gistfile1.txt

class AppComponent {
  @ViewChild('btnPlus') btnPlus;
  @ViewChild('btnMinus') btnMinus;
  ngOnInit() {
    const start = 0;
    var plus$ =  Observable.fromEvent(this.getNativeElement(this.btnPlus), 'click');
    var minus$ =  Observable.fromEvent(this.getNativeElement(this.btnMinus), 'click');
    var plusOrMinus$ = Observable.merge(plus$.mapTo(1), minus$.mapTo(-1));
    plusOrMinus$
    .scan((acc,curr) => acc+curr, start)
    .subscribe(x => console.log(x));
  }
  getNativeElement(element){
    return element.nativeElement;
  }
}
UPDATE
const start = 0;
    const min = 0;
    const max = 3
    const plus$ =  Observable.fromEvent(this.getNativeElement(this.btnPlus), 'click');
    const minus$ =  Observable.fromEvent(this.getNativeElement(this.btnMinus), 'click');
    const plusOrMinus$ = Observable.merge(plus$.mapTo(1), minus$.mapTo(-1));
    plusOrMinus$
    .scan((acc,curr) =>{
      if(acc === start){
        if(curr > 0){
          return acc+curr;
        }
      }
      if(acc > start){
        return acc+curr;
      }
      else{
        throw new Error('0 value');
      }
    },
    start)
    .catch((e,obs)=> obs.startWith(0))
    .subscribe(x => console.log(x));