Question

I have seen this link: Implementing Mutual Exclusion in JavaScript. On the other hand, I have read that there are no threads in javascript, but what exactly does that mean?

When events occur, where in the code can they interrupt?

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

Specifically, I am wondering about the effects of using functions called by setTimeout() and XmlHttpRequest‘s onreadystatechange on globally accessible variables.

Answer 1

Javascript is defined as a reentrant language which means there is no threading exposed to the user, there may be threads in the implementation. Functions like setTimeout() and asynchronous callbacks need to wait for the script engine to sleep before they’re able to run.

That means that everything that happens in an event must be finished before the next event will be processed.

That being said, you may need a mutex if your code does something where it expects a value not to change between when the asynchronous event was fired and when the callback was called.

For example if you have a data structure where you click one button and it sends an XmlHttpRequest which calls a callback the changes the data structure in a destructive way, and you have another button that changes the same data structure directly, between when the event was fired and when the call back was executed the user could have clicked and updated the data structure before the callback which could then lose the value.

While you could create a race condition like that it’s very easy to prevent that in your code since each function will be atomic. It would be a lot of work and take some odd coding patterns to create the race condition in fact.