Day 5: Express.js Framework - Node.js Tutorial

Express.js is a fast, unopinionated, minimalist web framework for Node.js. It provides a robust set of features for web and mobile applications.

What is Express.js?
Express is a web application framework that provides a thin layer of fundamental web application features. It simplifies the process of building web applications and APIs.

Key Features:
• Minimal and flexible web application framework
• Robust routing system
• Middleware support for request processing
• Template engine integration
• Static file serving
• Error handling
• HTTP helpers for redirections and caching

Installation and Setup:
Express.js is not a built-in module, so you need to install it using npm. Create a package.json file and install Express as a dependency.

Basic Express Application:
Creating an Express app involves requiring the express module, creating an app instance, defining routes, and starting the server to listen for requests.

Middleware:
Middleware functions are functions that have access to the request object, response object, and the next middleware function. They can execute code, make changes to request/response objects, and end the request-response cycle.

Routing:
Express provides a robust routing system that allows you to define routes based on HTTP methods and URL patterns. You can handle different routes and parameters easily.

Error Handling:
Express has built-in error handling mechanisms. You can define error-handling middleware to catch and handle errors gracefully.

Interactive Examples

Basic Express Application

Creating a simple Express.js application

JavaScript

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

// Middleware to parse JSON
app.use(express.json());

// Basic route
app.get('/', (req, res) => {
  res.send('<h1>Welcome to Express.js!</h1><p>This is a basic Express application.</p>');
});

// Route with parameters
app.get('/user/:id', (req, res) => {
  const userId = req.params.id;
  res.json({
    message: 'Get user with ID: ' + id + ',',
    userId: userId
  });
});

// POST route
app.post('/api/users', (req, res) => {
  const userData = req.body;
  res.status(201).json({
    message: 'User created successfully',
    user: userData
  });
});

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

// 404 handler
app.use((req, res) => {
  res.status(404).json({ error: 'Route not found' });
});

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

Output

Server running on port 3000

Explanation

This example shows a basic Express.js application with routing, middleware, error handling, and JSON parsing capabilities.