$ man environment-files

Environment Files

Files that store environment variables for local development.

by Shawn Tenam

.env files keep secrets out of Git. API keys, tokens, database URLs. They're local only. Each developer has their own. Vercel has its own set for production.

Create .env in your project root. Add API_KEY=value. Add .env to .gitignore. Load variables with process.env.API_KEY in code.