Javascript is said to be single-threaded. Also AJAX is said to be asynchronous.
Consider a scenario;
I have a button and on click of it, I make a AJAX call which takes 5-6 seconds. Now the UI would not be blocked and the user does some other action (say click on another button which is now executing some code, while the AJAX response has been returned). Now in this case, since the other code is being executed, when would the AJAX callback be executed? Would it have to wait OR can it be executed in a parallel thread ?