How to know which index is being edited inside an input on key press event?

Question

I am making an input 'type text' that accepts decimal numbers. The input currently validates a max integer value and a maximum of decimal digits.

For example the data type Decimal(4, 2) in the method onKeyPress. But I am havig problems when the maximum number of decimal digits is reached and the user is editing the integer part.

Is there a way to know the index that is being edited so I can tell if the user is editing the integer part or the decimal part?

Here is what I have for now:

function validar_decimales(event, oTxt, enteros, decimales){
    var key = event.keyCode;
    console.log(JSON.stringify(event));
    var puntos = 0;
    var ents = 0;
    var decs = 0;
    var pasoPunto = false;
    if(oTxt.value.length == 0 && key == 46)
        return false;
    var str = "";
    
    for(var i = 0; i < oTxt.value.length; i++){
        if(oTxt.value[i] == "."){
            pasoPunto = true;
            puntos++;
            continue;
        }
        if(!pasoPunto){
            ents++;
        }else{
            decs++;
        }
    }
    if(event.keyCode == 46){
        if(puntos > 0)
            return false;
    }else
        if (key <= 13 || (key >= 48 && key <= 57)){
            if(pasoPunto){
                return decs < decimales;
            }else
                return ents < enteros;
        }else
            return false;
}