Questions
3 of 6
How do you use an interceptor to transform every response into a standard envelope format in NestJS?
Difficulty: 5/10
interceptors, global providers, response formatting
Implement NestInterceptor with a generic type, use map() in the RxJS pipeline to wrap every handler return value in a standard object. Register globally via APP_INTERCEPTOR or app.useGlobalInterceptors() so every endpoint returns the same shape.
Response transform interceptor
Global registration options:
- 1
app.useGlobalInterceptors(new TransformInterceptor()) — no DI support.
- 2
{ provide: APP_INTERCEPTOR, useClass: TransformInterceptor } — full DI support, preferred.
- 3
The map() operator transforms every emitted value from the handler's observable.
- 4
null and undefined handler returns are also passed through map() — handle them if needed.
Scenario Questions
0-2 years experience
- 1We have a simple NestJS controller that returns a plain object. How would you add an interceptor so that every response is wrapped in { data: ..., success: true }?
- 2If you applied your interceptor globally, what would happen to a 404 error response? How can you ensure error responses keep their original shape?
- 3Where do you register an interceptor in NestJS so that it runs for all routes?
2-5 years experience
- 1You need to add pagination metadata to every list endpoint without changing each controller. Walk me through how you'd implement an interceptor to inject { meta: { page, limit }, data: [...] } and discuss any trade‑offs.
- 2During a code review you notice that after adding a response‑enveloping interceptor, a third‑party library that expects raw JSON is failing. How would you debug and fix the issue?
- 3Explain why using @UseInterceptors at the controller level versus setting the interceptor as a global provider might affect performance or testability.
5-8 years experience
- 1Our service handles high‑throughput streaming responses. How would you design an interceptor that wraps responses without introducing significant latency or breaking streaming?
- 2Consider a microservices architecture where some services return binary files (e.g., PDFs). How would you adapt a global response‑enveloping interceptor to handle both JSON and binary payloads correctly?
- 3What are the implications of using an interceptor that mutates the response object for NestJS’s built‑in exception filters and logging mechanisms? How would you mitigate any conflicts?
8+ years experience
- 1We are migrating a legacy monolith to a NestJS‑based platform and want to enforce a unified response envelope across all teams. What governance, tooling, and architectural patterns would you put in place to ensure consistency and allow teams to opt‑out when necessary?
- 2Discuss the long‑term maintenance impact of a global response‑enveloping interceptor on version upgrades of NestJS and third‑party libraries. How would you structure the code to minimize breakage?
- 3If multiple cross‑functional teams need different envelope schemas (e.g., one for internal APIs, another for public APIs), how would you design a flexible interceptor strategy that supports per‑module configuration while keeping a single source of truth?
Follow-up Questions
- How would you test that the interceptor correctly wraps both success and error payloads?
- What adjustments would you make if different modules need slightly different envelope structures?
- Can you talk about any performance or memory considerations when applying this globally?