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

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 help you to structure your code better. It takes a function and returns it after wrapping it with untracked, transaction and allowStateChanges. 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. Using the @action decorator with ES 5.1 setters (i.e. @action set propertyName) is not supported, however setters of computed properties are automatically actions.

Note: using action is mandatory when strict mode is enabled, see useStrict.

For an extensive introduction to action see also the MobX 2.2 release notes.

Two example actions from the contact-list project:

    @action    createRandomContact() {
            .set('Accept', 'application/json')
            .end(action("createRandomContact-callback", (error, results) => {
                if (error)
                else {
                    const data = JSON.parse(results.text).results[0];
                    const contact = new Contact(this, data.dob, data.name, data.login.username, data.picture)

async actions and runInAction.

action only affects the currently running function, not functions that are scheduled (but not invoked) by the current function! This means that if you have a setTimeout, promise.then or async construction, and in that callback some more state is changed, those callbacks should be wrapped in action as well! This is demonstrated above with the "createRandomContact-callback" action.

If you use async / await, this is a bit trickier as you cannot just wrap the async function body in action. In this situation runInAction can come in handy, wrap this around the places where you intend to update the state. (But don't make await calls in these blocks).


@action /*optional*/ updateDocument = async () => {
    const data = await fetchDataFromUrl();
    /* required in strict mode to be allowed to update state: */
    runInAction("update state after fetching data", () => {
        this.isSaving = true;

The usage of runInAction is: runInAction(name?, fn, scope?).

If you use babel, this plugin could help you to handle your async actions: mobx-deep-action.

Bound actions

The action decorator / function follows the normal rules for binding in javascript. However, Mobx 3 introduces action.bound to automatically bind actions to the targeted object. Note that (@)action.bound does, unlike action, not take a name parameter, the name will always be based on the property the action is bound to.


class Ticker {
    @observable this.tick = 0

    increment() {
        this.tick++ // 'this' will always be correct

const ticker = new Ticker()
setInterval(ticker.increment, 1000)


const ticker = observable({
    tick: 1,
    increment: action.bound(function() {
        this.tick++ // bound 'this'

setInterval(ticker.increment, 1000)

Note: don't use action.bind with arrow functions; arrow functions are already bound and cannot be rebound.