The use of map threw me off, too. The author mentions React as an example of that pattern. Not sure why they didn't use that. Express.js comes to mind, too, as

The use of map threw me off, too. The author mentions React as an example of that pattern. Not sure why they didn't use that. Express.js comes to mind, too, as a very grokkable example. (Edit: or perhaps the author considers Express.js an example of an imperative registration API?)