Original Author here. I fixed the Firefox bug and merged a pull request to allow answers to be transferred to other browsers. I will add a Show All Answers button soon for folks who just want to browse.
rcfox: These exercises teach functional programming concepts, and that includes composition as it applies to the Observer pattern. I use RX but you could also use bacon JS or build your own lightweight Observable library. You should check out rx.lite.js which is a minimal subset of the library. As for your concerns about including a library, I urge you to reconsider if you can afford the extra download size. The Observer pattern is ubiquitous in software and you'll be amazed how versatile Rx is. At Netflix we use it on the server and client for a wide variety of different tasks. Rx has more than paid for itself 10 times over.
rcfox: These exercises teach functional programming concepts, and that includes composition as it applies to the Observer pattern. I use RX but you could also use bacon JS or build your own lightweight Observable library. You should check out rx.lite.js which is a minimal subset of the library. As for your concerns about including a library, I urge you to reconsider if you can afford the extra download size. The Observer pattern is ubiquitous in software and you'll be amazed how versatile Rx is. At Netflix we use it on the server and client for a wide variety of different tasks. Rx has more than paid for itself 10 times over.