create custom function in jquery this function value can't get

Question

i am create check_agent_session() return response check below code

function check_agent_session(){
      var response = ''
      $.ajax({
          url:"<?php echo base_url() ?>agent/agent_session",
          success: function(res){
               response = $.parseJSON(res);
               response = response['message'];


             }
         });
        // alert(response);
         return response;
     }

  });

after i will put this function page document read

$(document).ready(function(e) {
alert(check_agent_session());
});

this function return value balnk. alert as blank.

Please guide me