jsGrid: How to pass additional variables from javascript to php using ajax

I'm using jsGrid for my project. View here for original source code

I want to pass an additional variable call $user_session to use for mysql select query in fetch.php but failed. Below is what i have been trying.

<script>

var user_session = "<?php echo $user_session; ?>"; //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

//......

 controller: {
  loadData: function(){
   return $.ajax({
    type: "GET",
    url: "fetch_data.php",
    data: {user_session:user_session} //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
   });
  },

 //......

Here's the fetch.php file

<?php

if($method == 'GET')
{
 $user_session = $_GET['user_session']; //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

 $query = "SELECT * FROM sample_data WHERE first_name=? ORDER BY id DESC";
 $statement = $connect->prepare($query);
 $statement->execute($user_session); //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 $result = $statement->fetchAll();
 foreach($result as $row)
 {
  $output[] = array(
   'id'    => $row['id'],   
   'first_name'  => $row['first_name'],
   'last_name'   => $row['last_name'],
   'age'    => $row['age'],
   'gender'   => $row['gender']
  );
 }
 header("Content-Type: application/json");
 echo json_encode($output);
}
//......
?>

What is the proper way to do this?

I'm able to do by this way.

<script>

//......

 controller: {
  loadData: function(filter){
    var user_session = "<?php echo $user_session; ?>"; //<<<<<<<<<<<<<<<<<<<<<<<<<<<
   return $.ajax({
    type: "GET",
    url: "fetch_data.php",
    data: {filter,
           user_session:user_session //<<<<<<<<<<<<<<<<<<<<<<<<<<<
          },
   });
  },

 //......
</script>

In fetch.php i do this.

<?php

if($method == 'GET')
{
 $user_session = $_GET['user_session'];//<<<<<<<<<<<<<<<<<<<<<<<<<<<

 $query = "SELECT * FROM sample_data WHERE first_name=? ORDER BY id DESC";
 $statement = $connect->prepare($query);
 $statement->execute([$user_session]); //<<<<<<<<<<<<<<<<<<<<<<<<<<<
 $result = $statement->fetchAll();
 foreach($result as $row)
 {
  $output[] = array(
   'id'    => $row['id'],   
   'first_name'  => $row['first_name'],
   'last_name'   => $row['last_name'],
   'age'    => $row['age'],
   'gender'   => $row['gender']
  );
 }
 header("Content-Type: application/json");
 echo json_encode($output);
}
//......
?>

For the security issue mentioned by @Andrea Golin, i will post another question.
Thanks.