jquery记录停留时间

function visit_timing()
    {
        var start;
        var end;
        var duration = 0;
        start = new Date();
            $(window).bind('beforeunload', function(e) {
                end = new Date();//用户退出时间
                duration = end.getTime() - start.getTime();
                duration = duration/1000;//取的是秒
                $.ajax({
                    type: 'POST',
                    async: false,//这块至关重要，用$post默认是true
                    url: 'url',
                    data: {a:1.vid,b:duration}
                });
            });
    }