Calculating total with discount

Original - javascript Code // Provide an indexOf function if none exists if (!Array.prototype.indexOf) { Array.prototype.indexOf = function (needle) { for (var i = 0; i < this.length; i++) { if (this[i] === needle) return i; } return -1; }; } // Run our code when the window is finished loading window.onload = function () { // Reference to our form element, and other things var form = document.forms["form1"], total = checked = 0, exempt = ["checkbox_group_4[]", "checkbox_group_5[]"]; // Bind up our calculate method when our form changes if (form.addEventListener) { form.addEventListener("change", calculate, false); } else if (form.attachEvent) { form.attachEvent("onchange", calculate); } else { form.onchange = calculate; } // Define our Calculate function function calculate () { // Reset total and checked total = 0, checked = 0; // Cycle over each item in our form for (var i = 0; i < form.length; i++) { // Handle only those that are checked if (form[i].checked) { // Add their value to the total value total += parseInt(form[i].value, 10); // If they aren't extempt from flatrate if (exempt.indexOf(form[i].name) < 0) { // Increment the number of eligible items checked += 1; } } } // If more than one eligible item is checked if (checked > 1) { total = 175; // Else if only one eligible item is checked } else if (checked > 0) { total = 100; } // Show the user the estimated value form["total"].value = total; } };

// Provide an indexOf function if none exists
if (!Array.prototype.indexOf) {
    Array.prototype.indexOf = function (needle) {
        for (var i = 0; i < this.length; i++) {
            if (this[i] === needle) return i;
        }
        return -1;
    };
}
// Run our code when the window is finished loading
window.onload = function () {
    // Reference to our form element, and other things
    var form = document.forms["form1"], total = checked = 0,
        exempt = ["checkbox_group_4[]", "checkbox_group_5[]"];
    // Bind up our calculate method when our form changes
    if (form.addEventListener) {
        form.addEventListener("change", calculate, false);
    } else if (form.attachEvent) {
        form.attachEvent("onchange", calculate);
    } else {
        form.onchange = calculate;
    }
    // Define our Calculate function
    function calculate () {
        // Reset total and checked
        total = 0, checked = 0;
        // Cycle over each item in our form
        for (var i = 0; i < form.length; i++) {
            // Handle only those that are checked
            if (form[i].checked) {
                // Add their value to the total value
                total += parseInt(form[i].value, 10);
                // If they aren't extempt from flatrate
                if (exempt.indexOf(form[i].name) < 0) {
                    // Increment the number of eligible items
                    checked += 1;
                }
            }
        }
        // If more than one eligible item is checked
        if (checked > 1) {
            total = 175;
        // Else if only one eligible item is checked
        } else if (checked > 0) {
            total = 100;
        }
        // Show the user the estimated value
        form["total"].value = total;
    }                          
}; 

Original - html Code <form name="form1"> <input type="checkbox" name="checkbox_group_1[]" value="100" /> <input type="checkbox" name="checkbox_group_2[]" value="100" /> <input type="checkbox" name="checkbox_group_3[]" value="100" /> <input type="checkbox" name="checkbox_group_4[]" value="20" /> <input type="checkbox" name="checkbox_group_5[]" value="20" /> <br/> <input type="text" name="total" /> </form>

<form name="form1">
    <input type="checkbox" name="checkbox_group_1[]" value="100" />
    <input type="checkbox" name="checkbox_group_2[]" value="100" />
    <input type="checkbox" name="checkbox_group_3[]" value="100" />
    <input type="checkbox" name="checkbox_group_4[]" value="20"  />
    <input type="checkbox" name="checkbox_group_5[]" value="20"  />
    <br/>
    <input type="text" name="total" />
</form>