CODE EXAMPLE FOR JAVASCRIPT

how to check if username already exists in database using javascript

<form onsubmit="return validacija()">
  <table>
    <tr>
      <td>Username:</td>
      <td><input type="text" id="user" name="user" list="list"></td>
      <datalist id="list">
                    <option value="Tilen">
                    <option value="Marko">
                    <option value="Teja">
                    <option value="Tisa">
                    <option value="Rok">
                    <option value="Luka">
                    <option value="Mojca">
                </datalist>
    </tr>
  </table>
</form>