I asked this question on Quora and got some interesting answers. Here i will be writing it in form of a blog post.

There are tons of JS Libraries, since JavaScript is one of hottest languages and a lot of developers work on it to develop rich client side as well as server side (NodeJS based) . So if your looking forward to use a MVC framework TodoMVC is a great place to start with. Next for more specialized ones, you can check http://jster.net or devrates.com.
Here are some good frameworks one should be familiar with such as AngularJS, BackBone JS, Mocha, Prototype JS.

Well, the key is get familiar with JavaScript, and learn/use these libraries that interests you most. Also going through the source code of these libraries is a very enlightening experience.

With that said, May the force be with you!