Using process.env in React

1. Install dotenv package
2. Create .env file in root directory
3. Add variables to the file like this:

REACT_APP_MOCK_API_URL=data1
REACT_APP_REST_KEY=data2

4. Read variables like this:

const key = process.env.REACT_APP_REST_KEY;
const mockUrl = process.env.REACT_APP_MOCK_API_URL;

Important!

Variable name MUST begin with "REACT_APP_"

(Initially I called variable MOCK_API_URL. It took me few hours to figure out why key contained value and mockUrl was undefined.)