Here's what the book preface has to say about audience:
Audience
If you aren’t at all familiar with JavaScript, this probably shouldn’t be your first book. Even if it is, don’t worry too much; we try to present fundamental JavaScript concepts in a way that should be understandable even for relative beginners. But, to be honest, this book will probably best fit people who already know some JavaScript and who wish to deepen their understanding of JavaScript as a language, as well as the browser as the environment in which JavaScript code is executed.
So if you know nothing about JavaScript,
you should probably learn a bit before tackling this book. But if you have even basic JavaScript skills, you can likely pick up what you need along the way, especially for the first half of the book.