const {when, fromEvent} = Rx.Observable;

​

const decObs = fromEvent(document.getElementById('dec'), 'click');

const incObs = fromEvent(document.getElementById('inc'), 'click');

const resetObs = fromEvent(document.getElementById('res'), 'click');

​

when(

  decObs.thenDo(x => prev => prev - 1),

  incObs.thenDo(x => prev => prev + 1),

  resetObs.thenDo(x => prev => 0)

).startWith(0).scan((prev, f) => f(prev)).subscribe(v => document.getElementById('out').innerHTML = v);