The main thing you'll need for this book is a computer and text editor. Your operating system will come with a text editor, any will do, but you will likely find that a text editor focused on developer needs will be most useful. There are many excellent developer-oriented text editors available for various operating systems, see https://github.com/showcases/text-editors and https://en.wikipedia.org/wiki/List_of_text_editors for some options. An Internet connection will be required to view the maps, and you'll also need a recent version of a modern web browser, such as Firefox, Google Chrome, Safari, Opera, or Internet Explorer (version 9 or higher).
For some advanced uses cases, you will also need Python, NodeJS, Java, and Git (a source code control management software). Installation instructions are provided for these additional tools.
No knowledge of Geographic Information Systems (GIS) is required, nor is extensive JavaScript experience. A basic understanding of JavaScript syntax and HTML/CSS will greatly aid in understanding the material, but is not required.