Router trong Express là gì? Cách tổ chức routes?

express.Router() tạo mini-app với routes và middleware riêng biệt — giải pháp chính để tổ chức code theo feature.

• Mounting pattern: app.use('/api/v1/users', userRouter) prefix tất cả routes trong router.
• Route-level middleware: router.use(authMiddleware) chỉ áp dụng cho routes trong router đó.
• Nested routers: adminRouter.use('/users', adminUserRouter) để nest sâu hơn.
• Versioned API pattern: app.use('/api/v1', v1Router); app.use('/api/v2', v2Router) — v1 và v2 hoàn toàn độc lập, dễ deprecate.
• Cấu trúc thực tế: routes/users.ts export router, routes/index.ts aggregate tất cả routers, app.ts chỉ app.use('/api', mainRouter).
• Lưu ý: router.use(middleware) sau router.get(...) thì route đó không được áp dụng middleware — phải đặt middleware TRƯỚC routes.

express.Router() creates a mini-app with its own routes and middleware — the primary way to organize code by feature.

• Mounting pattern: app.use('/api/v1/users', userRouter) prefixes all routes in the router.
• Route-level middleware: router.use(authMiddleware) applies only to routes within that router.
• Nested routers: adminRouter.use('/users', adminUserRouter) for deeper nesting.
• Versioned API pattern: app.use('/api/v1', v1Router); app.use('/api/v2', v2Router) — v1 and v2 are completely independent, easy to deprecate.
• Practical file structure: routes/users.ts exports a router, routes/index.ts aggregates all routers, app.ts simply does app.use('/api', mainRouter).

Pitfall: router.use(middleware) placed after router.get(...) means that route doesn't get the middleware applied — middleware must always be declared BEFORE the routes it should affect.