The Ultimate Introduction to ReactJS -- Building React from Scratch

I found this when I was gathering context on the whys of certain Javascript frameworks based on ideas from the world of functional reactive programming; eventhough, I had no intention of systematically working through the videos, nor even of knuckling down to learn React, it gave me some ahas!

1. [The Ultimate Introduction to ReactJS [Part 1] - Building React From Scratch ](https://www.youtube.com/watch?v=pTHCwUdGFkc)

2. [The Ultimate Introduction to ReactJS [Part 2] - Navigation, Event Flow, and Async Data Fetching ](https://www.youtube.com/watch?v=eS6MSegecH4)

I watched enough of the first video, while eating popcorn, to get a general answer to my why question; the second video, I have not watched at all, but include for the sake of completeness.