Introduction to Express

Express is the most popular web framework for Node.js. It wraps the built-in http module with a clean, minimal API for routing, middleware, and request/response handling.

Installing Express

npm install express

Your First Express Application

const express = require("express");

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

app.get("/", function (req, res) {
  res.send("Hello from Express!");
});

app.listen(PORT, function () {
  console.log("Server listening on port " + PORT);
});

express() returns an application object. You define routes on it using methods that match HTTP verbs: app.get(), app.post(), app.put(), app.delete(), and so on.

Sending Responses

Express adds convenient helper methods to the response object:

// Send a plain string
res.send("Hello!");

// Send JSON automatically (sets Content-Type header)
res.json({ message: "Success", data: [] });

// Send an HTTP status code and JSON
res.status(404).json({ error: "Not found" });

// Send an HTML file
res.sendFile("/absolute/path/to/index.html");

Reading Request Data

app.use(express.json());        // parse JSON request bodies
app.use(express.urlencoded({ extended: true })); // parse form data

app.post("/users", function (req, res) {
  const { name, email } = req.body;    // parsed body
  const search = req.query.filter;     // ?filter=active
  console.log(req.headers["content-type"]);
  res.status(201).json({ name, email });
});

Route Parameters

app.get("/users/:id", function (req, res) {
  const userId = req.params.id; // e.g. '42' from /users/42
  res.json({ userId });
});

Static Files

Serve an entire folder of static assets (HTML, CSS, images) with one line:

app.use(express.static("public"));

Files in the public/ directory are then accessible at the root URL.

Why Express over Raw http?

Task	Raw http	Express
Route by URL and method	Manual if/else	app.get('/path', handler)
Parse JSON body	Manual stream collection	express.json() middleware
Send JSON response	Manual headers + JSON.stringify	res.json(data)
Serve static files	Manual	express.static('public')

Express dramatically reduces boilerplate and is flexible enough to power anything from a simple REST API to a full-stack web application. The next lessons explore routing and middleware in greater depth.