Express Server Setup

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Express is the most popular Node.js web framework. It provides a minimal, flexible set of features for building web applications and APIs.

Installation

mkdir my-api && cd my-api
npm init -y
npm install express
npm install --save-dev nodemon

Add scripts to package.json:

{
  "scripts": {
    "start": "node src/index.js",
    "dev": "nodemon src/index.js"
  }
}

Hello World Server

const express = require("express");

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

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

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

Start the server:

npm run dev
# Visit http://localhost:3000 in your browser

Request and Response Objects

Request (`req`)

app.get("/example", (req, res) => {
  // URL info
  req.path;       // "/example"
  req.method;     // "GET"
  req.hostname;   // "localhost"
  req.protocol;   // "http"
  req.secure;     // false (true for HTTPS)

  // Query parameters — /search?q=hello&page=2
  req.query.q;    // "hello"
  req.query.page; // "2"

  // Route parameters — /users/:id
  req.params.id;  // from route

  // Headers
  req.headers["content-type"];
  req.get("Authorization");

  // Body (needs middleware)
  req.body;       // populated by express.json() middleware

  // Cookies
  req.cookies;    // needs cookie-parser middleware
});

Response (`res`)

app.get("/response-examples", (req, res) => {
  // Send various types
  res.send("Hello");                    // HTML/string
  res.json({ message: "Hello" });      // JSON
  res.json({ user: { id: 1, name: "Alice" } }); // JSON object
  res.status(201).json({ id: 1 });      // with status code
  res.redirect("/other-page");          // redirect
  res.sendFile("/path/to/file.pdf");    // send a file
  res.download("/path/to/file.pdf");    // download prompt
  res.end();                            // empty response

  // Set headers
  res.set("X-Custom-Header", "value");
  res.setHeader("Content-Type", "application/json");
});

Environment Variables with dotenv

npm install dotenv

PORT=4000
NODE_ENV=development
DATABASE_URL=mongodb://localhost:27017/myapp
API_KEY=sk-abc123

require("dotenv").config();

console.log(process.env.PORT);       // "4000"
console.log(process.env.NODE_ENV);   // "development"

Caution

Never commit .env files to version control. Add .env to .gitignore. Provide a .env.example file with placeholder values for other developers.

Project Structure

my-api/
├── src/
│   ├── index.js          # Entry point — server setup
│   ├── app.js            # Express app configuration
│   ├── routes/           # Route handlers
│   │   ├── users.js
│   │   └── products.js
│   ├── controllers/      # Business logic
│   │   ├── userController.js
│   │   └── productController.js
│   ├── middleware/        # Custom middleware
│   │   ├── auth.js
│   │   └── errorHandler.js
│   ├── models/           # Data models
│   │   └── User.js
│   └── utils/            # Helper functions
│       └── logger.js
├── tests/                # Test files
├── .env                  # Environment variables
├── .gitignore
└── package.json

App Configuration Pattern

const express = require("express");
require("dotenv").config();

const app = express();

// Middleware
app.use(express.json());                    // Parse JSON bodies
app.use(express.urlencoded({ extended: true })); // Parse form data
app.use(express.static("public"));          // Serve static files

// Routes
app.use("/api/users", require("./routes/users"));
app.use("/api/products", require("./routes/products"));

// Health check
app.get("/health", (req, res) => {
  res.json({ status: "ok", timestamp: new Date().toISOString() });
});

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

// Error handler
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: "Internal server error" });
});

module.exports = app;

const app = require("./app");

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

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

Organising Routes

const express = require("express");
const router = express.Router();

// GET /api/users
router.get("/", (req, res) => {
  res.json([{ id: 1, name: "Alice" }]);
});

// GET /api/users/:id
router.get("/:id", (req, res) => {
  res.json({ id: req.params.id, name: "Alice" });
});

// POST /api/users
router.post("/", (req, res) => {
  const { name, email } = req.body;
  // Create user logic
  res.status(201).json({ id: 2, name, email });
});

module.exports = router;

Running with Nodemon

Nodemon automatically restarts the server when file changes are detected:

npm run dev
# [nodemon] starting `node src/index.js`
# Server running on port 3000
# [nodemon] restarting due to changes...

Quick Reference

// Basic server
app.get("/path", (req, res) => res.send("ok"));
app.listen(PORT, () => console.log(`Running on ${PORT}`));

// Common middleware
app.use(express.json());
app.use(express.static("public"));

// Router pattern
const router = express.Router();
router.get("/", handler);
app.use("/api", router);

Practice Exercises

Basic Express server: Create a server with routes for /, /about, and /contact. Each should return a simple HTML string. Use nodemon for development.
Environment configuration: Set up a project with dotenv. Use environment variables for PORT, a custom APP_NAME, and an ADMIN_EMAIL. Log them on startup. Create a .env.example file.
Structured project: Create a project with the structure shown above. Move route handlers to separate files. Add a logger.js utility that logs requests with timestamps.