index.php

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

<form action="" method="post" id="form1">
<input type="text" name="fname" id="fname" value="fname value">
<input type="text" name="lname" id="lname" value="lname value">
<input type="button" value="Submit" id="submit">
</form>

<form action="" method="post" id="formError">
<input type="text" name="fname" id="fname" value="fname for error form">
<input type="text" name="lname" id="lname" value="lname for error form">
<input type="button" value="Submit For Errors" id="submitError">
</form>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js" integrity="sha512-894YE6QWD5I59HgZOGReFYm4dnWc1Qt5NtvYSaNcOP+u1T9qYdvdihz0PPSiiqn/+/3e7Jo4EaG7TubfWGUrMQ==" crossorigin="anonymous"></script>
<script src="./helpers.js"></script>
<script>
    $(document).ready(function() {
        $("#submit").on("click", function(event) {
          Ajax.post(event, $("#form1"), "dest.php", processResponse);
        });

        $("#submitError").on("click", function(event) {
          Ajax.post(event, $("#formError"), "error.php", processResponse);
        });
    });

    function processResponse(response) {
      if (response.failed) {
        console.log("Authentication failed!");
        // throw new Error("Authentication failed!"); // throwing will return out of processResponse no need to return if this will be caught somewhere else
        return null; // for demo purposes, or if the throwing is not the desired result
      }

      console.log("Response...", response);

      let now = new Date();
      now.setTime(now.getTime() + (30 * 24 * 60 * 60 * 1000))
      document.cookie = 'session_key=' + response.session_key + '; expires' + now.toUTCString();
      console.log('Autentificat cu succes! Cheie: ', response.message);
    }

</script>
</body>
</html>