When should I use the module pattern in Express?
For medium to large apps. For tiny apps with 3 routes, a flat structure (routes/, controllers/, models/) is fine. The module pattern adds folder nesting that is not worth it for tiny apps.
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More FAQs in Building Modular Express APIs With Routers and Services
Group each feature into a module: router, controller, service, model, validations in one folder (src/modules/users/). Mount modules in app.js. Each module is cohesive, independent, testable, and reusable.
Cohesion (everything for a feature in one folder), independence (a module can be extracted into a microservice later), testability (test the service without HTTP context), team scalability (different teams own different modules), and reusability across apps.
Business logic. Functions that use the model and utilities to do work. createUser hashes the password, checks for duplicates, and creates the user. Services are reusable and testable without an HTTP context.
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