Javascript Fetch API and MYSQL

My purpose is to select data from MYSQL database using Javascript Fetch API. I can do it using AJAX as follows:

File ‘getUser.php’:

<?php
$con = mysqli_connect('hostname','username','password','database');
$q = intval($_POST['q']);
if ($con) {
  $sql = "SELECT * FROM user WHERE id = '".$q."'";
  $result = mysqli_query($con, $sql);
  $row = mysqli_fetch_array($result)
  if ($row) {
      echo "$row[1]";
  }
}
?>

File ‘index.php’:

<script>
function showUser(str) {
  let xmlhttp = new XMLHttpRequest();
  xmlhttp.onreadystatechange = function() {
    if (this.readyState === 4 && this.status === 200) {
      ... = this.responseText;
    }
  };
  xmlhttp.open("POST", "getUser.php");
  xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xmlhttp.send("q="+str);
}
</script>

With Fetch API I tried this:

<script>
let file = "getUser.php";
fetch (file)
  .then(x => x.text())
  .then(y => ... = y);
</script>

But I cannot find how can I pass the value of ‘str’ to file ‘getUser.php’ so as to extract from the database the data for the specific person.