Thanks Bear, great answer!
Thanks for your question, Paras! It's true that the libraries help and we should take advantage. But at the end no library will write the code for you, so you'd better understand what you're doing and how it's working. People have always considered JavaScript tough, mainly because of the browser differences, and people have always tried to "ease the pain" letting you write JS without writing JS. The truth is JavaScript is not hard and it's about time we (to freely quote Douglass Crockford) man up and learn the language
Another reason: career development and job interviews. Good front-end engineers are hard to find. Often I've heard this complaint from people that hire: "the candidate claims they know JS. when asked how do I do this and that, the candidate says that with prototype.js, they... Turns out the candidate doesn't know JS, they know prototype.js" I mean even if you know
all the libraries (pretty much impossible), it's still better to know javascript, because you don't know which library the next project/contract/client/employer will be using. It may very well be an in-house developed one.
Two-three years ago I was actively looking for a job and went to several interviews. As a rule, every time I heard these two questions in this order: "Is javascript object-oriented? Good. How would you implement inheritance?" I hope that my book will help people ace those interviews! ;P