mergeMap vs switchMap vs concatMap

I was editing my Angular app after upgrading to Angular 17. I’m still using and learning about RxJS Observables¹. I learned an important lesson about the difference between RxJS operators mergeMap, switchMap, and concatMap

Marble Diagram

This marble diagram shows the difference. Given a starting observable that emits 1, 3, and 5… If you pipe it into mergeMap or switchMap or concatMap, which in turn yields an observable ² emitting 10, 10, 10, and then completing). Notice:…

• mergeMap emits when any of the 1st-order input emits, or its 2nd-order input emits…
• switchMap emits when any of the most recent 1st-order input emits, and any of its 2nd-order input emits…
  • note how the third 30 is not emitted because the more recent observable of 50’s begins to emit…
• concatMap emits when any of the inputs emit in order, which certain emissions will be delayed to match the order of the 1st-order observable
  • notice how the first 50 waits until the last 30 is emitted…

Marble Diagrams with Swirly

Design your own marble diagram with https://swirly.dev/, or check out the Swirly Github project here

I created the marble diagram above using the input below:

% An example application of the concatAll operator.
% Edit this code to redraw the diagram in real time.
[styles.a]
fill_color = #ff6944

[styles.b]
fill_color = #3ca2cc

[styles.c]
fill_color = #ff6944

[styles.f]
fill_color = #ff6944


[styles.i]
fill_color = #ff6944


[styles.m]
fill_color = #3ca2cc

[styles.k]
fill_color = #3ca2cc


[styles.o]
fill_color = #ff6944


--a------b--c---------|
title = 1st-Order
a := 1
b := 3
c := 5

  d-e-f-|
title = 2nd-Order
d := 10
e := 10
f := 10

%https://rxjs.dev/api/index/function/mergeMap
> mergeMap(i=> 10*i----10*i----10*i)

% mergeMap(i=> `d-e-f-|`)
%d := 10
%e := 10
%f := 10

--g-h-i--j-klmn-o-----|
g := 10
h := 10
i := 10
j := 30
k := 30
m := 30
l := 50
n := 50
o := 50



%https://rxjs.dev/api/operators/switchMap
> switchMap(i=> 10*i----10*i----10*i)

--g-h-i--j-kl-n-o-----|
g := 10
h := 10
i := 10
j := 30
k := 30
l := 50
n := 50
o := 50


%https://rxjs.dev/api/operators/concatMap
> concatMap(i=> 10*i----10*i----10*i)

--g-h-i--j-k-ml-n-o---|
g := 10
h := 10
i := 10
j := 30
k := 30
m := 30
l := 50
n := 50
o := 50

¹I know about the new async/state-management technique with signals, but I guess Observables still have a place; more on signals vs observables.

²An observable within an observable? That’s a “higher-order” observable!