Express.js #

Express.js is a popular web application framework for Node.js that simplifies the development of server-side applications and APIs. Here’s a basic overview of the Express framework in Node.js:

Installation: #

Install Express using npm, the Node.js package manager:

npm install express

Hello World Example: #

Create a simple Express app with a “Hello, World!” endpoint:

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

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

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Access the application by navigating to http://localhost:3000 in your web browser.

Routing: #

Express allows you to define routes for different HTTP methods and URLs:

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

app.post('/api/data', (req, res) => {
    // Handle POST request
});

Middleware: #

Middleware functions are functions that have access to the request, response, and the next function in the application’s request-response cycle.

app.use(express.json()); // Parse JSON requests
app.use(express.static('public')); // Serve static files from the 'public' directory

app.use((req, res, next) => {
    console.log('Middleware executed');
    next(); // Move to the next middleware or route handler
});

Template Engines: #

Express supports template engines like EJS, Pug, and Handlebars for rendering dynamic HTML.

npm install ejs

app.set('view engine', 'ejs');

app.get('/dynamic', (req, res) => {
    res.render('dynamic', { name: 'John' });
});

Query Parameters and Route Parameters: #

Access query parameters and route parameters in Express:

app.get('/user', (req, res) => {
    const userId = req.query.id;
    res.send(`User ID: ${userId}`);
});

app.get('/user/:id', (req, res) => {
    const userId = req.params.id;
    res.send(`User ID: ${userId}`);
});

Error Handling: #

Use error-handling middleware to catch and handle errors:

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Internal Server Error');
});

Express Router: #

Organize routes into separate files using Express Router:

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

router.get('/', (req, res) => {
    res.send('Router Home');
});

module.exports = router;

// app.js
const routes = require('./routes');
app.use('/router', routes);

Middleware Stack: #

Middleware can be specific to a route or applied globally:

const middleware1 = (req, res, next) => { /* ... */ };
const middleware2 = (req, res, next) => { /* ... */ };

app.get('/route1', middleware1, (req, res) => {
    // ...
});

app.get('/route2', [middleware1, middleware2], (req, res) => {
    // ...
});

app.use(middleware1); // Applies globally

RESTful APIs: #

Express is commonly used to build RESTful APIs, handling HTTP methods such as GET, POST, PUT, DELETE.

app.get('/api/users', (req, res) => {
    // Retrieve list of users
});

app.post('/api/users', (req, res) => {
    // Create a new user
});

Express.js provides a flexible and powerful foundation for building web applications and APIs in Node.js. Its simplicity, middleware system, and robust routing capabilities make it a go-to choice for developers when creating scalable and maintainable server-side applications.