastName))
  .scan(nameChanges => nameChanges + 1, 0)
  .subscribe(nameChanges => console.log("Changed name ", nameChanges, "times"))

Returns IObservableStream<T>



Converts an subscribable, observable stream (TC 39 observable / RxJS stream) into an object which stores the current value (as current). The subscription can be cancelled through the dispose method. Takes an initial value as second optional argument


  • observable IObservableStream<T>
  • initialValue


const debouncedClickDelta = MobxUtils.fromStream(Rx.Observable.fromEvent(button, 'click')
    .map(event => event.clientX)
    .scan((count, clientX) => count + clientX, 0)

autorun(() => {
    console.log("distance moved", debouncedClickDelta.current)



createViewModel takes an object with observable properties (model) and wraps a viewmodel around it. The viewmodel proxies all enumerable property of the original model with the following behavior:

  • as long as no new value has been assigned to the viewmodel property, the original property will be returned.
  • any future change in the model will be visible in the viewmodel as well unless the viewmodel property was dirty at the time of the attempted change.
  • once a new value has been assigned to a property of the viewmodel, that value will be returned during a read of that property in the future. However, the original model remain untouched until submit() is called.

The viewmodel exposes the following additional methods, besides all the enumerable properties of the model:

  • submit(): copies all the values of the viewmodel to the model and resets the state
  • reset(): resets the state of the viewmodel, abandoning all local modifications
  • resetProperty(propName): resets the specified property of the viewmodel
  • isDirty: observable property indicating if the viewModel contains any modifications
  • isPropertyDirty(propName): returns true if the specified property is dirty
  • model: The original model object for which this viewModel was created

You may use observable arrays, maps and objects with createViewModel but keep in mind to assign fresh instances of those to the viewmodel's properties, otherwise you would end up modifying the properties of the original model. Note that if you read a non-dirty property, viewmodel only proxies the read to the model. You therefore need to assign a fresh instance not only the first time you make the assignment but also after calling reset() or submit().


  • model T


class Todo {
  \@observable title = "Test"

const model = new Todo()
const viewModel = createViewModel(model);

autorun(() => console.log(viewModel.model.title, ",", viewModel.title))
// prints "Test, Test"
model.title = "Get coffee"
// prints "Get coffee, Get coffee", viewModel just proxies to model
viewModel.title = "Get tea"
// prints "Get coffee, Get tea", viewModel's title is now dirty, and the local value will be printed
// prints "Get tea, Get tea", changes submitted from the viewModel to the model, viewModel is proxying again
viewModel.title = "Get cookie"
// prints "Get tea, Get cookie" // viewModel has diverged again
// prints "Get tea, Get tea", changes of the viewModel have been abandoned


Like normal when, except that this when will automatically dispose if the condition isn't met within a certain amount of time.


  • expr
  • action
  • timeout number maximum amount when spends waiting before giving up (optional, default 10000)
  • onTimeout any the ontimeout handler will be called if the condition wasn't met within the given time (optional, default ())


test("expect store to load", t => {
  const store = {
    items: [],
    loaded: false
  fetchDataForStore((data) => {
    store.items = data;
    store.loaded = true;
    () => store.loaded
    () => t.end()
    () => t.fail("store didn't load with 2 secs")

Returns IDisposer disposer function that can be used to cancel the when prematurely. Neither action or onTimeout will be fired if disposed


MobX normally suspends any computed value that is not in use by any reaction, and lazily re-evaluates the expression if needed outside a reaction while not in use. keepAlive marks a computed value as always in use, meaning that it will always fresh, but never disposed automatically.


  • _1
  • _2
  • target Object an object that has a computed property, created by @computed or extendObservable
  • property string the name of the property to keep alive


const obj = observable({
  number: 3,
  doubler: function() { return this.number * 2 }
const stop = keepAlive(obj, "doubler")

Returns IDisposer stops this keep alive so that the computed value goes back to normal behavior



  • _1
  • _2
  • computedValue IComputedValue<any> created using the computed function


const number = observable(3)
const doubler = computed(() => number.get() * 2)
const stop = keepAlive(doubler)
// doubler will now stay in sync reactively even when there are no further observers
// normal behavior, doubler results will be recomputed if not observed but needed, but lazily

Returns IDisposer stops this keep alive so that the computed value goes back to normal behavior


queueProcessor takes an observable array, observes it and calls processor once for each item added to the observable array, optionally deboucing the action


  • observableArray Array<T> observable array instance to track
  • processor
  • debounce number optional debounce time in ms. With debounce 0 the processor will run synchronously (optional, default 0)


const pendingNotifications = observable([])
const stop = queueProcessor(pendingNotifications, msg => {
  // show Desktop notification
  new Notification(msg);

// usage:

Returns IDisposer stops the processor


chunkProcessor takes an observable array, observes it and calls processor once for a chunk of items added to the observable array, optionally deboucing the action. The maximum chunk size can be limited by number. This allows both, splitting larger into smaller chunks or (when debounced) combining smaller chunks and/or single items into reasonable chunks of work.


