text 20 min

MVC in Express

Why It Matters

Express lets you put everything in one file, but production APIs need boundaries. MVC is a simple mental model: routes map HTTP to controllers, controllers coordinate use cases, models or repositories handle data, and views render HTML when needed.

Core Concepts

  • Routes should describe URL shape and middleware, not contain all business logic.
  • Controllers translate HTTP input into service calls and translate results into HTTP responses.
  • Services hold business rules that should be testable without Express.
  • Repositories or models isolate database details.
  • Views are optional in API-only apps but useful for server-rendered pages.

Flow to Remember

Request data is parsed by middleware, route parameters reach a controller, the controller calls a service, the service uses a repository, and the controller sends a response.

Syntax and Examples

js
// routes/users.routes.js
import { Router } from 'express';
import { createUser } from '../controllers/users.controller.js';

export const usersRouter = Router();
usersRouter.post('/users', createUser);

// controllers/users.controller.js
import { registerUser } from '../services/users.service.js';

export async function createUser(req, res, next) {
  try {
    const user = await registerUser(req.body);
    res.status(201).json({ data: user });
  } catch (error) {
    next(error);
  }
}

Use Cases and Tradeoffs

  • Use MVC-style separation when route handlers start mixing validation, business rules, SQL, and response formatting.
  • Keep pure business decisions in services so unit tests do not need HTTP setup.
  • Use controllers as thin adapters; they are allowed to know Express exists.
  • For small apps, use fewer folders, but keep the same separation of responsibilities.

Common Mistakes

  • Creating huge controller classes that become a second application layer.
  • Letting services depend on req and res, which makes them hard to reuse.
  • Adding abstractions before the app has more than one real use case.
  • Returning database rows directly when public API fields need a stable contract.

Practical Challenge

Refactor a single-file /orders API into routes, controllers, and services. Unit test the service without importing Express.

Recap

  • MVC is a boundary pattern, not a folder-count requirement.
  • Controllers adapt HTTP to application logic.
  • Services become easier to test when they do not depend on Express.
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