How do you send an ajax request every time that a form input field changes?

Question

For example, there is an input field. Every time a user types a key into that field, it sends an AJAX request with whatever text is currently in that input, and does something

Answer 1

Just make an $.ajax() call every time the change event is fired! Like so:

$(document).on('keydown','input',function(){
    $.ajax({
        // options here
    });
});

Whilst the above will help achieve what you want, I must advise that it is not great practice to fire off constant AJAX requests as this can put a huge load on the server if you have a lot of traffic. You would be better off either validating every n seconds, or validating client side, or validating on submission...

UPDATE

It appears you do not want to catch the change event, you would like to know when anything is entered. Resultantly, I have changed my code to catch the keydown event. This will fire whenever a key is pressed down whilst focused on an input.