This below is the ajax code in javascript for wordpress. Wordpress recommends jQuery, but I use the code described below and I need this one (no jQuery). Ajax sends the code and saves it in the database, it works properly (I tested), but if I want to retrieve the response from the server (this.responseText) ..., I don't get any response, just an empty alert. Why?

somepage.php:

<?php
add_action( 'admin_footer', 'my_action_javascript' ); // Write our JS below here

function my_action_javascript() { 
?>

<script>
// Upload file

    
   var sendtext = "sendreceive";

      var formData = new FormData();
      formData.append("SENDTEXT", sendtext);
      formData.append("action", "my_action_send_receive"); 


      // Send request with data
      
  var xmlhttp=new XMLHttpRequest();
  xmlhttp.onreadystatechange=function() {
    if (this.readyState==4 && this.status==200) {
      alert("response from server"+this.responseText);
    }
  }
  xmlhttp.open("POST", "<?php echo admin_url( 'admin-ajax.php', 'relative' ); ?>", true);
  xmlhttp.send(formData);



</script>
<?php
}
?>

functions.php:

<?php 

add_action( 'wp_ajax_my_action_send_receive', 'send_receive' );

function send_receive() {

         if(isset($_POST['SENDTEXT'])){

         $sendtext = $_POST['SENDTEXT'];

         $response = $sendtext;
         echo $response;
         
         }
         
         exit;

    wp_die();
}

?>