Still broken

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();
}