Pure JavaScript Ajax Call

Below is sample form code to demonstrate the pure JavaScript ajax call.

  <form id="login" action="checkLogin.php" method="post">
    <input class="field-val" type="text" id="username" name="username" placeholder="Username" required>
    <input class="field-val" type="password" id="password" name="password" placeholder="Password" required>
    <button type="submit" name="checklogin" onClick="checkLogin(); return false;">Login</button>
  </form>

Pure JavaScript ajax call code snippet

function checkLogin(){
  var fieldVal = document.getElementsByClassName("field-val");
    var formContents = new FormData(); 
    for(var i=0; i<fieldVal.length; i++)
    {
        formContents.append(fieldVal[i].name, fieldVal[i].value);
    }
    var xmlHttp = new XMLHttpRequest();
        xmlHttp.onreadystatechange = function()
        {
            if(xmlHttp.readyState == 4 && xmlHttp.status == 200)
            {
              if(xmlHttp.responseText.trim()=='success'){
                  window.location = '/app/dashboard';
              }
              else{
                alert(xmlHttp.responseText);
              }
            }
        }
        xmlHttp.open("post", "checkLogin.php"); 
        xmlHttp.send(formContents); 
}

How to get and return values in the checkLogin.php

<?php
$username = $_POST['username'];
$password = $_POST['password'];
$login= false;
/*-- username and password matching logic starts --*/
$login= true;
/*-- username and password matching logic ends --*/
if ($login==true) { 
    echo "success"; 
} 
else
echo "Any Error Message will go here";
?>