What is the Express.js Router?

The Express Router is a mini-application that handles routing logic, allowing you to modularize your route definitions into separate files. Create a router: const router = express.Router();. Define routes on it: router.get('/', handler); router.post('/', handler);. Then mount it in the main app: app.use('/users', router). A GET to /users/ now matches router.get('/'). This is essential for organizing large applications — you create separate router files for each resource (routes/users.js, routes/posts.js) and mount them all in app.js. Routers can also have their own middleware, creating middleware scoped to specific route groups.