Question 1

What is Nest middleware vs Express middleware?

Accepted Answer

Nest middleware is framework-aware (implements NestMiddleware) but often wraps Express/Fastify style (req,res,next). Configured in modules via MiddlewareConsumer with route matching. Runs before guards — pipeline order awareness matters.

Question 2

Where do you apply middleware selectively?

Accepted Answer

In configure(consumer) with .forRoutes / .exclude, by controller or path. Global middleware applies everywhere — use for logging/correlation IDs. Keep auth preferably in guards for Nest idiomatic style.

Question 3

Middleware vs Guard vs Interceptor — which for auth?

Accepted Answer

Guards are the Nest-idiomatic authn/authz gate; middleware for low-level request shaping; interceptors for AOP around handlers. You can authenticate in middleware, but guards integrate better with roles/metadata. Know the official pipeline order.

Question 4

Debugging: middleware not running for a route.

Accepted Answer

Path mismatch, global prefix, versioning, or excluded routes; Fastify vs Express adapter differences. Log consumer config. Confirm route registration. Check middleware order.

Question 5

Lead: correlation ID middleware standards across services.

Accepted Answer

Accept inbound trace headers, create if missing, store in ALS, propagate outbound, and include in logs. Align with OpenTelemetry. Document header names. Make it a platform module, not per-service one-offs.

Middleware NestJS Interview Questions

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