Rxjs plus minus button with min max and not negative values - rxjs5

Question

How to avoid negative value ?
I've tried with filter but
if the value becomes negative
you should click twice to get
it positive.
const start = 0;
const min = 0;
const max = 3
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)
.filter(x => x > min)
.subscribe(x => console.log(x));
and if I wanted add a max value ?
UPDATE
This do the work for min
it's a little messy ^^
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));

Answer 1

If i understand your question correctly you want to prevent your inputs (+1/-1) from letting the total go out of bound (min, max). To do so you can use Math.min(nextVal, max) and Math.max(nextVal, min). Combined you will end up with Math.max(Math.min(nextVal, max), min). Next is using this in your .scan() function.
const incStream = Rx.Observable.fromEvent(document.getElementById('button_inc'), 'click').mapTo(1);
const decStream = Rx.Observable.fromEvent(document.getElementById('button_dec'), 'click').mapTo(-1);
const min = 0;
const max = 3;
Rx.Observable.merge(incStream, decStream)
.scan((acc, curr) => Math.max(Math.min(acc + curr, max), min), 0)
.startWith(0)
.distinctUntilChanged()
.subscribe(val => document.getElementById('result').value = val);
<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.0.3/Rx.js"></script>
<input type='button' value='+1' id='button_inc' />
<input type='button' value='-1' id='button_dec' />
<input type='text' id='result' />
The .startWith(0) is used to have a 0 value emitted to the stream before your first scan value arrives. .distinctUntilChanged() is used to not update the result with the same value (for instance when reaching max when using +1 multiple times).