Observable that tracks its observers count. Run at https://observable-playground.github.io/gist/4a7415f3528aa125fb686204041138cb

index.js

const { chart } = require('rp-api');
const { Observable, timer, merge } = require('rxjs');
const { take } = require('rxjs/operators');

function createTrackedObservable(onEmpty) {
  let count = 0;
  return new Observable(() => {
    count++;
    return ()=>{
      count--;
      if (count === 0 && onEmpty) {
        onEmpty();
      }
    };
  });
}

console.log('Start');
const onEmpty = () => console.log('Done');
const source$ = createTrackedObservable(onEmpty);

const timer$ = timer(0, 5);
const tracked$ = merge(source$, timer$);

tracked$
  .pipe(take(5))
  .subscribe(chart.createRxObserver());

setTimeout(()=>{
  tracked$
    .pipe(take(2))
    .subscribe(chart.createRxObserver());
}, 10);