To understand .env.development.local , we must first break down its components:
Your team’s .env.development points to a shared staging database. You, however, are testing a new migration script and need to point to localhost:5432/my_local_db . Instead of modifying the shared file (and risking committing the change), you add DATABASE_URL=postgres://localhost/my_local_db to .env.development.local . When you switch to production mode, this file is completely ignored. .env.development.local
To truly understand the role of .env.development.local , let’s visualize how a typical tool (like Create React App or Vite) resolves variables. To understand