how to check if username already exists in database using javascript

1function validacija() {
2
3  let user = document.getElementById('user');
4  let listOptions = document.querySelectorAll("#list option");
5
6
7  if (user.value.length <= 20 && user.value.length >= 3) {} else {
8    alert("Username has to be between 3-20 characters.")
9  }
10
11
12  for (let i = 0; i < listOptions.length; i++) {
13    if (listOptions[i].value === user.value) {
14      alert('The name already exist')
15    }
16  }
17  return false;
18}

1<form onsubmit="return validacija()">
2  <table>
3    <tr>
4      <td>Username:</td>
5      <td><input type="text" id="user" name="user" list="list"></td>
6      <datalist id="list">
7                    <option value="Tilen">
8                    <option value="Marko">
9                    <option value="Teja">
10                    <option value="Tisa">
11                    <option value="Rok">
12                    <option value="Luka">
13                    <option value="Mojca">
14                </datalist>
15    </tr>
16  </table>
17</form>