How to separate concerns across layers — Intermediate

Misc / Other Coding & architecture practices

Routers wire URLs, controllers handle HTTP, services own data—avoid SQL or business rules in route files.

Learn: Layers exist so each file has one reason to change. If SQL appears in a router, move it to a service. If HTTP status logic sprawls across services, return errors and map them in the controller.

Supplementary examples

Controller maps service error to 404

try {
  const row = await service.read(id);
  res.json({ data: row });
} catch (err) {
  next(err); // errorHandler reads err.status
}

Course example

// Good: router only connects URL to controller
router.route("/").get(controller.list).post(controller.create);

// Avoid: database logic inline in router
// router.get("/", async (req, res) => {
//   const rows = await knex("users").select();  // move to service
// });

Additional references & examples

Thinkful — Organizing Express (pastes example)
Router/controller structure demonstration project.
Express.js guide
Routing, middleware order, and request/response lifecycle.