The most interesting project of that sort which I've come across is Cody Brocious'
Dyna6502Asm which is a dynamically recompiled fork of 6502asm.
Note that with git, taking a fork is a natural way to do some development even if you expect to contribute it back. And on github, you can publish by maintaining a branch with the special name 'gh-pages' - as
Cody has in this case
(It's not too hard to find a mail address for the author of 6502asm)
(Of course
visual6502 is also javascript, but really it's a switch level simulator which happens to load a 6502 netlist at present, and with some 6502 specifics in the user interface. I think you're looking for a 6502 model in javascript. Python is quite a similar language, so
py65 is worth a look too - there's an outstanding ticket for improving the 65c02 model, or you could venture into a
GUI for it...)
Cheers
Ed