"There are tens or hundreds useful emacs packages on the web that are not bundled with emacs. Some are for special purposes such as making emacs act as a mp3 player, some others are written by third-parties, others are in the process of being bundled with the next release of emacs.
When you download a emacs package, typically there's a README/INSTALL file of install instructions. Or, the elisp file itself will contain installation guides in the header. Follow those. Here we give general instructions on installing packages..."
*Via Newsgroups: comp.lang.lisp, comp.emacs - Jul 8 2008
*NB: js2-mode http://code.google.com/p/js2-mode/