How do I capture response of form.submit

An Ajax alternative is to set an invisible <iframe> as your form's target and read the contents of that <iframe> in its onload handler. But why bother when there's Ajax?

Note: I just wanted to mention this alternative since some of the answers claim that it's impossible to achieve this without Ajax.

The non-jQuery vanilla Javascript way, extracted from 12me21's comment:

var xhr = new XMLHttpRequest();
xhr.open("POST", "/your/url/name.php"); 
xhr.onload = function(event){ 
    alert("Success, server responded with: " + event.target.response); // raw response
}; 
// or onerror, onabort
var formData = new FormData(document.getElementById("myForm")); 
xhr.send(formData);

For POST's the default content type is "application/x-www-form-urlencoded" which matches what we're sending in the above snippet. If you want to send "other stuff" or tweak it somehow see here for some nitty gritty details.

You won't be able to do this easily with plain javascript. When you post a form, the form inputs are sent to the server and your page is refreshed - the data is handled on the server side. That is, the submit() function doesn't actually return anything, it just sends the form data to the server.

If you really wanted to get the response in Javascript (without the page refreshing), then you'll need to use AJAX, and when you start talking about using AJAX, you'll need to use a library. jQuery is by far the most popular, and my personal favourite. There's a great plugin for jQuery called Form which will do exactly what it sounds like you want.

Here's how you'd use jQuery and that plugin:

$('#myForm')
    .ajaxForm({
        url : 'myscript.php', // or whatever
        dataType : 'json',
        success : function (response) {
            alert("The server says: " + response);
        }
    })
;