Express.js Basics

Express.js Basics

Express.js is the most popular Node.js web framework. It gives you a clean API for building web servers and APIs without reinventing the wheel.

Setting Up Express

npm install express

const express = require("express");
const app = express();

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

app.listen(3000, () => {
  console.log("Server running on http://localhost:3000");
});

Middleware

Middleware functions have access to the request, response, and next function. They run before route handlers:

// Built-in middleware
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

// Custom middleware
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();  // Pass control to next middleware
});

// Static files
app.use(express.static("public"));

Key Concept: Middleware is the most powerful feature of Express. Authentication, logging, CORS, body parsing — they are all middleware. You can even write your own.

Serving JSON APIs

app.get("/api/users", (req, res) => {
  res.json([
    { id: 1, name: "Alice" },
    { id: 2, name: "Bob" }
  ]);
});

app.post("/api/users", (req, res) => {
  const { name, email } = req.body;
  // Save to database...
  res.status(201).json({ id: 3, name, email });
});