  • observableArray Array<T> observable array instance to track
  • processor
  • debounce number optional debounce time in ms. With debounce 0 the processor will run synchronously (optional, default 0)
  • maxChunkSize number optionally do not call on full array but smaller chunks. With 0 it will process the full array. (optional, default 0)


const trackedActions = observable([])
const stop = chunkProcessor(trackedActions, chunkOfMax10Items => {
}, 100, 10)

// usage:
// when both pushes happen within 100ms, there will be only one call to server

Returns IDisposer stops the processor


Returns the current date time as epoch number. The date time is read from an observable which is updated automatically after the given interval. So basically it treats time as an observable.

The function takes an interval as parameter, which indicates how often now() will return a new value. If no interval is given, it will update each second. If "frame" is specified, it will update each time a requestAnimationFrame is available.

Multiple clocks with the same interval will automatically be synchronized.

Countdown example: https://jsfiddle.net/mweststrate/na0qdmkw/


  • interval (number | "frame") interval in milliseconds about how often the interval should update (optional, default 1000)


const start = Date.now()

autorun(() => {
  console.log("Seconds elapsed: ", (mobxUtils.now() - start) / 1000)


deprecated this functionality can now be found as flow in the mobx package. However, flow is not applicable as decorator, where asyncAction still is.

asyncAction takes a generator function and automatically wraps all parts of the process in actions. See the examples below. asyncAction can be used both as decorator or to wrap functions.

  • It is important that asyncAction should always be used with a generator function (recognizable asfunction_or_name` syntax)
  • Each yield statement should return a Promise. The generator function will continue as soon as the promise settles, with the settled value
  • When the generator function finishes, you can return a normal value. The asyncAction wrapped function will always produce a promise delivering that value.

When using the mobx devTools, an asyncAction will emit action events with names like:

  • "fetchUsers - runid: 6 - init"
  • "fetchUsers - runid: 6 - yield 0"
  • "fetchUsers - runid: 6 - yield 1"

The runId represents the generator instance. In other words, if fetchUsers is invoked multiple times concurrently, the events with the same runid belong toghether. The yield number indicates the progress of the generator. init indicates spawning (it won't do anything, but you can find the original arguments of the asyncAction here). yield 0 ... yield n indicates the code block that is now being executed. yield 0 is before the first yield, yield 1 after the first one etc. Note that yield numbers are not determined lexically but by the runtime flow.

asyncActions requires Promise and generators to be available on the target environment. Polyfill Promise if needed. Both TypeScript and Babel can compile generator functions down to ES5.

N.B. due to a babel limitation, in Babel generatos cannot be combined with decorators. See also #70


  • arg1
  • arg2


import {asyncAction} from "mobx-utils"

let users = []

const fetchUsers = asyncAction("fetchUsers", function* (url) {
  const start = Date.now()
  const data = yield window.fetch(url)
  users = yield data.json()
  return start - Date.now()

fetchUsers("http://users.com").then(time => {
  console.dir("Got users", users, "in ", time, "ms")
import {asyncAction} from "mobx-utils"

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

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

	*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"

Returns Promise


Like normal when, except that this when will return a promise that resolves when the expression becomes truthy


  • fn
  • timeout number maximum amount of time to wait, before the promise rejects


await whenAsync(() => !state.someBoolean)

Returns any Promise for when an observable eventually matches some condition. Rejects if timeout is provided and has expired


expr can be used to create temporarily views inside views. This can be improved to improve performance if a value changes often, but usually doesn't affect the outcome of an expression.

In the following example the expression prevents that a component is rerender each time the selection changes; instead it will only rerenders when the current todo is (de)selected.


  • expr


const Todo = observer((props) => {
    const todo = props.todo;
    const isSelected = mobxUtils.expr(() => props.viewState.selection === todo);
    return <div className={isSelected ? "todo todo-selected" : "todo"}>{todo.title}</div>


Creates a function that maps an object to a view. The mapping is memoized.

See: https://mobx.js.org/refguide/create-transformer.html


  • transformer
  • onCleanup