Express.js Fundamentals

Express.js Fundamentals

Express is the de‑facto minimal web framework for Node.js. It helps you spin up HTTP servers quickly while keeping the core API tiny.

1️⃣ Install & scaffold

mkdir my‑api && cd my‑api
npm init -y
npm install express

Create index.js:

const express = require('express');
const app = express();
const PORT = process.env.PORT || 3000;

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

Run with node index.js.

2️⃣ Routing

app.get('/', (req, res) => {
  res.send('Welcome to the homepage!');
});

app.get('/api/users/:id', (req, res) => {
  const user = { id: req.params.id, name: 'Alice' };
  res.json(user);
});

Supported verbs: app.get, app.post, app.put, app.delete, etc.

3️⃣ Middleware

// built‑in JSON parser
app.use(express.json());

// simple logger middleware
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

Error‑handling middleware has four arguments:

app.use((err, req, res, next) => {
  console.error(err);
  res.status(500).json({ error: 'Server error' });
});

4️⃣ Static files

app.use('/static', express.static('public'));

All files in the public folder are served under /static.

5️⃣ Simple API example

let todos = [{ id:1, title:'Learn Express', done:false }];
app.get('/api/todos', (req, res) => res.json(todos));
app.post('/api/todos', (req, res) => {
  const newTodo = { id: Date.now(), ...req.body };
  todos.push(newTodo);
  res.status(201).json(newTodo);
});

Express is intentionally unopinionated – you can add templating engines, ORMs, authentication, etc., via a rich ecosystem of middleware.