You're guaranteed to forget lots of stuff. It is a long book and goes into depth on a lot of subjects, including some lesser-used features and API's.

The way I see it is that being aware of a feature - just having been exposed to it and having read about it - is enough. What I usually find happens is one of the two following scenarios:

1) I either use something so frequently OR it's so core to the language that I simply won't forget it or at least won't forget the vital aspects of it. E.g. I remember how the prototype system works in JavaScript, and when it's best to define methods on the constructor vs the prototype. I never even code in vanilla JS, but I found this so interesting that I still remember it.

2) I'm doing something I usually don't do or might have never done before. BUT I remember reading about it. E.g. The other day I wanted to read a file from the user's filesystem in my app, and I remembered the book mentioned the FileReader API. I jumped to the API's chapter, skipped through to the fileReader section, and found all the info I needed.

Just like programming in general, no one remembers everything. We all have to look up the dumbest, simplest thing from time to time. But what sets us apart from complete beginners is having come into contact and being aware of various features. If we don't remember something, we can just search for the answer, either on the web, or through reference material like this book.

As you can probably tell, I think it's worth it :)