Express routing

Express maps HTTP methods and path patterns to handler functions. Routes can be declared on the app or on `express.Router()` instances mounted at prefixes — this keeps resource boundaries clear and enables modular route trees.

					const users = express.Router({ mergeParams: true });
users.get('/:id', getUser);
users.post('/', createUser);
app.use('/api/users', users);
				

Path params (`:id`), optional params, and regex segments capture URL data. `mergeParams: true` lets nested routers see parent params. Method-specific handlers (`app.get`, `app.post`) run only for matching verbs; `app.all` matches every method.

On interviews: explain route registration order (first match wins), mounting routers for feature modules, and why handlers should stay thin — validation and domain logic belong in middleware or services.

Common pitfalls: overlapping routes where a broad pattern shadows a specific one, forgetting `mergeParams` in nested routers, and putting business logic directly in route callbacks.

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• Group routes by resource with Router mounting.
• Keep handlers thin; delegate to services.
• Name params consistently across nested routers.
• Document which routes are public versus protected.