JavaScript and Jquery: Interactive Front-End Web Development text in the Java course?

I have noticed the course links to this book throughout, however the link takes you to a section to purchase the book. Is the expectation is that we will buy this book, or is the course supposed to be linking you to a readable chapter?

It’s supplementary (optional) material you can purchase if you want. The course is self-sufficient without the book, but it just provides more background/reference material.