switchMap not double-flattening its internals

[dariomannu]

Whether this is a bug is open to debate (more like a subtle GOTCHA, in fact), but it's been causing a few inglorious debugging parades that may be worth some discussion.

The journey starts with a switchMap step that makes a humble fetch call, as we rely on the fact that Promises, as well as async functions, normally get flatMapped back into Observables, so we can conveniently do the following:

const stream = new Subject().pipe( switchMap(async data=> { const res = await fetch(data).then(j=>j.json()); return res; }) );

What happens in real life is you start refactoring and perhaps you change the internals of the above step, replacing your promise with an Observable:

const stream = new Subject().pipe( switchMap(async data=> { const res = from(data).pipe( blah() ); return res; }) );

All seems fine, right? switchMap is known to flatten internal streams, as well...

Nope, what happens instead is you notice some really weird behaviour first, spend a few hours deep debugging, wondering why are you suddenly getting an observable somewhere else in your app when you were expecting a value.

Turns out, whilst refactoring that async function above in which you turned a Promise into an Observable, you most certainly forgot to remove the async keyword, as well!!!
Chances are it wasn't even the first time you did.

So, your switchMap step started returning a Promise<Observable<T>> instead of a Promise<T> or Observable<T>.

Hard luck, sure, one may argue; switchMap is working as expected by performing ONE flattening step, as designed.

Solutions?

Double-flattening doesn't look like the most elegant/viable/acceptable solution, but at least we may have the chance to make some improvements?

Perhaps raising a warning from switchMap (and mergeMap, etc), when such a situation is detected?

Anybody ever been in this situation?