Lesson 2 of 5 · 15–25 minutes

Express basics

🏠The everyday picture

A ready-built counter with labelled trays, a stamp and a cash box, instead of building the counter yourself from planks.

📘What it really is

Express is the most widely used Node.js web framework: small, flexible and well documented.

// src/app.ts
import express from "express";

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

app.get("/health", (req, res) => {
  res.json({ status: "ok" });
});

// src/server.ts
import { app } from "./app.ts";
import { config } from "./config.ts";

app.listen(config.port, () => {
  console.log(`API listening on http://localhost:${config.port}`);
});

express.json() reads JSON request bodies into req.body.
res.json(value) sends JSON with the right content type; res.status(201).json(...) sets a status first.
Keeping app (the routes) separate from server (listening on a port) lets tests use the app without starting a real server (Module 46).
Similar frameworks: Fastify (faster, built-in validation), Hono (tiny, runs in many places), NestJS (large, structured). The ideas are the same.

🖱️Try it

Set up app.ts and server.ts as above. Run npm run dev and visit /health. Add a GET / route that returns the API's name and version.

✅Quick check

What does app.use(express.json()) do?

Show the answer

It reads JSON request bodies so they're available as req.body.

Why separate app.ts from server.ts?

Show the answer

Tests can use the app's routes directly without starting a server on a port.