Wait for all observables in mergeMap to complete before emitting a custom value

Question

I want to process a list of observables concurrently using flatMap and then to emit a single empty value when all inner observables are processed. Is there an elegant way to achieve this, i.e. using a single operator?

Here's the example:

const { of, from } = Rx.Observable;

from([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
  .mergeMap(number => multiply(number), 2) // processing two numbers at a time
  .last() // waiting for all inner observables to complete
  .map(_ => undefined) // casting a value returned by last() to an empty value
  .subscribe()
;

function multiply(number) {
  return of(number * 2) // multiplying the number
    .delay(200) // adding a slight delay
  ;
}

I know I can use toArray() or last() to wait for all inner observables to complete, but then I will need to cast it to an empty value using map() operator (as in my example above).

I guess, I'm looking for an operator with the following semantics: emit X when source observable completes, e.g.:

from([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
  .mergeMap(number => multiply(number), 2)
  .emitOnComplete(undefined)
  .subscribe(console.log) // we should get undefined here
;

Answer 1

The most convenient way that I've managed to find so far is combining ignoreElements operator with endWith:

from([1, 2, 3, 4, 5])
  .mergeMap(number => multiply(number), 2)
  .ignoreElements()
  .endWith(undefined)
  .subscribe(console.log) // getting undefined here
;

However, it would be really nice to have a single operator for this.

Answer 2

There's actually one sneaky solution using reduce() that emits only once when its source observable completes. You can use it to ignore all values and just return the seed value:

from([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
  .pipe(
    mergeMap(number => multiply(number), 2),
    reduce((acc, value) => acc, undefined),
  )
  .subscribe(console.log);

Live demo: https://stackblitz.com/edit/rxjs-tx6bbe

Btw, funny fact: The same trick with reduce() is used inside Angular's Router package (just without the seed value).