What does mergeMap do?What does mergeMap do?

AngularMiddle~1 min read

In short (as in an interview)

mergeMap is an RxJS operator that subscribes to every inner Observable and runs them in parallel, merging the results into a single stream.

It cancels nothing and queues nothing.

In detail

mergeMap (also known as flatMap) is a mapping operator that:

takes a value from the source Observable
creates an inner Observable
subscribes to all inner Observables at once
merges their results

Signature:

source$.pipe(
  mergeMap(value => observable$)
)

Example

import { of } from 'rxjs';
import { delay, mergeMap } from 'rxjs/operators';

of(1, 2, 3).pipe(
  mergeMap(v => of(v).pipe(delay(1000)))
).subscribe(console.log);

Result:

1
2
3

They all arrive at roughly the same time, after 1 second, because they run in parallel.

Visually

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

mergeMap:
inner a:     -----
inner b:     -----
inner c:     -----

result:      --- --- ---

When mergeMap is used

When:

order doesn't matter
the operations are independent
you need parallelism
operations must not be canceled

Angular examples:

sending analytics
logging
batch requests
loading several resources
background operations

For example:

from(ids).pipe(
  mergeMap(id => this.api.loadUser(id))
)

The loads will run in parallel.

An important nuance (frequently asked)

mergeMap can lead to:

race conditions
overloading the API
subscription leaks

That's why concurrency is sometimes limited:

mergeMap(fn, 3)

No more than 3 active Observables at a time.

Difference from the other map operators

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

This is an almost mandatory table for an Angular interview.

A typical follow-up question

This is a favorite question:

Why is mergeMap dangerous for HTTP?

Answer: because you can trigger many parallel requests, run into a data race, or overload the server.

A mini cheat sheet for the interview

You can remember it like this:

mergeMap - for independent operations concatMap - when order matters switchMap - for cancelable requests exhaustMap - to guard against repeated actions

Short Answer

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