concatMap

Reference · 23

concatMap

concatMap

rxjs.dev

Runs inner streams sequentially, waiting for the previous one to complete.

Signature

concatMap(project): OperatorFunction

Marble diagram

outer:    --a---b---|
inner(a): --1-2-3|
result:   --1-2-3-----1-2-3-|

What it does

Equivalent to mergeMap(fn, 1): the next inner starts only after the previous one completes. Guarantees strict output order.

When to use

Ordered database writes, a queue of user actions, migrations, wizard steps. Whenever order matters more than speed.

Gotcha

If a previous inner never completes, the later ones never start. Infinite sources (interval without take) break the logic.

See also

mergeMap, switchMap, concat