As an example of an open source project you could look into I would recommend to check the dojo sources. It's easier to grasp the advanced concepts there than e.g. in JQuery since there is a much wider code base for many different aspects. There is a lot to go through in the dojox packages. The code is also nicely documented, and recently they added nice online documentation, too, something that was missing for a long time. I learned a lot by peeking into dojo's internals, so I can only recommend it.
You should pay attention to their way of coding object-orientedly and how it differs from what plain JS offers you.
The concept of "Closures" is something you could look up, then find examples in existing code and finally use them in your own.
"Ajax" is another buzzword and concept you should be familiar with, it lets you do all the nice things in your browser that some years ago were only possible in desktop applications.
Some book recommendations