Question

Are there any JavaScript editors that can validate JavaScript syntax on the fly? Things like missing brackets, etc.

Answer 1

Personally, I like to use emacs with flymake, which can use Spidermonkey, JsLint or Rhino to do the validation. This does the sort of on the fly syntax checking I think you are after. If you want a liberal validation, you might want to use Spidermonkey as it is the execution engine from firefox, so it will reject anything firefox rejects. If you want something a little stricter, Crockford’s JsLint enforces good style.
_{(source: mnemonikk.org)}

Emacs is not for everyone however. If you prefer something point-and-click, you might want to check out IntelliJ’s IDEA. It can do on the fly syntax checking, along with a raft of other features. Sadly, javascript support is not available in the free version.

_{(source: jetbrains.com)}

A free alternative to IntelliJ is Netbeans, which purports to offer the same javascript functionality baked into the core. I don’t have experience with it, but it is free, open source, and reputedly improving in recent versions
netbean's javascript syntax highlighting http://netbeans.org/images_www/articles/61/ide/javascript/background_parser.png

If you’re a Windows or ASP.NET type developer you might be more interested in Microsoft Visual Web Developer Express 2010. It supports this features, and probably has better support for ASP.NET development than the other tools I’ve mentioned.