As a front-end developer (if you’re working with CSS you are a front-end developer, make no mistakes about that), you need to be at least familiar, if not very well versed in JavaScript.
Admittedly, starting to learn JavaScript can be a bit hard and thus it’s tempting to jump into a library that will make your work a lot easier. Don’t do it just yet. Andrew Burgess explains all JavaScript core concepts in digestible chunks with very good examples.
If you’re making your first steps in JavaScript, or if you want to reassess your basic knowledge, this book is a must read.