Express Fundamentals

Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications.

Why Express?

• Minimal and unopinionated: Freedom to structure your application
• Robust routing: Powerful routing mechanism
• Middleware: Extensive middleware ecosystem
• Performance: Built on Node.js for fast I/O operations

Installation

npm install express

Your First Express Server

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

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

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

Request Flow in Express

graph LR
    A[Client Request] --> B[Express App]
    B --> C[Middleware 1]
    C --> D[Middleware 2]
    D --> E[Route Handler]
    E --> F[Response]
    F --> G[Client]

Routing

Basic Routes

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

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

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

// DELETE request
app.delete('/users/:id', (req, res) => {
  res.status(204).send();
});

Route Parameters

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

// Query parameters
app.get('/search', (req, res) => {
  const { q, limit } = req.query;
  res.json({ query: q, limit: limit || 10 });
});

Middleware

Middleware functions have access to request and response objects.

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

// Built-in middleware
app.use(express.json()); // Parse JSON bodies
app.use(express.static('public')); // Serve static files

// Custom middleware
const validateUser = (req, res, next) => {
  if (!req.body.name) {
    return res.status(400).json({ error: 'Name required' });
  }
  next();
};

app.post('/users', validateUser, (req, res) => {
  res.json({ message: 'User created', name: req.body.name });
});

Response Methods

app.get('/examples', (req, res) => {
  // Send JSON
  res.json({ key: 'value' });
  
  // Send text
  res.send('Hello');
  
  // Set status
  res.status(404).json({ error: 'Not found' });
  
  // Redirect
  res.redirect('/new-url');
  
  // Download file
  res.download('/path/to/file.pdf');
});