Neat! I hope physics classes today are getting more into using such web interactive modeling. Or, that the new wave of interactive books are leveraging this kind of ability.
http://www.walter-fendt.de/ph14e/
Which are Java applets. Not quite as good as modern JS would be, but given the era they were written, they were pretty cool things.
Although they are also not done in javascript, but a mix of java and flash.
(All are Java applets.)