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What is Express.js?

Introduction to Express.js:

Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It facilitates the rapid development of Node-based web applications.

Key Features:

Fast and lightweight
Middleware support
Routing
Template engines
Debugging

Use Cases:

Express.js is used to build single-page, multi-page, and hybrid web applications. It is an integral part of the MEAN stack (MongoDB, Express.js, AngularJS, Node.js).

Benefits for Developers:

Express.js simplifies server-side coding, supports a variety of HTTP utility methods, and enables the efficient organization of web applications into an MVC architecture.

Setting Up an Express.js Server

Installation:

To set up Express.js, you need Node.js installed on your machine. Use npm to install Express.js.


        // Install Express.js
        npm install express --save

Basic Server Setup:

Create a basic server using Express.js to handle HTTP requests and responses.


        const express = require('express');
        const app = express();
        const port = 3000;

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

        app.listen(port, () => {
          console.log(`Server is running at http://localhost:${port}`);
        });

Explanation:

The above code initializes an Express application, sets up a route to handle GET requests to the root URL, and starts the server on port 3000.

Express.js Middleware

Understanding Middleware:

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.


        // Example of Middleware
        app.use((req, res, next) => {
          console.log('Time:', Date.now());
          next();
        });

Explanation:

This middleware logs the time of the request. The next() function is used to pass control to the next middleware function.

Routing in Express.js

Defining Routes:

Express.js provides a robust routing mechanism to define application endpoints and how they respond to client requests.


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

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

Explanation:

The above code defines two routes: one for handling GET requests to "/about" and another for POST requests to "/submit".

Using Template Engines with Express.js

Template Engines:

Express.js supports various template engines like Pug, EJS, and Handlebars to render dynamic HTML pages.


        // Using Pug as template engine
        app.set('view engine', 'pug');

        app.get('/home', (req, res) => {
          res.render('index', { title: 'Home Page', message: 'Welcome to Express.js!' });
        });

Explanation:

In this example, Pug is used as the template engine. The route "/home" renders an HTML page using a Pug template named 'index'.

Error Handling in Express.js

Error Handling Middleware:

Express.js provides an error-handling middleware to catch and manage errors in the application.


        // Error handling middleware
        app.use((err, req, res, next) => {
          console.error(err.stack);
          res.status(500).send('Something broke!');
        });

Explanation:

The error-handling middleware logs the error stack and sends a 500 status response to the client.

Deployment of Express.js Applications

Deployment Considerations:

When deploying Express.js applications, consider using process managers like PM2, cloud platforms like Heroku, and services like Docker for containerization.


        // Start Express server with PM2
        pm2 start app.js

Explanation:

Using PM2 ensures your application remains online by managing and monitoring the application processes.