Is there any reason to use a synchronous XMLHttpRequest?

Question

It seems most everyone does asynchronous requests with XMLHttpRequest but obviously the fact that there is the ability to do synchronous requests indicates there might be a valid reason to do so. So what might that valid reason be?

Answer 1

Synchronous XHRs are useful for saving user data. If you handle the beforeunload event you can upload data to the server as the user closes the page.

If this were done using the async option, then the page could close before the request completes. Doing this synchronously ensures the request completes or fails in an expected way.

Answer 2

I think they might become more popular as HTML 5 standards progress. If a web application is given access to web workers, I could foresee developers using a dedicated web worker to make synchronous requests for, as Jonathan said, to ensure one request happens before another. With the current situation of one thread, it is a less than ideal design as it blocks until the request is complete.