promise-pool | Map-like , concurrent promise processing | Reactive Programming library

For e.g. I have a DB operation that's async. While waiting for the DB operation to complete, the thread moves on to some other statements and starts executing them. Once the DB operation is complete, the execution engine picks it up again and starts executing the functions provided after promise resolve. Is this s fair statement to make?

Yes, more or less. Although it's unlikely the engine itself that does this. Somewhere deeper in your database library there's likely a TCP socket. The engine will queue up event executions when data arrives in these sockets, and eventually this trickles down to your database library resolving the promise.

How then do libraries like promise-pool work? E.g. I start 10 different DB operations in async mode. Will they work similar to (1), just wait for resolve and carry on from there? If those subsequent operations after promise is synchronous, will promises get "queued up" to get executed?

On a high level the way these libraries work is that they are given 'some function that returns a promise'. When given many, it keeps count how many of these functions have been called, and for which the promise has not resolved yet. If it's given more of these functions than the size of the pool, it keeps these functions in an array of sorts and call them when that number reduces.

By using the same "promise pool" what will happen if I start 10 different transactions that are synchronous and compete for CPU time (say finding out first 100K numbers in Fibonacci series)? Will they just queued up and are not really executed in parallel?

Javascript engines execute code synchronously. Something like a fibonacci sequence will execute all at once, which will cause the triggering of any io and timing events to be delayed.

As a closing note, I found it helpful to read the source of a Promise. They're only a few dozen lines of code and there's nothing inherently part of a promise that is async. Aside perhaps that when a callback passed to then() is supposed to be triggered, it will always happen slightly later.

We may shorten it a bit: