In your emacs config:
(autoload 'js2-mode "js2-mode" nil t)
- (add-to-list 'auto-mode-alist '("\\.js$" . js2-mode))
+ (add-to-list 'auto-mode-alist '("\\.js\\'" . js2-mode))
See <http://code.google.com/p/js2-mode/wiki/InstallationInstructions> for
additional details.