Is JavaScript single threaded? If not, how do I ge

Question

I have a web page with DIVs with a mouseover handler that is intended to show a pop-up information bubble. I don't want more than one info bubble to be visible at a time. But when the user moves the mouse rapidly over two items, I sometimes get two bubbles. This should not happen, because the code for showing a pop-up cancels the previous pop-up.

If this were a multi-threaded system then the problem would be obvious: there are two threads trying to show a pop-up, and they both cancel existing pop-ups then pop up their own pop-ups. But I assumed JavaScript is always run single-threaded, which would prevent this. Am I wrong? Are event handlers running asynchronously, in which case I need synchronized access to shared data, or should I instead be looking for bugs in the library code for cancelling pop-ups?

Edited to add:

• The library in question is SIMILE Timeline and its Ajax library;
• The event handler does call SimileAjax.DOM.cancelEvent(domEvt), which I assume based on the name cancels the bubbling of events;
• Just to make thing s more complicated, what I am actually doing is starting a timeout that if not cancelled by a moustout shows the pop-up, this being intended to prevent pop-ups flickering annoyingly but annoyingly having the reverse effect.

I'll have another poke at it and see if I can work out where I am going wrong. :-)

Answer 1

FYI: As of Firefox 3 there is a change pretty much relevant to this discussion: execution threads causing synchronous XMLHttpRequest requests get detached (this is why the interface doesn't freeze there during synchronous requests) and the execution continues. Upon synchronous request completion, its thread continues as well. They won't be executed at the same time, however relying on the assumption that single thread stops while a synchronous procedure (request) happening is not applicable any more.