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6 of 6
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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06 / 06

How would you implement a request-scoped audit log interceptor that captures both the request and response body in NestJS?

Use Scope.REQUEST so a new interceptor instance is created per request — safe for storing request-level state. Inject the REQUEST token to access the request object. Use tap() to log the response body after the handler completes and catchError() to log failures before re-throwing.

Request-scoped audit interceptor
Design decisions for this pattern:
  1. 1

    Scope.REQUEST ensures a fresh instance per request — no state leaks between requests.

  2. 2

    @Inject(REQUEST) gives direct access to the request object including headers, body, and user.

  3. 3

    tap() logs on success without changing the response value.

  4. 4

    catchError() logs the error but re-throws — exception filters still format the error response.

  5. 5

    Register via APP_INTERCEPTOR with useClass so NestJS manages the REQUEST scope correctly.