I am planning to use Mockjax for one of my offline applications. Will Mockjax capture the ajax calls made even by extjs or other javascript libarires?
That depends... if you are referring to the original mockjax jQuery plugin then it will only work with jQuery's $.ajax() method calls. That said, there are other libraries out there called "Mockjax" which do similar things.
If you need a more "generic" solution that will work with other libraries (or just vanilla JavaScript) then I would recommend looking into the Sinon library.