Question 1

What are Nest interceptors?

Accepted Answer

AOP wrappers around handler execution using RxJS Observables — logging, mapping, caching, timeouts. Implement NestInterceptor with intercept(context, next). Can alter request/response streams.

Question 2

Interceptor vs middleware vs filter — response mapping?

Accepted Answer

Interceptors map successful responses; exception filters map errors; middleware is lower-level and less Nest-context rich. Use ClassSerializerInterceptor for entity shaping. Keep mapping explicit when possible.

Question 3

How do you implement a timeout interceptor?

Accepted Answer

Race the handler stream with a timeout operator and throw RequestTimeoutException. RxJS timeout on next.handle(). Document limits. Don't timeout long uploads without care.

Question 4

Caching interceptor pitfalls?

Accepted Answer

Caching personalized or authz-sensitive responses; weak keys; memory growth; stale data. Key by user/tenant when needed. Prefer HTTP/Redis caches with TTLs. Bypass on mutations.

Question 5

Scenario: need to measure handler duration and status.

Accepted Answer

Logging interceptor recording start time and tap/finalize on the observable stream. Include correlation IDs. Avoid logging huge bodies. Export metrics histograms.

Question 6

Lead: when interceptors become a dumping ground — redesign?

Accepted Answer

Extract clear cross-cutting modules, limit interceptor count, prefer domain events for business side-effects. Too many nested interceptors obscure control flow. Enforce review for new global interceptors.

Interceptors NestJS Interview Questions

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