First and most importantly, learn how to write in pure JavaScript. Have an understanding of what libraries are built from.

tl;dr – Find an application you like to make, and remake it in every new library you want to learn.

I teach a lot of people to write in Javascript. And the thing I have noticed when they need to learn a second framework is that they always get stuck up on why the new language can’t do something their old one can.

I don’t think that is productive. So, after some serious trial and error, I figured out what I think is the best way to write in a new library.

Find a simple tool you enjoy building. Something that has the basics of a simple application. API support, data binding(or lack of), repeating elements, promises, etc.

Make sure you can build it in your first library with your eyes closed. When you can do that, try your hand at building the exact same tool in the new library.

Compare each step with a feature you are used to, and use google to help you find the best practices that compare to them in your new library.

In Angular, you keep JSON data and logical code in services, and make API calls from a factory. How would you structure that with Backbone? Would you keep your logic inside a model?

Good luck making yourself marketable and well rounded.