Questions
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1What is an interceptor in NestJS and what can it do that guards and middleware cannot?
2What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?
3How do you use an interceptor to transform every response into a standard envelope format in NestJS?
4How do you implement a caching interceptor that short-circuits the handler in NestJS?
5How do interceptors handle errors vs exception filters in NestJS — what is the division of responsibility?
6How would you implement a request-scoped audit log interceptor that captures both the request and response body in NestJS?
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What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?

tap() is a side-effect operator that does not change the emitted value — used for logging and auditing. map() transforms the emitted value — used to wrap responses or rename fields. catchError() catches errors in the stream — used to remap domain exceptions into HTTP exceptions or provide fallback values.

All three operators in one interceptor
When to use each operator:
  1. 1

    tap(fn) — logging, metrics, auditing. The stream value passes through unchanged.

  2. 2

    map(fn) — response wrapping, field renaming, stripping sensitive properties.

  3. 3

    catchError(fn) — remapping lower-level errors (TypeORM, Mongoose) into NestJS HTTP exceptions.

  4. 4

    Operators execute in the order they are listed in pipe().

  5. 5

    catchError must return an Observable — use throwError(() => new Error()) to re-throw.