React is a JavaScript library, so it’s fair to say that we’ll be using a whole heck of a lot of JavaScript in this bootcamp. You ought to be familiar with the fundamentals of JavaScript before class begins.
Lean JavaScript is another brand-new, free interactive tutorial that replaced an older Codecademy course.
I highly recommend completing JavaScript 30 by Wes Bos. It’s a series of thirty 20-minute videos, meant to be completed over the course of 30 days. That will take some planning on your part, obviously; you can’t wait until the last second to dive into this stuff. But it’s not a huge daily time commitment, and it will help a great deal.