What does exhaustMap do?

AngularMiddle~1 min read

The last operator from the "big four" RxJS map operators.

In short (as in an interview)

exhaustMap is an RxJS operator that ignores new values until the current inner Observable completes.

It's used to guard against repeated user actions (for example, a double click).

In detail

exhaustMap works like this:

a value arrives from the source Observable
an inner Observable is created
while it runs, all new values are ignored
once it completes, the next value can be accepted

Example

import { fromEvent } from 'rxjs';
import { exhaustMap } from 'rxjs/operators';

fromEvent(button, 'click').pipe(
  exhaustMap(() => this.http.post('/save', data))
).subscribe();

If the user clicks:

click click click click

only one HTTP request executes, the rest are ignored.

Visually

source:      a----b----c

exhaustMap:
inner a:     ----------
b ignored
c ignored

result:      ----------

When exhaustMap is used

Very typical Angular use cases:

form submit
login
payment
preventing double-clicks
guarding against repeated HTTP requests

For example:

this.loginClicks$.pipe(
  exhaustMap(() => this.authService.login())
)

Difference from other map operators

This is an almost mandatory question at an Angular interview.

Operator	Behavior
switchMap	cancels the previous one
concatMap	queue
mergeMap	in parallel
exhaustMap	ignores new ones

An important nuance

exhaustMap does not cancel the current operation, it simply ignores new ones.

This is the main difference from switchMap.

A typical follow-up question

Very often asked:

switchMap vs exhaustMap?

Short answer:

switchMap → cancels the previous one
exhaustMap → ignores new ones

A real Angular use case

fromEvent(button, 'click').pipe(
  exhaustMap(() => this.api.save())
)

Protects against:

double submit
duplicate requests
accidental clicks

A mini cheat sheet for map operators

In an interview, you can answer like this:

switchMap - switches concatMap - queue mergeMap - parallel exhaustMap - ignores

Short Answer

Interview ready

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