I know "enough to spend most of my time working with it for my day job"--the huge swaths of javascript that my coworker set up to run our site uses a lot of functions-that-return-other-functions and JSON and other things that seem kind of advanced but cool and useful. I know there are gaps in my knowledge, but I can find my way around well enough to read the code that's there, modify it, add my own, and look up the things that I'm not sure about.