Ajax :: Query Database If Email Already Registered?

I'm making a little CMS, and I need to use AJAX to query the DB to see if an email is already registered. Here's my code so far:

The javascript:
Code:
<script>
function getEmail(){
var ajaxRequest;
try{
ajaxRequest = new XMLHttpRequest();
} .....

I'll test it by entering javascript:alert(getEmail()) in my browser, and it returns undefined. When I replace return 'valid/taken' to alert valid/taken, it will alert the values correctly, but for some reason it has to be entered twice (I have to hit enter twice in javascript:getEmail() for it to be correctly outputted). I'm relatively new to ajax and I think it might have something to do with the readyState not being correct.