Getting Started with Express

Writing routing ladders and body parsers by hand gets old fast. The Node.js for Beginners series now adds Express — the minimal framework that turns the raw http module into a readable application. This chapter covers installing it, structuring a project, sending responses, and the two pieces of built-in middleware you will use in every app.

What Express actually is

Express sits on top of node:http. It does not replace the request and response objects so much as decorate them: you still write the same handler functions, but path matching, body parsing, headers and static files come out of the box. Its entire vocabulary is small — app, route, middleware, response helpers — and the project for the rest of this series is built on it. Express 5 is the current major version; the examples here work on both 4 and 5, which is why no deprecated syntax appears.

Install and hello

mkdir todo-api && cd todo-api
npm init -y
npm pkg set type=module
npm install express

// src/server.js
import express from 'express';

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

app.get('/', (req, res) => {
  res.json({ message: 'Welcome to the Todo API', docs: '/api/todos' });
});

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

node src/server.js
curl -s http://localhost:3000/

Expected output:

Todo API listening on http://localhost:3000
{"message":"Welcome to the Todo API","docs":"/api/todos"}

In a CommonJS project the first line becomes const express = require('express'); and everything else stays identical — the API is the same in both module systems. Note also process.env.PORT ?? 3000 from chapter 8: platforms assign the port, your laptop defaults it.

Project shape

Keeping listening code separate from route code is the single most valuable structural habit: tests can import the app without opening a port.

Ways to send a response

app.get('/api/status', (req, res) => {
  res.status(200).json({ ok: true });          // JSON with a chosen status
});

app.post('/api/things', (req, res) => {
  res.status(201).location('/api/things/1').end();  // created, no body
});

app.get('/legacy', (req, res) => {
  res.redirect('/api/status');                 // 302 to somewhere sane
});

Expected output: {"ok":true} at /api/status, an empty 201 with a Location header from the POST route, and a redirect from /legacy. Express also offers res.send(stringOrBuffer), which picks the content type for you — handy for HTML pages, while res.json is the default for APIs.

Two built-ins you will never skip

• express.json() — parses JSON request bodies into req.body. Without it, req.body is undefined — the number-one Express bug for newcomers.
• express.static('public') — serves files from a folder: HTML, CSS, images, favicons. Chapter 17 uses it to host the to-do frontend.

app.use(express.json());
app.use(express.static('public'));

Both are middleware in the plainest sense: they run for every request, do one job, and pass control on. Chapter 14 gives the full picture.

Development convenience

Restarting by hand is a tax. Node's built-in watcher covers it with zero dependencies:

"scripts": {
  "start": "node src/server.js",
  "dev": "node --watch src/server.js"
}

Run npm run dev and every save restarts the process. A classic alternative is nodemon (npm install --save-dev nodemon), which adds a few niceties such as ignoring data folders — either is fine, but keep the tool in dev dependencies.

Common mistakes

• Missing express.json() — req.body stays undefined and validation fails mysteriously.
• Calling app.listen at module load — importing the app in a test also binds the port; guard or split the files.
• Registering routes after the catch-all — order decides which handler wins (chapter 13).
• Hard-coding port 3000 in production — read process.env.PORT or the platform's bind fails.
• Using res.send and res.json interchangeably — a raw string with a JSON shape but no content type confuses clients.

Practice checklist

1. Scaffold the project exactly as shown and confirm the welcome JSON with curl.
2. Add GET /api/health returning { status: 'up' }.
3. Switch the entry file to CommonJS and prove the app still runs.
4. Add the dev script and verify a save triggers a restart.
5. Delete express.json(), POST some JSON, and observe undefined — then restore it.

One route is a demo; an application needs a real routing scheme. Next: Routes and Route Parameters.