I'm always surprised that reading source code is such a rare occurrence or suggestion in this world. If you consider yourself even in the vicinity of "advanced", I think you're capable of learning great stuff by looking at anything on github. Something big, but well organized. Lo-dash, Node, Mocha, etc.

Of course you won't understand everything on first read. But you will learn really good things if you know how to look at something confusing and identify what it is you don't understand; then you can go find didactic materials to fill in the gaps.

This is an amazing way to learn. You get to see how some of the most used, or cutting-edge tools are implemented (for free!). You learn about the tools themselves and thus gain expertise in them. And, you gain the meta-skill of working in unfamiliar territory, which I think is a fundamental key to being considered "advanced".