Systeme asynchrone d,envoie de message dans la bdd avec ajax php mysql

document.getElementById("envoi").addEventListener("submit", function(e) {
  e.preventDefault();
 
  var data = new FormData(this);
  alert("data");
 
  var xhr = new XMLHttpRequest();
 
  xhr.onreadystatechange = function(){
    if (this.readyState == 4 && this.status == 200) {
      console.log(this.response);
      var res = this.response;
     
    } else if (this.readyState == 4) {
      alert("Une erreur est survenue...");
    }
  };
 
  xhr.open("POST", "send.php", true);
  xhr.responseType = "json";
  // xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhr.send(data);
 
  return false;
});​

if (isset($_POST['submit'])) {
if(isset ($_POST['message']) and !empty($_POST['message'])) {
 $message = htmlspecialchars($_POST['message']);

 $ins = $bdd->prepare('INSERT INTO messages(message) VALUES (?)');
          $ins->execute(array($message)); }}?> 

 <form method="POST" class="envoi" id="envoi">
          
         <textarea  classe='message' placeholder="Votre message" name="message"></textarea>
         <br /><br />
         <input class="submit" name="submit" type="submit" value="envoie"   />
         <br /><br />
         <?php if(isset($error)) { echo '<span style="color:red">'.$error.'</span>'; } ?>
          
   
   </form>