[mhd]

If four years would be enough to change a lot about the core JavaScript language, there wouldn't be a need for the book in the first place. It's still quite modern enough, and will be until a major EcmaScript overhaul is propagated and accepted by all common browsers…

It's not about modern web techniques, which is why it aged that well. For those, I'd personally recommend pulling apart some modern webapps or libraries, or reading modern tutorials. Usually you're somewhat outdated once your book reached its publisher… (Having said that, JavaScript Patterns is also pretty neat)

[TazeTSchnitzel]

And if you want more of Crockford's advice, he's done a talk about ES5 changes (which you should really watch, they are great)