Condition within beforeunload event not working?

Question

I just want to check condition within beforeunload event like other guy ask.My Code is here=>

 $(window).bind('beforeunload', function (e) {       
            if (CheckCondition())
                return "Are you sure to go?";
        });

function CheckCondition()
        {
            $.ajax({
                url: '/Field/CheckBeforeChangeURL',
                type: "GET",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (r) {
                    //Bool value return from server side
                    return r;
                },
                async: true,
                processData: false,
                cache: false
            });
        }

It still go without asking.I already try this beforeunload ignore condition but still not working.