Is-there an opposite of the `race` operator in RxJS?

Question

I have two observables and I want listen to the one that emits its first value last, is there an operator for this ? Something like that :

let obs1 = Rx.Obse         


        

Answer 1

Using RxJS 6 and ReplaySubject:

function lastOf(...observables) {
  const replayable = observables
    .map(o => {
      let r = o.pipe(multicast(new ReplaySubject(1)));
      r.connect();
      return r;
    });
  const racing = replayable
    .map((v, i) => v.pipe(
      take(1),
      mapTo(i),
    ))
    ;
  return of(...racing).pipe(
    mergeAll(),
    reduce((_, val) => val),
    switchMap(i => replayable[i]),
  );
}

Use:

const fast = interval(500);
const medium = interval(1000);
const slow = interval(2000);

lastOf(fast, slow, medium).subscribe(console.log);