Not a ton to say in this chapter. The organization is pretty straightforward, leading to the discussion of observables at the end.
You may want to mention observables and RxJS in the introduction, because that's where the chapter is heading. As far as I can tell, there's no other structure that will allow lazy mapping in the way you've laid it out here, so you should warn readers that you're dealing with a library outside the ECMA spec, even if it is one that everybody's heard of, and uses.
Not a ton to say in this chapter. The organization is pretty straightforward, leading to the discussion of observables at the end.
You may want to mention observables and RxJS in the introduction, because that's where the chapter is heading. As far as I can tell, there's no other structure that will allow lazy mapping in the way you've laid it out here, so you should warn readers that you're dealing with a library outside the ECMA spec, even if it is one that everybody's heard of, and uses.
You might also consider providing a reference to an outside source where readers can learn more about RxJS. (This one, for example: https://pragprog.com/book/smreactjs/reactive-programming-with-rxjs )
There are a bunch of short paragraphs in this chapter that could be combined without harming the meaning.
Reducing Time section
I suggest bumping this up to a level-1 heading.