Introduction to Express.js

// Install Express
// npm install express

const express = require('express');
const app = express();
const PORT = 3000;

// Define routes
app.get('/', (req, res) => {
  res.send('<h1>Welcome to Express!</h1>');
});

app.get('/about', (req, res) => {
  res.send('<h1>About Page</h1>');
});

app.get('/api/users', (req, res) => {
  res.json([
    { id: 1, name: 'Alice' },
    { id: 2, name: 'Bob' }
  ]);
});

// Start the server
app.listen(PORT, () => {
  console.log(`Express server running on http://localhost:${PORT}`);
});

const express = require('express');
const app = express();

app.use(express.json());   // WITHOUT this line, req.body is undefined

// GET /books/42?format=short
app.get('/books/:id', (req, res) => {
  const id = Number(req.params.id);            // params are STRINGS
  if (!Number.isInteger(id)) {
    return res.status(400).json({ error: 'id must be a number' });
  }

  const format = req.query.format || 'full';   // query values are strings too
  const auth = req.headers.authorization;      // header names are lowercased

  return res.status(200).json({ id, format, authenticated: Boolean(auth) });
});

// POST /books  with a JSON body
app.post('/books', (req, res) => {
  const { title, author } = req.body;          // may be missing, or any type
  if (typeof title !== 'string' || typeof author !== 'string') {
    return res.status(400).json({ error: 'title and author are required' });
  }
  return res.status(201).json({ id: 1, title: title.trim(), author: author.trim() });
});

app.listen(process.env.PORT || 3000);

const express = require('express');
const app = express();

// Built-in middleware: Parse JSON request bodies
app.use(express.json());

// Custom middleware: Log every request
app.use((req, res, next) => {
  console.log(`${new Date().toISOString()} ${req.method} ${req.url}`);
  next(); // Pass control to the next middleware
});

// Route handler (also middleware)
app.get('/', (req, res) => {
  res.json({ message: 'Hello World' });
});

// Order is simply the order they are written:
//   1. express.json() parses the body
//   2. the logger logs the request
//   3. the route handler sends the response

// TOO LATE — every route registered above answers without ever seeing this.
// No error is printed. It just never runs for them.
app.use((req, res, next) => {
  console.log('never reached by the routes above');
  next();
});

app.listen(process.env.PORT || 3000);

// src/app.js — builds the application; never listens
const express = require('express');
const bookRoutes = require('./routes/books');

const app = express();

app.use(express.json());
app.use('/api/books', bookRoutes);

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

module.exports = app;      // note: no app.listen() anywhere in this file


// server.js — the only file that starts anything
const app = require('./src/app');
const connectDb = require('./src/db');

const PORT = process.env.PORT || 3000;

async function start() {
  try {
    await connectDb();                  // fail BEFORE accepting traffic
    app.listen(PORT, () => console.log(`Listening on port ${PORT}`));
  } catch (err) {
    console.error('Startup failed:', err.message);
    process.exit(1);                    // crash loudly rather than limp on
  }
}

start();