[nathanallen]

I've enjoyed working with riot.js (https://muut.com/riotjs/). It's a "minimalist" front end framework, written in vanilla javascript. It's easy to pick up, and it's not super opinionated. You can read/absorb the library in one sitting -- so you'll have a good sense of what's going on under the hood.

Another great resource is You Might Not Need jQuery (http://youmightnotneedjquery.com/). Take what you need, leave the bloat behind!

[CHsurfer]

I picked riot.js to learn javascript as it seemed to rely most heavily on solid javascript program paradigms rather than abstractions introduced by the framework. So far I'm happy, with this choice.