This guy wrote a Gameboy emulator in Javascript.

This is the most accessible guide to the internals of an emulator I've ever read. I don't know whether it's efficient or complete at all, but it actually makes sense when scanning through the source.

Tetris was pretty buggy for me in Chrome, but it runs.

It's really a testament to how fast modern Javascript engines [and CPUs] are. You would've had a damned hard time running something like this a short while ago.

Edit: Oops, I meant to put this in General Discussion... Move it if you mind, sorry.

The documentation is quite interesting. I like the approach with concept, application, and example code. It would be nice to see more documentation like this for other languages and emulators, but it must have taken quite some time to write up. I don't have much interest in a javascript emulator though. It seems more of a novelty thing rather than practical as it can't compare to the features and flexibility of the standalone emulators.

Oh some of us can find uses for well documented Game Boy Emulators....

I also want to noe that Javascript emulators are probably the easiest to improve, given that no compilers are needed; all that is needed to write the code is a text editor; and the tools are free and will assuredly remain so. (you can't say that about the C# and VB-based emulators).