Cancelling some async side effects is a common requirement of Epics. While there are several ways of doing this depending on your requirements, the most common way is to have your application dispatch a cancellation action and listen for it inside your Epic.

This can be done with the takeUntil() RxJS operator:

import { ajax } from 'rxjs/ajax';

const fetchUserEpic = action$ => action$.pipe(
  mergeMap(action => ajax.getJSON(`/api/users/${action.payload}`).pipe(
    map(response => fetchUserFulfilled(response)),

Here we placed the takeUntil() inside our mergeMap(), but after our AJAX call; this is important because we want to cancel only the AJAX request, not stop the Epic from listening for any future actions. Isolating your observable chains like this is an important concept you will use often. If this isn't clear, you should consider spending some time getting intimately familiar with RxJS and generally how operator chaining works. Ben Lesh has a great video that explains how Observables work and even covers isolating your chains!

This example uses mergeMap (aka flatMap), which means it allows multiple concurrent FETCH_USER requests. If you instead want to cancel any pending request and instead switch to the latest one, you can use the switchMap operator.

Cancel and Do Something Else (Emit a Different Action)

Sometimes you want to not only cancel a side effect (such as an AJAX call), but also do something else, like emit a totally different action.

You can achieve that using the aptly named race operator. It allows you to literally "race" between streams; whichever one emits a value first wins! The losing streams are unsubscribed, cancelling any operation they were performing.

For example, let's say that we make an AJAX call when someone dispatches FETCH_USER, but if someone dispatches FETCH_USER_CANCELLED we cancel that pending AJAX request and instead emit a totally different action - in this case, to increment a counter:

import { ajax } from 'rxjs/ajax';

const fetchUserEpic = action$ => action$.pipe(
  mergeMap(action => race(
      map(response => fetchUserFulfilled(response))
      map(() => incrementCounter()),

We also need to use take(1), because we only want to listen for the cancellation action once while we're racing the AJAX call.

This brings up a worthwhile consideration: instead of following up on the cancellation event with a separate action, could you just idiomatically repurpose the original cancellation action being absorbed by your reducers? In other words, is it better to rely on a single action that triggers both the cancellation itself, and what happens afterward, or to define two unrelated actions that reflect your intent? Optimal use cases vary from one implementation to another.

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