AJAX卸载未触发

Question

I need to run a process when the user exists or leaves a page. I have the following code that I received from another SO question I posted:

 <script>
var unloaded = false;

$(window).on('beforeunload', unload);
$(window).on('unload', unload);

function unload() {

    if (!unloaded) {


        $.ajax({
            type: 'post',
            async: false,
            url: '/Function/Left/@ViewBag.ID',
            success: function () {
                unloaded = true;
                $('body').css('cursor', 'default');
            },
            timeout: 5000
        });
    }
}

The problem is that the /Function/Left event is NEVER fired. Is something wrong?