// RxJS v6+
import { empty } from 'rxjs';
​
//output: 'Complete!'
const subscribe = empty().subscribe({
  next: () => console.log('Next'),
  complete: () => console.log('Complete!')
});

// RxJS v6+
import { interval, fromEvent, merge, empty } from 'rxjs';
import { switchMap, scan, takeWhile, startWith, mapTo } from 'rxjs/operators';
​
const countdownSeconds = 10;
const setHTML = id => val => (document.getElementById(id).innerHTML = val);
const pauseButton = document.getElementById('pause');
const resumeButton = document.getElementById('resume');
const interval$ = interval(1000).pipe(mapTo(-1));
​
const pause$ = fromEvent(pauseButton, 'click').pipe(mapTo(false));
const resume$ = fromEvent(resumeButton, 'click').pipe(mapTo(true));
​
const timer$ = merge(pause$, resume$)
  .pipe(
    startWith(true),
    // if timer is paused return empty observable
    switchMap(val => (val ? interval$ : empty())),
    scan((acc, curr) => (curr ? curr + acc : acc), countdownSeconds),
    takeWhile(v => v >= 0)
  )
  .subscribe(setHTML('remaining'));