form action with javascript

I have a form that must execute a javascript function on submit, the function then posts data to my php send mail file and the mail is sent. But it only works in fire fox. The form action does not seem to be doing anything in IE, my question is: Is this the correct way to call a function from an external file from the form action:

action="javascript:simpleCart.checkout()"

simpleCart is the .js file and checkout() is the function.

Tips appreciated, struggling to understand why it would work in firefox but not IE, chrome or safari.

<form name=form onsubmit="return validateFormOnSubmit(this)" enctype="multipart/form-data" action="javascript:simpleCart.checkout()" method="post">

A form action set to a JavaScript function is not widely supported, I'm surprised it works in FireFox.

The best is to just set form action to your PHP script; if you need to do anything before submission you can just add to onsubmit

Edit turned out you didn't need any extra function, just a small change here:

function validateFormOnSubmit(theForm) {
    var reason = "";
    reason += validateName(theForm.name);
    reason += validatePhone(theForm.phone);
    reason += validateEmail(theForm.emaile);

    if (reason != "") {
        alert("Some fields need correction:\n" + reason);
    } else {
        simpleCart.checkout();
    }
    return false;
}

Then in your form:

<form action="#" onsubmit="return validateFormOnSubmit(this);">