What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?

Q: What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?

@Injectable() export class FullPipelineInterceptor implements NestInterceptor { intercept(ctx: ExecutionContext, next: CallHandler): Observable { return next.handle().pipe( tap(val => this.audit.log(val)), // side effect — value unchanged map(val => ({ success: true, data: val })), // transforms the value catchError(err => { if (err instanceof EntityNotFoundError) throw new NotFoundException(err.message); // remap domain error throw err; // re-throw unknown errors }), ); } } tap(fn) — logging, metrics, auditing. The stream value passes through unchanged. map(fn) — response wrapping, field renaming, stripping sensitive properties. catchError(fn) — remapping lower-level errors (TypeORM, Mongoose) into NestJS HTTP exceptions. Operators execute in the order they are listed in pipe(). catchError must return an Observable — use throwError(() => new Error()) to re-throw.

Difficulty: 6/10

RxJS operators, NestJS interceptors, error handling

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
tap(fn) — logging, metrics, auditing. The stream value passes through unchanged.
2
map(fn) — response wrapping, field renaming, stripping sensitive properties.
3

What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?

Difficulty: 6/10

RxJS operators, NestJS interceptors, error handling

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
tap(fn) — logging, metrics, auditing. The stream value passes through unchanged.
2
map(fn) — response wrapping, field renaming, stripping sensitive properties.
3

What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?

What is the difference between tap(), map(), and catchError() in an interceptor's RxJS pipeline?

Scenario Questions

Follow-up Questions