I don't understand your concern. When you say 'Upon calling something_async, the code starts waiting and blocks further execution until it is ready', you're describing a synchronous call. Indeed window.confirm is a synchronous function--nothing is executed until it returns.

And yes, both asynchronous and synchronous operations have their place, umm, except perhaps in Haskell where things get funky. Chaining async operations together in javascript can be tedious, but its easy enough to write a helper function to handle it for you. Every javascript app I've written that uses AJAX always has some form of createCallback(context, function, arguments) function.