Lesson 02

Router boundaries

Group related routes behind routers with explicit dependencies.

routing
routers
boundaries

Good Code

src/reviews/router.ts

import { Router } from "express";
import type { ReviewsService } from "./service";

export function createReviewsRouter(service: ReviewsService) {
  const router = Router();

  router.get("/", async (_req, res) => {
    const reviews = await service.listOpenReviews();
    res.json({ data: reviews });
  });

  router.post("/", async (req, res) => {
    const review = await service.createReview(req.body);
    res.status(201).json({ data: review });
  });

  return router;
}

Bad Code

src/app.ts

import express from "express";
import { reviewService, userService, billingService } from "./services";

const app = express();

app.get("/reviews", async (_req, res) => {
  res.json(await reviewService.listOpenReviews());
});
app.post("/reviews", async (req, res) => {
  res.json(await reviewService.createReview(req.body));
});
app.get("/users", async (_req, res) => {
  res.json(await userService.listUsers());
});
app.post("/billing/retry", async (req, res) => {
  res.json(await billingService.retryCharge(req.body.invoiceId));
});

Review Notes

What to review

Good Code

The good version keeps review routes in a focused router and receives the service dependency explicitly.

Bad Code

The bad version piles unrelated resources into the app entry point and reaches into shared service singletons.

Takeaways

Routers should own one resource area and receive services instead of reaching into globals.