RxJS reduce doesn't continue

The problem isn't in flatMap; it's in the way reduce works.

reduce reads in a whole stream and reduces it to a single value, emitted only when the source stream is closed. If your from(files) stream doesn't end, then reduce will never output its value.

Try using scan instead; it emits each intermediate step and seems to be what you're looking for.

If files is an array, then reduce should terminate if the observable returned from fileReader does. So for this code, the problem was that fileReader returned an observable that didn't complete.