Js2-mode is an interesting case of an author (Steve Yegge) going against the fold of how Emacs is supposed to interact with programming modes -- by writing his own parser and syntax highlighter and eschewing the built-in way, which is to use Emacs's font locking, which itself is driven by regular expressions and the odd lisp function.

It's a shame Emacs lacks the facility to properly capture the more complex requirements of languages today - especially files with mixed content, such as HTML, Javascript and templating tools. The few that do support this do it using a variety of clever hacks to work around this problem; and still, using regular expressions to capture the nuances and complexities of some languages make it an error-prone way of writing major modes in Emacs. Not to mention you lose out on the many advantages of having an AST in memory for code jumping, refactoring, syntax checking and what have you.

Emacs could do with a shake-up in this space. The proposed solution (Semantic Mode / CEDET) has never really caught on, even though it adds much of the same functionality to Emacs; my suspicion is that this is because Semantic/CEDET is ridiculously complex and lacking in documentation.