http://blog.alexanderdickson.com/javascript-nes-emulator-par...
Part 2 will be posted when it's done. :)
If you want to get something done much quicker, have a go at emulating the Chip-8 VM.
I also have a post, but it's less hand-holding than the first.
http://blog.alexanderdickson.com/javascript-chip-8-emulator
Good luck!