Lesson 06

Interceptors for response and logging work

Use interceptors for cross-cutting response mapping, timing, and logging around controller method execution.

interceptors
logging
responses

Good Code

request-timing.interceptor.ts

import {
  CallHandler,
  ExecutionContext,
  Injectable,
  NestInterceptor,
} from "@nestjs/common";
import { tap } from "rxjs";

@Injectable()
export class RequestTimingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler) {
    const startedAt = Date.now();

    // The interceptor measures every decorated route the same way.
    return next.handle().pipe(
      tap(() => {
        const request = context.switchToHttp().getRequest();
        logger.info({ path: request.url, durationMs: Date.now() - startedAt });
      }),
    );
  }
}

Bad Code

reviews.controller.ts

import { Controller, Get } from "@nestjs/common";

@Controller("reviews")
export class ReviewsController {
  @Get()
  async findAll() {
    // Repeating timing code in each route invites drift.
    const startedAt = Date.now();
    const result = await this.reviews.findAll();
    logger.info({ durationMs: Date.now() - startedAt });
    return { data: result };
  }
}

Review Notes

What to review

Good Code

The good version puts request timing in an interceptor that wraps the controller call. The controller can focus on returning reviews.

Bad Code

The bad version repeats timing and response wrapping inside a route. Other routes can drift in field names, timing behavior, or log context.

Takeaways

Interceptors should wrap route execution without forcing every controller method to repeat the same timing or response code.