By describing all possible events that can occur in the application (users taking action, or events triggered by other things like an Ajax request returning), and then subscribing individual components (essentially registering event listener functions), problems with application state rarely occur and debugging becomes much easier.

With a tightly-coupled web application where each component depends on interlinking chains of data, walking through the code can be a nightmare. But as long as you follow practices like this to decouple the code, it’s possible to keep the JavaScript very clean, readable and editable even at immense sizes and levels of complexity.

Great article. Like Nick, we’ve started to use the State Machine model in some of our front end work. However, this was before we’d investigated YUI, knowing about the custom event object would have made our lives a little easier!