If you find yourself doing substantial hacking on complicated websites, you’ll probably find yourself needing a decent major mode for editing JavaScript. There are at least six different JavaScript major modes out there — which one should you use? Let’s take a look at them!

A survey of the various JavaScript major modes

generic-x.el (which comes with Emacs) contains javascript-generic-mode, which doesn’t handle indentation well, and fails to correctly highlight C-style (/* … */) comments. But it’s fine for short-term use: since it ships with Emacs, it’s already there when you need it.

To enable it, add (require 'generic-x) to ~/.emacs. This will automatically associate files with .js extensions with javascript-generic-mode.

Peter Kruse’s javascript-mode.el is antiquated. It hasn’t been updated in many years, and relies on hilit19, an obsolete syntax highlighting library. Definitely not recommended.

My recommendation: Karl Landström’s javascript.el appears to be the nicest JavaScript mode out there. It handles indentation well, and both C- and C++-style (// …) comments. A winner! To use, place javascript.el in a directory inside load-path and add the following to your ~/.emacs:

(autoload 'javascript-mode "javascript" nil t)
(add-to-list 'auto-mode-alist '("\\.js\\'" . javascript-mode))

Joost Diepenmaat has a slight tweak of Karl’s mode which contains a few font-lock fixes for quoted strings and regular expressions. I haven’t tried it myself.

There’s also ecmascript-mode.el, which is a derived mode from java-mode. It doesn’t appear to do anything that javascript.el doesn’t do, and it unconditionally makes font-lock-builtin-face bold, regardless of your personal settings. Thanks to krupan and Mathias for the link.

MozRepl comes with its own js-mode.el, which appears to be a very minimal variant on cc-mode. Thanks to RetroJ for pointing this out.

In addition to all of the above, there’s the javascript-mode.el which comes with XEmacs, which I’m told works in Emacs as well (and is even bundled with Aquamacs). Thanks to Reed and Xah for the link.

Robustness tip: simple failover

Say you’ve installed javascript.el on machine A but not on machine B. Ideally, your ~/.emacs file should detect if javascript.el is installed: if it is, it should use it, but if it isn’t, it should fall back on javascript-generic-mode from generic-x.el. This is pretty easy stuff. First, require generic-x as above. Then check for the presence of javascript.el, and configure it if it’s present:

(require 'generic-x)

(when (locate-library "javascript")
  (autoload 'javascript-mode "javascript" nil t)
  (add-to-list 'auto-mode-alist '("\\.js\\'" . javascript-mode)))

This way, you can copy your ~/.emacs file to some other machine, and you know it won’t break.