set up react environment create-react-app code example

Example 1: update create react app

npm install react-scripts@latest

Example 2: create-react-app enviroment variables

process.env.NODE_ENV gives you either "development" or "production" or "test"
When you run npm start it is equal to develpment
When you run npm run build it is equal to production
When you run npm test it is equal to test