First Express 5 Server: Routes, Requests, and Responses

Express is the most widely used web framework in the Node.js ecosystem. It builds on the http module and adds readable routing, middleware, and handy helpers like res.json. Version 5 is the current version: it is the one npm install express installs.

A minimal server

// src/server.js (ES modules project: "type": "module" in package.json)
import express from "express";

const app = express();
app.use(express.json());           // parses JSON bodies -> req.body

app.get("/health", (req, res) => {
  res.json({ status: "ok" });      // status 200 by default, JSON Content-Type
});

const port = Number(process.env.PORT ?? 3000);
app.listen(port, () => console.log(`API at http://localhost:${port}`));

Run it with node --watch src/server.js and test with curl http://localhost:3000/health.

Reading the request

const notes = [{ id: 1, titre: "Groceries", fait: false }];

// Path parameter: /notes/42 -> req.params.id === "42" (a STRING)
app.get("/notes/:id", (req, res) => {
  const id = Number(req.params.id);
  const note = notes.find((n) => n.id === id);
  if (!note) return res.status(404).json({ error: "Note not found" });
  res.json(note);
});

// Query string: /notes?fait=true -> req.query.fait === "true"
app.get("/notes", (req, res) => {
  const { fait } = req.query;
  const resultat = fait === undefined ? notes : notes.filter((n) => String(n.fait) === fait);
  res.json(resultat);
});

// JSON body: requires app.use(express.json())
app.post("/notes", (req, res) => {
  const note = { id: notes.length + 1, titre: req.body.titre, fait: false };
  notes.push(note);
  res.status(201).location(`/notes/${note.id}`).json(note);
});

Three sources of data, three objects: req.params, req.query, req.body. All of them come from the client: never trustworthy, always to be validated (lesson 3).

Essential status codes

Code	Usage
200 OK	Successful read or update
201 Created	Resource created (with the Location header)
204 No Content	Successful deletion, no body (res.status(204).end())
400 Bad Request	Invalid data
401 / 403	Not authenticated / not authorized
404 Not Found	Resource does not exist
500	Server error

Each request must receive a single response. The return in front of res.status(404)... avoids sending a second one (the Cannot set headers after they are sent error).

Grouping with express.Router

// src/routes/notes.js
import { Router } from "express";
export const notesRouter = Router();
notesRouter.get("/", listerNotes);
notesRouter.get("/:id", lireNote);
notesRouter.post("/", creerNote);

// src/server.js
app.use("/api/notes", notesRouter);   // GET /api/notes/3 -> lireNote

What changes with Express 5

• Errors from async handlers are automatically passed to the error handler (lesson 4).
• Paths are stricter: a wildcard must be named (/fichiers/*chemin), and an optional segment is written /livres{/:format}.
• Without express.json(), req.body is undefined (and no longer {}).
• req.query is read-only; app.del() and res.sendfile() are gone.

Key takeaways

A route = method + path + handler. Read params, query, and body while converting them, respond only once with the right status, and organize resources with Router.