Beginner level would be best. I'm taking my first steps into JavaScript programming which (up to now) I've been doing just by adapting bits of code that I've found in web pages etc. I find the basics easy enough to follow but already, it seems clear that there's a lot going on "under the hood". For example 'document' appears to be some sort of predefined variable and I'm sure there must be others too, as well as predefined functions. A good book would probably be the best place for me to start.