What should be path of learning a ReactJS(facebook) for AngularJs developer?

I’m AngularJS(1.5,2 and 4) developer. Now, I want to learn ReactJS from scratch.

Can anyone help me what should be learning curve for me to start
ReactJS?

I’m lfeeling very difficult to learn it while comparing with AngularJS 2?

What are the main things I should be focused in ReactJS?

Source: AngularJS