<p>I have seen this link: <a href="http://www.developer.com/lang/jscript/article.php/3592016" rel="noreferrer">Implementing Mutual Exclusion in JavaScript</a>.
On the other hand, I have read that there are no threads in javascript, but what exactly does that mean? </p>

<p>When events occur, where in the code can they interrupt?</p>

<p>And if there are no threads in JS, do I need to use mutexes in JS or not?</p>

<p>Specifically, I am wondering about the effects of using functions called by <code>setTimeout()</code> and <code>XmlHttpRequest</code>'s <code>onreadystatechange</code> on globally accessible variables. </p>

JavaScript is single threaded... though Chrome may be a new beast (I think it is also single threaded, but each tab has it's own JavaScript thread... I haven't looked into it in detail, so don't quote me there).
However, one thing you DO need to worry about is how your JavaScript will handle multiple ajax requests coming back in not the same order you send them.  So, all you really need to worry about is make sure your ajax calls are handled in a way that they won't step on eachother's feet if the results come back in a different order than you sent them.
This goes for timeouts too...
When JavaScript grows multithreading, then maybe worry about mutexes and the like....