Introduction to Express.js

What is 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, powering millions of applications worldwide.

Installation

# Create a new project
mkdir my-express-app
cd my-express-app

# Initialize package.json
npm init -y

# Install Express
npm install express

# For TypeScript projects
npm install express
npm install -D typescript @types/express @types/node ts-node nodemon

# Initialize TypeScript
npx tsc --init

Your First Express Server

// app.js (JavaScript)
const express = require('express');
const app = express();
const PORT = 3000;

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

// JSON response
app.get('/api/status', (req, res) => {
  res.json({ status: 'running', timestamp: new Date() });
});

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

// Run with: node app.js

Express with TypeScript

// src/app.ts
import express, { Request, Response } from 'express';

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

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

// Type-safe route
app.get('/', (req: Request, res: Response) => {
  res.json({ message: 'Hello from TypeScript Express!' });
});

// Route with typed parameters
interface UserParams {
  id: string;
}

app.get('/users/:id', (req: Request<UserParams>, res: Response) => {
  const { id } = req.params;
  res.json({ userId: id });
});

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

// tsconfig.json
{
  "compilerOptions": {
    "target": "ES2020",
    "module": "commonjs",
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true
  }
}

// package.json scripts
{
  "scripts": {
    "dev": "nodemon --exec ts-node src/app.ts",
    "build": "tsc",
    "start": "node dist/app.js"
  }
}

Project Structure

# Recommended Express.js project structure
my-express-app/
├── src/
│   ├── app.ts              # Express app setup
│   ├── server.ts           # Server entry point
│   ├── routes/
│   │   ├── index.ts        # Route aggregator
│   │   ├── users.ts        # User routes
│   │   └── products.ts     # Product routes
│   ├── controllers/
│   │   ├── userController.ts
│   │   └── productController.ts
│   ├── middleware/
│   │   ├── auth.ts         # Authentication
│   │   ├── errorHandler.ts # Error handling
│   │   └── validation.ts   # Request validation
│   ├── models/
│   │   └── User.ts         # Data models
│   ├── services/
│   │   └── userService.ts  # Business logic
│   └── utils/
│       └── logger.ts       # Utility functions
├── tests/
├── package.json
├── tsconfig.json
└── .env

Basic App Setup Pattern

// src/app.ts - Separate app from server
import express from 'express';
import cors from 'cors';
import helmet from 'helmet';
import morgan from 'morgan';
import { userRoutes } from './routes/users';
import { errorHandler } from './middleware/errorHandler';

const app = express();

// Security middleware
app.use(helmet());
app.use(cors());

// Logging
app.use(morgan('dev'));

// Body parsing
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

// Routes
app.use('/api/users', userRoutes);

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

// Error handling (must be last)
app.use(errorHandler);

export default app;

// src/server.ts - Entry point
import app from './app';

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

app.listen(PORT, () => {
  console.log(`🚀 Server running on port ${PORT}`);
});

Essential Dependencies

# Core dependencies
npm install express cors helmet morgan dotenv

# TypeScript dependencies
npm install -D typescript @types/express @types/cors @types/morgan @types/node

# Development tools
npm install -D nodemon ts-node

# Common additions
npm install express-validator   # Request validation
npm install express-rate-limit  # Rate limiting
npm install compression         # Gzip compression

# package.json
{
  "dependencies": {
    "cors": "^2.8.5",
    "dotenv": "^16.0.0",
    "express": "^4.18.2",
    "helmet": "^7.0.0",
    "morgan": "^1.10.0"
  }
}