now the console confirms whether it is the string null or null type

var flag = true;
var student_id = $('input[name="student_id"]').val();//input
            //alert(student_id);
            $.ajaxSetup({
                  async: false//
                  });
            $.post("<%=basePath%>user/queryByID", {student_id : student_id},function(data){
                //alert(data);
                if (!data) {//
                    alert("");
                    flag = false;
                    //alert(flag);
                }
            });
        //alert("======"+flag);
        return flag;