of emits a value after take(n) should've already unsubscribed

[OmerGronich]

Hello,

I ran into the following behavior which seems like a bug.

In this stream, I have a take(3) so I expected to see 3 logs from each concatMap.
Why am I seeing a fourth value emission from the first concatMap?
Important thing to note, if you try to return a synchronous value from the concatMap, like of([v, v]) instead of a promise the fourth value emission disappears which makes me think it has something to do with promises inside observables and schedulers.

stackblitz link:
https://stackblitz.com/edit/rxjs-9cqjn6?file=index.ts

[voliva]

I think this is expected:

• of emit all of the values synchronously.
• The first concatMap keeps all of those emissions on a buffer, as it needs to run the map function for each one of them.
• For each one, it waits for a microtask to complete before moving to the next one.
• The second concatMap does the same: It will wait for a microtask to complete before moving to the next one.
• Then take(3) will unsubscribe once the second concatMap has emitted for the 3rd time.
• But before that happens, the first concatMap had values on their buffer, so they will start mapping it before the second concatMap emits the 3rd value.

I hope this explains it 🙏