make sure user cannot enter input into two different boxes javascript code example

Example: dont allow user to insert number greater than particular number

function isNumberKey(e) {
    var currentChar = parseInt(String.fromCharCode(e.keyCode), 10);
    if(!isNaN(currentChar)){
        var nextValue = $("#txthour").val() + currentChar; //It's a string concatenation, not an addition

        if(parseInt(nextValue, 10) <= 12) return true;
    }

    return false;
}