Queue operator for RxJS - rxjs

Question

Is there an operator in RxJS that would allow me to buffer items and let them out one by one whenever a signal observable fires? Sort of like bufferWhen, but instead of dumping the whole buffer on each signal it would dump a certain number per signal. It could even dump the number that gets emitted by the signal observable.
Input observable: >--a--b--c--d--|
Signal observable: >------1---1-1-|
Count in buffer: !--1--21-2-121-|
Output observable: >------a---b-c-|

Answer 1

Yes, you can use zip to do what you want:
const input = Rx.Observable.from(["a", "b", "c", "d", "e"]);
const signal = new Rx.Subject();
const output = Rx.Observable.zip(input, signal, (i, s) => i);
output.subscribe(value => console.log(value));
signal.next(1);
signal.next(1);
signal.next(1);
.as-console-wrapper { max-height: 100% !important; top: 0; }
<script src="https://unpkg.com/rxjs#5/bundles/Rx.min.js"></script>
In fact, zip is used as an example in this GitHub issue that pertains to buffering.
If you want to use the signal's emitted value to determine how many buffered values are to be released, you could do something like this:
const input = Rx.Observable.from(["a", "b", "c", "d", "e"]);
const signal = new Rx.Subject();
const output = Rx.Observable.zip(
input,
signal.concatMap(count => Rx.Observable.range(0, count)),
(i, s) => i
);
output.subscribe(value => console.log(value));
signal.next(1);
signal.next(2);
signal.next(1);
.as-console-wrapper { max-height: 100% !important; top: 0; }
<script src="https://unpkg.com/rxjs#5/bundles/Rx.min.js"></script>

Answer 2

window can be used to separate the timeline. And takeLast is used to hold the output.
let signal = Rx.Observable.interval(1000).take(4);
let input = Rx.Observable.interval(300).take(10).share();
let output = input
.do(value => console.log(`input = ${value}`))
.window(signal)
.do(() => console.log(`*** signal : end OLD and start NEW subObservable`))
.mergeMap(subObservable => {
return subObservable.takeLast(100);
})
.share()
output.subscribe(value => console.log(` output = ${value}`));
Rx.Observable.merge(input.mapTo(1), output.mapTo(-1))
.scan((count, diff) => {
return count + diff;
}, 0)
.subscribe(count => console.log(` count = ${count}`));
.as-console-wrapper { max-height: 100% !important; top: 0; }
<script src="https://unpkg.com/rxjs/bundles/Rx.min.js"></script>
Result:
22:28:37.971 *** signal : end OLD and start NEW subObservable
22:28:38.289 input = 0
22:28:38.292 count = 1
22:28:38.575 input = 1
22:28:38.576 count = 2
22:28:38.914 input = 2
22:28:38.915 count = 3
<signal received>
22:28:38.977 output = 0
22:28:38.979 count = 2
22:28:38.980 output = 1
22:28:38.982 count = 1
22:28:38.984 output = 2
22:28:38.986 count = 0
22:28:38.988 *** signal : end OLD and start NEW subObservable
22:28:39.175 input = 3
22:28:39.176 count = 1
22:28:39.475 input = 4
22:28:39.478 count = 2
22:28:39.779 input = 5
22:28:39.780 count = 3
<signal received>
22:28:39.984 output = 3
22:28:39.985 count = 2
22:28:39.986 output = 4
22:28:39.988 count = 1
22:28:39.989 output = 5
22:28:39.990 count = 0
22:28:39.992 *** signal : end OLD and start NEW subObservable
22:28:40.075 input = 6
22:28:40.077 count = 1
22:28:40.377 input = 7
22:28:40.378 count = 2
22:28:40.678 input = 8
22:28:40.680 count = 3
22:28:40.987 input = 9
22:28:40.990 count = 4
<input completed>
22:28:40.992 output = 6
22:28:40.993 count = 3
22:28:40.995 output = 7
22:28:40.996 count = 2
22:28:40.998 output = 8
22:28:40.999 count = 1
22:28:41.006 output = 9
22:28:41.007 count = 0