Controllers and services

📘What it really is

• A controller translates between HTTP and the service:

// src/routes/enquiries.ts
enquiriesRouter.patch("/:id/close", async (req, res) => {
  const input = closeSchema.parse(req.body);           // Lesson 4
  const enquiry = await enquiryService.close(req.params.id, input.note, req.user);
  res.json(enquiry);
});

• A service holds the business rules and knows nothing about HTTP (no req or res):

// src/services/enquiryService.ts
export async function close(id: string, note: string, user: User): Promise<Enquiry> {
  const enquiry = await enquiryRepo.findById(id);
  if (!enquiry) throw new NotFoundError("Enquiry not found");
  if (enquiry.status === "closed") throw new ConflictError("Enquiry is already closed");
  const updated = { ...enquiry, status: "closed" as const, closedBy: user.id, closedNote: note };
  return enquiryRepo.save(updated);
}

• Services throw meaningful errors (Lesson 5), which the error handler turns into status codes.
• Because services are plain functions, they can be tested directly, and reused by other entry points (a scheduled job, a command-line tool).