There is a slight difference between `map`...`concatAll` and `concatMap`
in that the latter (`concatMap`) will ensure that the computations are
executed in-order and only if needed while the former may execute the
`map` body of all items if they do not emit immediately. That is, if the stream
is
`from([a, b, c]).pipe(map(v => of(v).pipe(delay(1))), concatAll(), first())`
the `map` body will execute for all of `a`, `b`, and `c`.
However, the following will only execute the `concatMap` body for `a`
`from([a, b, c]).pipe(concatMap(v => of(v).pipe(delay(1))), first())`
See https://stackblitz.com/edit/rxjs-cvwxyxfixes#38691
PR Close#38780
9c51ba321e