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.
Learn JavaScript is another 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.