How do you structure a large Express.js application?

A scalable Express project typically follows a layered architecture: Routes (routes/): define endpoints, validate input, call controllers. Controllers (controllers/): handle HTTP request/response, delegate to services. Services (services/): contain business logic, orchestrate operations. Models (models/): define database schemas. Middleware (middleware/): reusable middleware like auth, validation. Config (config/): environment configuration. Utils (utils/): helper functions. Entry point app.js wires everything together. This separation of concerns makes each layer independently testable and maintainable. Each router file handles one resource, and app.js imports and mounts all routers.