.....
 echo '[';
   while($row = $rs->fetch_assoc())
            {
              echo  $row['name'].'","'; 
            }
echo ']';
.....

        <?php
               echo 'var arr =xmlhttp.responseText;';
                               ?>

                        console.log(arr[0]); // получаем [ 

   while($row = $rs->fetch_assoc())
            {
              echo  '"'.$row['Prep'].'",';
            }
            echo '""';
        }

         <?php
               echo 'var arr ="[" + xmlhttp.responseText + "]";';
                               ?>
console.log(arr); // ["Дмитрий","Ира","Вася",""]
console.log(arr[0]); // [
console.log(arr[1]); // "
console.log(arr[2]); // Д