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!