Simple bulletin board using ajax, insert doesn't work

<divid="content">
    <div style="text-align:right"><a href="logout.php">Logout</a></div>;
    <form action="javascript:void(0)" method="post" id="form">
        <textarea name="message" id="message" cols="70" rows="15" placeholder="Please leave a note">/textarea><br>
        <input type="submit" value="write" id="write">
    </form>
</div>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
  <!--Reading jQuery Library-->
  <script type="text/javascript">
// For messages
    $(function(){
            // button click
            $('#write').on('click', function(){
                $.ajax({
                    url: 'index_do.php',
                    type: 'POST',
                    data: {
                        'message': $('#message').val()
                    }
                })
                // Invoked when Ajax request is successful
                .done((data)=>{
                    $('.result') .html(data);
                    console.log(data);
                })
                // Invoked when Ajax request fails
                .fail((data)=>{
                    $('.result') .html(data);
                    console.log(data);
                })
                // Ajax request invokes either successful or unsuccessful
                .always((data)=>{

                });
            });
        });
  </script>

<?php
    header('Content-type:text/plain; charset=UTF-8');
    $message=h(isset($_POST["message"]);
    echo$message;//for confirm
    $db->query("INSERT INTO tb(mes) VALUES('$message')");
    $db->query("UPDATE`tb`");
    echo$message;
?>