If you buy this book, you get:
The current price is introductory. It will increase as more content is added.
On the homepage of this book, there are extensive previews for all versions of this book.
For example, you may see:
That is called the type signature of
Number.isFinite(). This notation, especially the static types
Why is this notation being used? It helps give you a quick idea of how a function works. The notation is explained in detail in a 2ality blog post, but is usually relatively intuitive.
Explains how to best read the content.
Points to additional, external, content.
Gives a tip related to the current content.
Asks and answers a question pertinent to the current content (think FAQ).
Warns about pitfalls, etc.
Provides additional details, complementing the current content. It is similar to a footnote.
Mentions the path of a test-driven exercise that you can do at that point.
Indicates that there is a quiz for the current (part of a) chapter.
Thanks to Allen Wirfs-Brock for his advice via Twitter and blog post comments. It helped make this book better.
More people who contributed are acknowledged in the chapters.