What is Express.js?

Introduction to Express.js

Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for building web and mobile applications. It's the de facto standard server framework for Node.js.

Learning Objectives

Understand what Express.js is and its role in backend development
Learn about the middleware architecture
Understand routing and HTTP methods
Recognize when Express is the right choice for your project

Why Express.js?

Minimal & Fast

Unopinionated framework with minimal overhead for maximum performance

Flexible

Use any database, template engine, or architecture pattern you prefer

Battle-Tested

Powers millions of applications with extensive middleware ecosystem

Express.js Core Concepts

Your First Express Server

Here's how simple it is to create a web server with Express:

javascript
const express = require("express")
const app = express()
const PORT = 3000

// Define a route
app.get("/", (req, res) => {
  res.send("Hello World!")
})

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

Key Insight

Express abstracts the complexity of Node.js's built-in HTTP module, making it incredibly easy to build web servers and APIs.

HTTP Methods and Routing

Express provides methods for all HTTP verbs:

GET - Retrieve Data

javascript
// Get all users
app.get('/api/users', (req, res) => {
  res.json(users);
});

// Get user by ID
app.get('/api/users/:id', (req, res) => {
  const user = users.find(u => u.id === req.params.id);
  if (!user) return res.status(404).json({ error: 'Not found' });
  res.json(user);
});

Middleware Architecture

Middleware functions are the backbone of Express. They have access to the request, response, and the next middleware in the chain.

javascript
// Custom logging middleware
const logger = (req, res, next) => {
  console.log(`${req.method} ${req.url} - ${new Date().toISOString()}`)
  next() // Pass control to next middleware
}

// Apply middleware
app.use(logger)

// 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

Middleware Flow

Request→Middleware 1→Middleware 2→Route Handler→Response

Express vs Other Frameworks

Feature	Express	Fastify	NestJS
Learning Curve	Easy	Easy	Moderate
Performance	Good	Excellent	Good
TypeScript	@types/express	Built-in	Native
Architecture	Minimal	Minimal	Opinionated
Ecosystem	Massive	Growing	Strong

When to Use Express

Quick Start Example

Here's a complete Express API example:

javascript
const express = require("express")
const cors = require("cors")
const helmet = require("helmet")

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

// Middleware
app.use(helmet()) // Security headers
app.use(cors()) // Enable CORS
app.use(express.json()) // Parse JSON

// In-memory data store
let todos = [
  { id: 1, title: "Learn Express", completed: false },
  { id: 2, title: "Build an API", completed: false },
]

// Routes
app.get("/api/todos", (req, res) => {
  res.json(todos)
})

app.get("/api/todos/:id", (req, res) => {
  const todo = todos.find((t) => t.id === parseInt(req.params.id))
  if (!todo) return res.status(404).json({ error: "Todo not found" })
  res.json(todo)
})

app.post("/api/todos", (req, res) => {
  const todo = {
    id: todos.length + 1,
    title: req.body.title,
    completed: false,
  }
  todos.push(todo)
  res.status(201).json(todo)
})

app.patch("/api/todos/:id", (req, res) => {
  const todo = todos.find((t) => t.id === parseInt(req.params.id))
  if (!todo) return res.status(404).json({ error: "Todo not found" })

  if (req.body.title) todo.title = req.body.title
  if (req.body.completed !== undefined) todo.completed = req.body.completed

  res.json(todo)
})

app.delete("/api/todos/:id", (req, res) => {
  const index = todos.findIndex((t) => t.id === parseInt(req.params.id))
  if (index === -1) return res.status(404).json({ error: "Todo not found" })

  todos.splice(index, 1)
  res.status(204).send()
})

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

// Start server
app.listen(PORT, () => {
  console.log(`🚀 Server running at http://localhost:${PORT}`)
})

Next Steps

Continue Learning

Now that you understand what Express.js is, let's set up your development environment and build your first Express server!

Next Lesson:Setting Up Express Development Environment