Why split app.js and server.js in Express?
So you can import the Express app in tests without listening on a port, and so you can swap the server startup (HTTP, HTTPS, cluster) without touching the app. app.js creates the app; server.js starts it.
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More FAQs in Express Server Folder Structure Best Practices
Separate folders for config, middlewares, models, routes, controllers, and utils. Keep app.js and server.js separate. One file per concern. This is the cleanest layout for a growing Express backend.
Env var loading with dotenv, exported as a typed config object. By convention, all env access goes through this file so other files do not read process.env directly. This keeps config centralized.
Thin. Parse the request, call a service or model, send the response. If a controller has complex logic, move that logic to a services folder. Controllers orchestrate; they do not contain business logic.
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