What is an interceptor in NestJS and what can it do that guards and middleware cannot?

Q: What is an interceptor in NestJS and what can it do that guards and middleware cannot?

@Injectable() export class LoggingInterceptor implements NestInterceptor { intercept(ctx: ExecutionContext, next: CallHandler): Observable { const start = Date.now(); const req = ctx.switchToHttp().getRequest(); console.log(`--> ${req.method} ${req.url}`); return next.handle().pipe( tap(() => console.log(`<-- ${req.method} ${req.url} ${Date.now() - start}ms`)), ); } } Transform the response — wrap in an envelope, rename fields, strip sensitive data. Short-circuit the handler — return a cached observable before calling next.handle(). Measure execution time — spans from before next.handle() to after the stream completes. Remap exceptions — use catchError() to convert domain errors into HTTP errors. Add retry logic — use retryWhen() to automatically retry failed handler calls. Override the return value — next.handle() returns an Observable you control fully.

Difficulty: 6/10

request/response transformation, logging, exception handling

An interceptor implements NestInterceptor and wraps handler execution using RxJS Observable. It runs both before and after the controller handler. Its unique capabilities include transforming the response, measuring execution time, caching (short-circuiting before the handler), overriding exceptions, and adding pre/post logic around the full handler call.

Logging interceptor measuring handler duration

Unique capabilities of interceptors:

1
Transform the response — wrap in an envelope, rename fields, strip sensitive data.
2
Short-circuit the handler — return a cached observable before calling next.handle().
3
Measure execution time — spans from before next.handle() to after the stream completes.
4
Remap exceptions — use catchError() to convert domain errors into HTTP errors.
5
Add retry logic — use retryWhen() to automatically retry failed handler calls.
6
Override the return value — next.handle() returns an Observable you control fully.

Scenario Questions

0-2 years experience

2-5 years experience

1You notice that a request to the payment service sometimes returns a 500 error after the controller has already sent a response. How would you use an interceptor to capture and modify the error before it reaches the client?

Follow-up Questions