การใช้ javascript คำนวณค่า จาก getElementById ต้องทำยังงัยอ่ะครับ

 function CalNetWeight(TotalW, CarW, NetW) 
{

            var txTotalWEdit = document.getElementById(TotalW).value;
            var txCarWEdit = document.getElementById(CarW).value;
            var txNetWEdit = document.getElementById(NetW).value;

            var Total = parseFloat(document.getElementById(TotalW).value);
            var Car = parseFloat(document.getElementById(CarW).value);
            var net;

            net = Total - Car;

            if (txTotalWEdit.value == "0") {

                txCarWEdit.style.display = 'none';
                 
            }
            else {

                txCarWEdit.style.display = ' ';
            }

}