Express.js Framework

What is Express.js?

Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It’s the most popular Node.js framework.

Basic Setup

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

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Routing

// GET request
app.get('/users', (req, res) => {
  res.json({ users: [] });
});

// POST request
app.post('/users', (req, res) => {
  res.status(201).json({ message: 'User created' });
});

// PUT request
app.put('/users/:id', (req, res) => {
  res.json({ message: `User ${req.params.id} updated` });
});

// DELETE request
app.delete('/users/:id', (req, res) => {
  res.json({ message: `User ${req.params.id} deleted` });
});

// Route parameters
app.get('/users/:id', (req, res) => {
  const userId = req.params.id;
  res.json({ userId });
});

// Query parameters
app.get('/search', (req, res) => {
  const { q, page } = req.query;
  res.json({ query: q, page });
});

Middleware

// Application-level middleware
app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next();
});

// Built-in middleware
app.use(express.json()); // Parse JSON bodies
app.use(express.urlencoded({ extended: true })); // Parse URL-encoded bodies
app.use(express.static('public')); // Serve static files

// Third-party middleware
const cors = require('cors');
app.use(cors());

const morgan = require('morgan');
app.use(morgan('dev'));

Request and Response

app.get('/api/data', (req, res) => {
  // Request properties
  console.log(req.method); // GET
  console.log(req.url); // /api/data
  console.log(req.headers);
  console.log(req.body);
  console.log(req.params);
  console.log(req.query);
  
  // Response methods
  res.send('Text response');
  res.json({ data: 'value' });
  res.status(404).send('Not found');
  res.redirect('/other-page');
  res.download('file.pdf');
  res.sendFile('/path/to/file.html');
});

Router

// users.routes.js
const express = require('express');
const router = express.Router();

router.get('/', (req, res) => {
  res.json({ users: [] });
});

router.post('/', (req, res) => {
  res.status(201).json({ message: 'Created' });
});

module.exports = router;

// app.js
const userRoutes = require('./routes/users.routes');
app.use('/api/users', userRoutes);

Error Handling

// Error handling middleware
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: err.message });
});

// Async error handling
app.get('/async', async (req, res, next) => {
  try {
    const data = await fetchData();
    res.json(data);
  } catch (err) {
    next(err);
  }
});

Template Engines

// EJS
app.set('view engine', 'ejs');
app.get('/', (req, res) => {
  res.render('index', { title: 'Home' });
});

// Pug
app.set('view engine', 'pug');
app.get('/', (req, res) => {
  res.render('index', { title: 'Home' });
});

Complete Example

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

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

// Routes
app.get('/', (req, res) => {
  res.send('Home Page');
});

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

app.post('/api/users', (req, res) => {
  const user = req.body;
  res.status(201).json(user);
});

// Error handling
app.use((err, req, res, next) => {
  res.status(500).json({ error: err.message });
});

// Start server
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

Interview Tips

• Explain Express purpose: Web framework for Node.js
• Show routing: Different HTTP methods
• Demonstrate middleware: Request/response processing
• Discuss error handling: Error middleware
• Mention features: Routing, middleware, templating

Summary

Express.js is a minimal Node.js web framework providing routing, middleware, and HTTP utilities. Use app.get/post/put/delete for routes, app.use for middleware, and error handling middleware for errors. Popular for building REST APIs and web applications.