Undefined in the PHP return to the front with JSON

I am trying to create a session validation with Jquery, AJAX, JSON and PHP. I have this code.

This is the generic function that I invoke in each query, I modified the code because apparently I was doing the return inside the AJAX function, I created a variable to store the response and I have returned the variable, but it follows without working, if I do the console.log within the .done correctly launches the response, but when I exit the AJAX, I lose the value of the variable.

  function ajaxConn(incomingUrl,incomingData)
  {
    var returnValue;
    $.ajax({
      url:'backend/'+incomingUrl,
      method:'POST',
      data: JSON.stringify(incomingData),
      contentType: "application/json; charset=UTF-8"
    }).done(function(response){
          returnValue = response.resp;
    });
    console.log(returnValue);
    return returnValue;
  }

This is the invocation form, in the data comes a series of validations of the inputs:

var data = {user:user,pass:pass};
var answer = ajaxConn('core_files/startSession.php',data);

All this is my initial code in PHP with which I receive the data:

require 'connection.php';
date_default_timezone_set('America/Mexico_City');
mysqli_set_charset($conn,"utf8");
header('Access-Control-Allow-Origin: *');
header('Content-Type: application/json');
$data = json_decode(file_get_contents('php://input'), true);
$user =  mysqli_real_escape_string($conn,$data["user"]);
$pass =  mysqli_real_escape_string($conn,$data["pass"]);
$response = array();

After a series of queries and ciphers depending on the condition, I get a Y for successful validation or an N for a login failure that encapsulates an array and passes it to JSON format and return it.

$response["resp"] = 'N';
echo json_encode($response);

If in the AJAX success I make a console.log(response.resp); , the character returned by the PHP returns to me well. But if I try to make a console.log(answer) out of the AJAX function, in the place where it was invoked, it returns undefined . I'm supposed to be giving it return response.resp I would have to return the lyrics.

What is failing me: (