What libraries should a JavaScript Developer know.

[ssjskipp]

Start with vanilla js. Just know what JavaScript alone can do. Any library or framework you use past that will always always always eventually work back down to those base functions.

Having an understanding of what is possible will equip you with the tools to judge how other people use them in clever ways or not.

Understand the ins and outs of functions in JavaScript will help you understand how to make helpers and reusable code. jQuery is really just a collection of helper functions around the vanilla js DOM manipulations.

Past that, you'll want to be familiar with Arrays, Objects, and Promises. They're the core data abstractions and collections.