Form Submit Execute JavaScript Best Practice?

Attach an event handler to the submit event of the form. Make sure it cancels the default action.

Quirks Mode has a guide to event handlers, but you would probably be better off using a library to simplify the code and iron out the differences between browsers. All the major ones (such as YUI and jQuery) include event handling features, and there is a large collection of tiny event libraries.

Here is how you would do it in YUI 3:

<script src="http://yui.yahooapis.com/3.4.1/build/yui/yui-min.js"></script>
<script>
    YUI().use('event', function (Y) {
        Y.one('form').on('submit', function (e) {
            // Whatever else you want to do goes here
            e.preventDefault();
        });
    });
</script>

Make sure that the server will pick up the slack if the JavaScript fails for any reason.

Use the onsubmit event to execute JavaScript code when the form is submitted. You can then return false or call the passed event's preventDefault method to disable the form submission.

For example:

<script>
function doSomething() {
    alert('Form submitted!');
    return false;
}
</script>

<form onsubmit="return doSomething();" class="my-form">
    <input type="submit" value="Submit">
</form>

This works, but it's best not to litter your HTML with JavaScript, just as you shouldn't write lots of inline CSS rules. Many Javascript frameworks facilitate this separation of concerns. In jQuery you bind an event using JavaScript code like so:

<script>
$('.my-form').on('submit', function () {
    alert('Form submitted!');
    return false;
});
</script>

<form class="my-form">
    <input type="submit" value="Submit">
</form>

I know it's a little late for this. But I always thought that the best way to create event listeners is directly from JavaScript. Kind of like not applying inline CSS styles.

function validate(){
    //do stuff
}
function init(){
    document.getElementById('form').onsubmit = validate;
}
window.onload = init;

That way you don't have a bunch of event listeners throughout your HTML.