WARNING: Do not store any secrets (such as private API keys) in your React app!

Environment variables are embedded into the build, meaning anyone can view them by inspecting your app's files.

outside our src folder create .env file and add it to gitignore
REACT_APP_WEATHER_API_KEY=123456
inside where you want to access the variable 
const API_KEY = process.env.REACT_APP_WEATHER_API_KEY;

make sure to restart the server