How to alert user before leaving webpage and run ajax function if user presses “ok”?

I have this function that is supposed to ask the user if they are sure that they want to leave or refresh the website and if they say yes and leave the ajax function is executed. My problem is if the ajax function is alone the user is given no warning but if anything else is in the code the ajax function doesn’t work.

<script type="text/javascript">
            window.onbeforeunload = function () {
                return "";
                $.ajax({url: "checkout.php", type : 'post', data : { page_left: 1}});
            }
       </script>