Question

Possible Duplicate:
Why does setTimeout(fn, 0) sometimes help?

Reading jQuery 1.8 source,

WHY does it do setTimeout with 0 ms delay ?
(instead of just executing the callback ?)

https://github.com/jquery/jquery/blob/1.8.0/src/ajax/xhr.js#L196

                if ( !s.async ) {                       
                    callback();
                } else if ( xhr.readyState === 4 ) {


                    // (IE6 & IE7) if it's in cache and has been
                    // retrieved directly we need to fire the callback

         //-------->// WHY do setTimeout with 0 ms delay ?
                    setTimeout( callback, 0 );
                } else {
                    handle = ++xhrId;

Answer 1

This is a workaround for a peculiarity of IE6 and IE7 which can retrieve an AJAX result from cache without firing the XMLHTTPRequest callback, immediately setting its readyState property to 4, instead.

However the API contract for $.ajax requires that it return immediately for an asynchronous request (i.e. without calling the programmer-supplied callbacks).

Hence $.ajax call tests for those cached results, and then fakes the required asynchronous callback using setTimeout.

The $.ajax call completes, and as soon as the browser re-enters its event processing loop it’ll find the (immediately expired) timer event and call its callbacks.