ajax project using php and jquery

PHP
<script>
var objXMLHttpRequest = new XMLHttpRequest();
objXMLHttpRequest.onreadystatechange = function() {
  if(objXMLHttpRequest.readyState === 4) {
    if(objXMLHttpRequest.status === 200) {
          alert(objXMLHttpRequest.responseText);
    } else {
          alert('Error Code: ' +  objXMLHttpRequest.status);
          alert('Error Message: ' + objXMLHttpRequest.statusText);
    }
  }
}
objXMLHttpRequest.open('GET', 'request_ajax_data.php');
objXMLHttpRequest.send();
</script>
<!doctype html>
<html>
<head>
<script src="https://code.jquery.com/jquery-3.3.1.js" integrity="sha256-2Kok7MbOyxpgUVvAk/HJ2jigOSYS2auK4Pfzbm7uH60=" crossorigin="anonymous"></script>
</head>
<body>
<form id="loginform" method="post">
    <div>
        Username:
        <input type="text" name="username" id="username" />
        Password:
        <input type="password" name="password" id="password" />    
        <input type="submit" name="loginBtn" id="loginBtn" value="Login" />
    </div>
</form>
<script type="text/javascript">
$(document).ready(function() {
    $('#loginform').submit(function(e) {
        e.preventDefault();
        $.ajax({
            type: "POST",
            url: 'login.php',
            data: $(this).serialize(),
            success: function(response)
            {
                var jsonData = JSON.parse(response);
  
                // user is logged in successfully in the back-end
                // let's redirect
                if (jsonData.success == "1")
                {
                    location.href = 'my_profile.php';
                }
                else
                {
                    alert('Invalid Credentials!');
                }
           }
       });
     });
});
</script>
</body>
</html>
<?php
if (isset($_POST['username']) && $_POST['username'] && isset($_POST['password']) && $_POST['password']) {
    // do user authentication as per your requirements
    // ...
    // ...
    // based on successful authentication
    echo json_encode(array('success' => 1));
} else {
    echo json_encode(array('success' => 0));
}
function AjaxCallWithPromise() {
    return new Promise(function (resolve, reject) {
        const objXMLHttpRequest = new XMLHttpRequest();
 
        objXMLHttpRequest.onreadystatechange = function () {
            if (objXMLHttpRequest.readyState === 4) {
                if (objXMLHttpRequest.status == 200) {
                    resolve(objXMLHttpRequest.responseText);
                } else {
                    reject('Error Code: ' +  objXMLHttpRequest.status + ' Error Message: ' + objXMLHttpRequest.statusText);
                }
            }
        }
 
        objXMLHttpRequest.open('GET', 'request_ajax_data.php');
        objXMLHttpRequest.send();
    });
}
 
AjaxCallWithPromise().then(
    data => { console.log('Success Response: ' + data) },
    error => { console.log(error) }
);
Source