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Introduction

Express.js is a minimal and flexible Node.js web application framework that provides robust features for building web and mobile applications. It simplifies the process of handling routes, requests, responses, and middleware in Node.js.

1. Why Use Express.js?

  • Simplifies building server-side applications with Node.js
  • Provides routing, middleware, and templating support
  • Lightweight and fast
  • Widely used in production apps and APIs

2. Installation

First, initialize a Node.js project:

mkdir express-app
cd express-app
npm init -y
````

Install Express:

```bash
npm install express

3. Basic Express Server

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

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

app.listen(PORT, () => {
  console.log(`Server running on http://localhost:${PORT}`);
});
  • express() creates an Express app
  • app.get(path, callback) handles GET requests
  • res.send() sends a response
  • app.listen() starts the server

4. Handling Different Routes

app.get("/about", (req, res) => {
  res.send("About Page");
});

app.post("/submit", (req, res) => {
  res.send("Form Submitted");
});

5. Middleware

Middleware functions are functions that run before reaching the final route handler. They can modify req and res.

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

6. Serving Static Files

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

Now you can serve files like public/index.html.

7. JSON Data Handling

app.use(express.json());

app.post("/data", (req, res) => {
  res.json({ received: req.body });
});
  • express.json() parses JSON request bodies

8. Express Routing in Separate Files

// routes/user.js
const express = require("express");
const router = express.Router();

router.get("/", (req, res) => res.send("User List"));
router.get("/:id", (req, res) => res.send(`User ${req.params.id}`));

module.exports = router;

// app.js
const userRoutes = require("./routes/user");
app.use("/users", userRoutes);

Visiting /users/ → "User List" Visiting /users/42 → "User 42"

  • Lightweight but powerful
  • Large ecosystem of middleware
  • Easy integration with databases like MongoDB, MySQL, PostgreSQL
  • Perfect for building REST APIs and full-stack apps