const interval = Rx.Observable.interval(1000);

const delayedInterval = interval.delay(10).take(4);

​

const exhaustSub = Rx.Observable

    .merge(

        // delay 10ms, then start interval emitting 4 values

        delayedInterval,

        // emit immediately

        Rx.Observable.of(true)

  )

  /*

   *  The first emitted value (of(true)) will be mapped 

   *  to an interval observable emitting 1 value every 

   *  second, completing after 5.

   *  Because the emissions from the delayed interval 

   *  fall while this observable is still active they will be ignored.

   *

   *  Contrast this with concatMap which would queue, 

   *  switchMap which would switch to a new inner observable each emission,

   *  and mergeMap which would maintain a new subscription for each emitted value.

   */

  .exhaustMap(_ => interval.take(5))

  // output: 0, 1, 2, 3, 4

  .subscribe(val => console.log(val))