Threads in Javascript

complex things. One of the things it really has to do is to wait for the
browser to load something.

Because Javascript is all event-driven, the closest I seem to be able to
come is this:

loadMyFile("http://...insert url here...", function () {
alert("the file has loaded!");
});

That is, loadMyFile() executes asynchronously but registers a callback to
execute once the thing's loaded.

Unfortunately, while this works, it's a pig to program for. What I'd like is
for loadMyFile() to be synchronous: that is, it doesn't return until the
file has completely loaded.

(And I want to wait for other things apart from the file loading.)

It would appear that in order to do this I'm going to need multiple threads
of execution, somehow. I need to be able to have my main thread block, and
get woken up later by another thread. Is this at all possible?

Some experimentation would appear to suggest that all Javascript code on a
particular page, *including* code running in embedded iframes, appears to
execute in a single thread. Javascript on another page may or may not
execute in a different thread on my browser, but that's not terribly
important because I can't communicate across pages anyway.

Any suggestions?

I have a big, complicated Javascript program that's doing some rather
complex things. One of the things it really has to do is to wait for the
browser to load something.

Because Javascript is all event-driven, the closest I seem to be able to
come is this:

loadMyFile("http://...insert url here...", function () {
alert("the file has loaded!");
});

That is, loadMyFile() executes asynchronously but registers a callback to
execute once the thing's loaded.

Unfortunately, while this works, it's a pig to program for. What I'd like is
for loadMyFile() to be synchronous: that is, it doesn't return until the
file has completely loaded.

If you are using XMLHttpRequest (Mozilla, Netscape, latest Safari and
Opera) or Microsoft.XMLHTTP (IE/Win) then you can however use the third
argument to the open method set to false to have a synchronous request.

My current RPC system is a home-grown thing based around persistent
connections; it works scarily well, and doesn't appear to actually break
any standards (although they get bent quite a lot).
Then what are you doing, loading stuff into a hidden iframe that calls
back into the parent window?
How portable is XMLHttpRequest? If I were to switch to using that, which
would be rather simpler, how badly am I going to restrict myself in browser
choice?

Then what are you doing, loading stuff into a hidden iframe that calls
back into the parent window?
Yup. Been done, has it?

[...] It is supported in...