Questions
4 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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How do you implement a caching interceptor that short-circuits the handler in NestJS?

Return a cached observable before calling next.handle() to bypass the controller handler entirely. Calling next.handle() is what actually invokes the handler — returning an observable without calling it means the handler is skipped. Store the result with tap() after the first real invocation.

Short-circuiting cache interceptor
Key concepts behind short-circuiting:
  1. 1

    next.handle() is a lazy Observable — calling it is what invokes the controller handler.

  2. 2

    Returning of(cachedValue) before calling next.handle() skips the handler entirely.

  3. 3

    tap() is used to store the result after the first real handler execution.

  4. 4

    This is the exact mechanism behind NestJS @nestjs/cache-manager CacheInterceptor.

  5. 5

    For production caching use @nestjs/cache-manager with TTL and Redis backend.