Express Application Structure and Routing

Required tools

• Node.js (20.x or 22.x LTS)
• npm (10.x (bundled with Node.js))

Setup

1. Create a new folder `learning-api` and run `npm init -y` inside it.
2. Run `npm install express`.
3. Add `"type": "module"` to the generated package.json so `import`/`export` syntax works.
4. Create the files below in the structure shown, then run `node src/server.js`.

Project structure

learning-api/
  src/
    server.js
    routes/
      courses.routes.js
      enrollments.routes.js
  package.json

Starter files

import { Router } from "express";

const router = Router();

const COURSES = [
  { id: 1, title: "HTML & CSS Fundamentals" },
  { id: 2, title: "JavaScript Fundamentals" },
];

router.get("/", (req, res) => {
  res.json(COURSES);
});

router.get("/:id", (req, res) => {
  // TODO: find the course by id (req.params.id) and respond with it,
  // or respond with a 404 if no course matches.
});

export default router;

import { Router } from "express";

const router = Router();
const ENROLLMENTS = [];

router.get("/", (req, res) => {
  res.json(ENROLLMENTS);
});

router.post("/", (req, res) => {
  // TODO: push a new enrollment (req.body) onto ENROLLMENTS and
  // respond with 201 and the created enrollment.
});

export default router;

import express from "express";
import coursesRouter from "./routes/courses.routes.js";
import enrollmentsRouter from "./routes/enrollments.routes.js";

const app = express();
app.use(express.json());

app.use("/courses", coursesRouter);
app.use("/enrollments", enrollmentsRouter);

const PORT = 3001;
app.listen(PORT, () => {
  console.log("Learning API listening on port " + PORT);
});

Requirements

• Course and enrollment routes live in their own separate router modules, not all in server.js
• GET /courses/:id returns the matching course, or a 404 status if no course has that id
• POST /enrollments accepts a JSON body and adds a new enrollment, responding with 201 and the created record
• The server starts without errors and logs the port it's listening on

Commands to run

• Start the server

  node src/server.js

• Test a GET request (in a second terminal)

  curl http://localhost:3001/courses

• Test a POST request (in a second terminal)

  curl -X POST http://localhost:3001/enrollments -H "Content-Type: application/json" -d "{\"courseId\":1}"

Expected behavior

GET /courses returns the two seed courses as JSON. GET /courses/1 returns that specific course; GET /courses/999 returns a 404. POST /enrollments with a JSON body creates and returns a new enrollment with status 201.

Verify it yourself

• curl http://localhost:3001/courses/1

  Expected: A JSON object for the HTML & CSS Fundamentals course

• curl -i http://localhost:3001/courses/999

  Expected: HTTP status 404, since no course has that id

• curl -i -X POST http://localhost:3001/enrollments -H "Content-Type: application/json" -d "{\"courseId\":1}"

  Expected: HTTP status 201 with the newly created enrollment echoed back in the response body

Troubleshooting

• "Cannot use import statement outside a module" error on startup — Confirm "type": "module" is present in package.json — without it, Node expects CommonJS require() syntax instead of import.
• req.body is undefined inside the POST handler — Confirm `app.use(express.json())` is registered in server.js before the routes that need to read a JSON body — without it, Express never parses the incoming body.
• GET /courses/999 returns a generic error page instead of a clean 404 — Make sure the route handler explicitly checks whether a matching course was found and calls res.status(404).json(...) rather than letting an undefined value flow into res.json().

Extension challenge

Add a GET /enrollments/:id route, and a DELETE /enrollments/:id route that removes an enrollment, responding 404 if the id doesn't exist.