…I added variable to .env.development like this API_URL=http://localhost:3000/

…I restarted server

…I googled for an hour

…And I still was facing that process.env.API_URL is undefined

Somehow I missed this from official documentation

Note: You must create custom environment variables beginning with REACTAPP. Any other variables except NODE_ENV will be ignored to avoid accidentally exposing a private key on the machine that could have the same name. Changing any environment variables will require you to restart the development server if it is running.

That’s smart, but 🤬

Anyways, don’t forget to add REACT_APP_ prefix to your variables After renaming my environment variable to REACT_APP_API_URL I could carry on with my tasks 😊

Happy coding!