This guy wrote an article about it:
http://stuck-in-windows.blogspot.com/2009/02/fud-over-javascript-and-gpl.html
In his article, he cites the gnu website FAQ, which has Q&A:
(Q) A company is running a modified version of a GPL'ed program on a
web site. Does the GPL say they must release their modified sources?
(A) The GPL permits anyone to make a modified version and use it
without ever distributing it to others. What this company is doing is
a special case of that. Therefore, the company does not have to
release the modified sources.
The FAQ then references the GNU Affero GPL which does require the release of all website source code.
