If you want to really understand the language, do what I did, and write a compiler (http://jurassic.codeplex.com/). You'll learn about fun oddities, like why the lexical scope introduced by the catch block is different from any other type of scope and why declaring variables inside an eval is different from declaring variables in the parent scope.
Because start-up require people to do all kind of stuff. You'll learn front-end and (some) back-end and will have bigger picture, which generally is "A Good Thing".
I currently work at a largish consulting firm and get all those things, plus I don't have to settle for "long hours and low pay". Perhaps "work at a company that lets you work on interesting problems and learn new skills" would be far better advice. There is no reason why that has to be a start-up, nor is there a guarantee that a start-up will give you those things.
True, there's no guarantee. There's no guarantee that you'll find "work at a company that lets you work on interesting problems and learn new skills". Everything here is subjective. :]
"Practice" is his overall message. He's just listing (semi-?)practical ways to do that, for people who can't force themselves to sit at a computer and mess around with random things.
