The Javascript party line is that you don't need locking, because no threads, no safety, no problem: every window or tab has its own Javascript context, and that context is guaranteed to behave as if it is single-threaded: sequential execution, timers fire on return to the event loop, processes cannot communicate with each other. Except then they added localStorage. Oh, what's this? Is it shared memory that can be accessed asynchronously by multiple contexts? Why yes it is! So now you really do need locking. Except then they didn't add mutexes, or even an atomic-read-and-increment operator. Hooray. And they also didn't add any IPC, like a socket that one process can write to and that others can select() on. Hooray. So let's say you've got an app, it spans multiple documents, and you want a sound to play in reaction to some external, asynchronous event if any of your documents are open. But if three windows are open, you only want that sound to play once […more…]