I discovered the hard way that prior ReactJS knowledge was required to dive into creating custom components for SharePoint and PowerApps.
Here are three free resources I used to learn it:
- Intro to ES6 – https://scrimba.com/learn/introtoes6
- React JS Fundamentals – https://scrimba.com/learn/learnreact
- React Advance – https://scrimba.com/learn/react
- React Class Components – https://scrimba.com/learn/classcomponents
- CSS Flexbox – https://scrimba.com/learn/flexbox
I did find egghead.io‘s free resources and codecademy.com‘s free trial but Scrimba would be my personal favourite.