Interceptor Patterns

Cross-cutting concerns handled through NestJS interceptors.

Active Interceptors

Interceptor	Purpose
TransformInterceptor	Standardize response format
ActivityLogInterceptor	Audit trail for mutations
LazyTransactionInterceptor	Transaction management
TimeoutInterceptor	Request timeout handling
CacheInterceptor	Response caching

TransformInterceptor

Wraps all responses in a standard format:

@Injectable()
export class TransformInterceptor<T> implements NestInterceptor<
  T,
  Response<T>
> {
  intercept(
    context: ExecutionContext,
    next: CallHandler,
  ): Observable<Response<T>> {
    return next.handle().pipe(
      map((data) => ({
        data,
        statusCode: context.switchToHttp().getResponse().statusCode,
        timestamp: new Date().toISOString(),
      })),
    );
  }
}

ActivityLogInterceptor

Records entity changes for audit:

@Injectable()
export class ActivityLogInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler) {
    const request = context.switchToHttp().getRequest();
    const before = this.captureState(request);

    return next.handle().pipe(
      tap((result) => {
        this.logActivity({
          action: request.method,
          entity: this.getEntityName(context),
          before,
          after: result,
          userId: request.user?.id,
        });
      }),
    );
  }
}

TimeoutInterceptor

@Injectable()
export class TimeoutInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler) {
    return next.handle().pipe(
      timeout(30000), // 30 second timeout
      catchError((err) => {
        if (err instanceof TimeoutError) {
          throw new RequestTimeoutException();
        }
        throw err;
      }),
    );
  }
}

Applying Interceptors

// Global
app.useGlobalInterceptors(new TransformInterceptor());

// Controller-level
@UseInterceptors(CacheInterceptor)
@Controller('employee')
export class EmployeeController {}

// Method-level
@UseInterceptors(ActivityLogInterceptor)
@Put(':id')
async update() {}

Related Pages

• Guard System — guards
• Request Lifecycle — full request flow
• Audit Logging — audit trail