If I had to define this book in a single word, I'd had to go with "unfocused".
Now, with that title, you'd expect to learn about the principles that drove the design of things like "ReactJS". But it doesn't. This is not about Reactive Programming. It's about ReactJS. And it's not about Reactive Programming, it's about Reactive Funcional Programming.
Well, you'd still expect it to come with some conclusions about ReactJS, right? Wrong again.
Most of the time you'll spend reading things that have absolutely no relation with reactive programming, functional reactive programming or even ReactJS. There is a long rant about C++ which ends with no conclusion at all and gives no pointers on how it connects to the whole. There is another discussion about INTERCAL which leads to nowhere -- maybe, except, the author's bank account for the number of words.
At some point, the author finally discusses a bit of functional programming talking about
reduce, but it goes nowhere from there and a whole chapter with 10+ pages have a single paragraph about real, focused talk about functional programming; the rest is just more rambling going to nowhere.
If it was possible to run tests over the content of the book, the amount of content out of a coverage on a BDD about Reactive Programming would point that about 90% of it is never tested. It's content that talks absolutely nothing about reactive programming, with large portions being repeated over and over again (which makes me, once again, wonder why Packt pays for reviewers when this kind of bullshit happens).
You want a review in a single phrase? Ok, that phrase would be "stay away from this book".