Add environment variables to create-react-app React APP

1. Environment variables in Create-react-app projects must be prefixed with REACT_APP_, for example: REACT_APP_API_URL = http://localhost:3001/posts/
2. Another special environment variable is NODE_ENV, we can read it from process.env.NODE_ENV, the value will be development, test, or production.
3. Environment variable is injected into code at build time.
4. Access it using process.env.REACT_APP_VAR_NAME in JS code.
5. Access it in the HTML using %REACT_APP_VAR_NAME%.

Add environment variables in .env file

Create .env file under the project’s root folder.

Add temporary environment variables in Shell

for example: REACT_APP_NOT_SECRET_CODE=abcdef npm start.

Also see https://create-react-app.dev/docs/adding-custom-environment-variables/