When should I move to the module pattern?
For medium to large apps. Group each feature into a module (src/modules/users/ with router, controller, service, model, validations). Mount modules in app.js. For tiny apps, a flat structure is fine.
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More FAQs in Express API Structure Roadmap: From Spaghetti to Modular
Start with app.js, add a routes folder, mount routers in app.js, add controllers, add a services layer, add middlewares, add validations, add models and utils folders, add a config folder, version the API, and move to the module pattern for large apps.
When you have more than ~10 routes. Move them into a routes/ folder with one file per feature. Mount each router in app.js with app.use('/path', router). app.js becomes thin.
When controllers get long. Move logic into services/. Controllers parse the request, call a service, send the response. Services do the work and are reusable and testable without an HTTP context.
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