I did as you suggested and found that the error was mine.
Following where the changes that I did :-
1. Remove the <script> tags in ajax.js
2. Replace with xmlHttpRequest.setRequestHeader for xmlHttpRequest.setRequestHandler in ajax.js.
One of the major typo mistakes i did
3. I wrote to call doPost inside the same resulting a unwanted recurring loop, I meant to call the doPost from doGet.