I think the ideal would be introducing testing toward the beginning (perhaps after you give them the initial basic mental model) so that someone has a recommend

I think the ideal would be introducing testing toward the beginning (perhaps after you give them the initial basic mental model) so that someone has a recommendation for the testing library they should use which will work for the rest of the book. Then, every so often when you notice that something is hard to test, have a sidebar to help them along. This does kinda eliminate having a chapter on it though, unless you have something how to migrate from legacy frontend code to react/redux (which actually would be pretty valuable, though it sounds like a tall order to me).