var $ = function (id) {
return document.getElementById(id);
}
var calculateTax = function () {
var subtotal = parseFloat($("subtotal").value);
var tax_rate = parseFloat($("tax_rate").value);
var isValid = true;

if (isNaN(subtotal)) {
alert("Subtotal must be numeric");
}
else if (isNaN(tax_rate)) {
alert("Tax rate must be numeric");
}
else if (subtotal <= 0) {
alert("Subtotal must not be negative");
}
else if (tax_rate <= 0) {
alert("Tax rate must not be negative");
}
else {
var sales_tax = calculateTax(subtotal, tax_rate);
$("sales_tax").value = sales_tax.toFixed(1);
var total = subtotal + sales_tax;
$("total").value = total.toFixed(1);
}
}
var clear = function () {
$("subtotal").value = "";
$("tax_rate").value = "";
$("sales_tax").value = "";
$("total").value = "";
}
window.onload = function () {
$("calculate").onclick = calculateTax;
$("clear").onclick = clear;
$("subtotal").focus();
}