// An observable that emits 10 multiples of 100 every 1 second
const source$ = Observable.interval(1000)
  .take(10)
  .map(x => x * 100);
/**
 * returns a promise that waits `ms` milliseconds and emits "done"
 */
function promiseDelay(ms) {
  return new Promise(resolve => {
    setTimeout(() => resolve('done'), ms);
  });
}

// using it in a switchMap
source$.switchMap(x => promiseDelay(x)) // works
  .subscribe(x => console.log(x)); 

source$.switchMap(promiseDelay) // just a little more terse
  .subscribe(x => console.log(x)); 

// or takeUntil
source$.takeUntil(doAsyncThing('hi')) // totally works
  .subscribe(x => console.log(x))

// or weird stuff you want to do like
Observable.of(promiseDelay(100), promiseDelay(10000)).mergeAll()
  .subscribe(x => console.log(x))