javascript - can a synchronous (blocking) ajax call block the browser's UI?

Question

This question is jQuery oriented, but not necessarily exclusively.

The short question:

Can synchronous ajax call block a normal button from being clicked on? My testings show it doesn't happen, but maybe another browser makes problems.

The long question:

In another question I've asked, how to block on ajax call (I want it to block), the guys said that the browser will block in some cases.

Actually even the docs of jQuery say: Note that synchronous requests may temporarily lock the browser, disabling any actions while the request is active.

I'm trying to:
1. Understand how/why that would happen.
2. Assess the probability of that to happen.

From my understanding, which might be incorrect:
I imagine that "locking" will happen to the page's UI if I, as the dev, didn't build/update the UI before the ajax call itself, which "blocks" the javascript VM, thus delaying the UI build/update. Correct or not?

Answer 1

Yes it will. While a synchronous request is outstanding, the browser waits for it to return. The probability of that happening is 100%, but if your request returns fast enough, it might not be that noticeable. Thing is, you can't count on a request returning quickly.

That's the point of ajax calls. The first a mean asynchronous, which means, "doesn't block".

Since ajax calls are asynchronous, your code can be anywhere when the request returns, which means you need a way to handle the response when the request does return. That is why you use callbacks to handle the response.

Why don't you try it?