This article was originally published on AI Study Room . For the full version with working code examples and related articles, visit the original post. Environment Variables: The Complete Guide for Developers Environment variables connect your code to the outside world — database URLs, API keys, feature flags. Misconfiguring them is one of the most common causes of production incidents and security breaches. Here's the complete guide to managing them correctly. The Hierarchy of Config Layer Where Example Never Commit? Default values Code (as fallback) PORT ?? 3000 Commit (with safe defaults) Local dev overrides .env.local DATABASE_URL=localhost Yes (.gitignore) CI/CD Platform secrets DATABASE_URL=staging-db Yes (platform-managed) Production Platform secrets / vault DATABASE_URL=prod-db Yes (platform-managed) Public config NEXT_PUBLIC_* vars NEXT_PUBLIC_API_URL OK (intentionally public) Rules for Environment Variables Never commit secrets to Git. Use .gitignore for .env.local, .env.*.local.…