Some JS knowledge is required. But doesn't need to be latest greatest ES2016. If you know about loops and functions in JavaScript
you should be fine. If you stopped writing and keeping up with JavaScript 10 years ago you should be fine. The first 3 chapters build on ancient JavaScript that has been around since IE6. Chapter 4 adds React-specific syntax (JSX). From there new JS features (some not even in the browsers yet) are introduced.
In way the book can help you catch up with JS if you've been under a rock for a while