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Setting up an Express.js Application

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.

Why Use Express.js?

Simplifies server-side coding by providing a clean API.
Supports middleware to handle HTTP requests.
Facilitates routing and templating.

Installing Express.js

Using npm for Installation:

To install Express.js, you need Node.js installed on your machine. Use npm (Node Package Manager) to install it globally or locally in your project.


$ npm install express --save

Global vs Local Installation:

A global installation makes the package available system-wide, while a local installation restricts it to the project directory.

Creating a Basic Express.js Server

Server Setup:

Once Express is installed, you can create a simple server to handle HTTP requests. This involves setting up the server and defining routes.


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

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

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

Understanding the Code:

This code sets up an Express application that listens for GET requests on the root URL ('/') and responds with 'Hello World!'.

Middleware Functions in Express.js

Role of 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.


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

Chaining Middleware:

Middleware can be chained to handle a request. It's important to call the next() function to pass control to the next middleware.

Routing in Express.js

Defining Routes:

Express allows you to define routes for different HTTP methods and URLs. You can create dynamic routes with parameters.


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

Route Parameters:

Route parameters are named URL segments that are used to capture values specified at their position in the URL.

Handling Errors in Express.js

Error Handling Middleware:

Error-handling middleware is defined in the same way as other middleware, except it has four arguments instead of three: (err, req, res, next).


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

Importance of Error Handling:

Proper error handling ensures that your application can gracefully handle unexpected situations without crashing.

Serving Static Files in Express.js

Static Middleware:

To serve static files such as images, CSS files, and JavaScript files, use the express.static built-in middleware function.


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

Directory Structure:

Ensure that your static files are organized in a 'public' directory or any directory of your choice, specified in express.static.

Using Template Engines with Express.js

Integrating Template Engines:

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


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

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

Benefits of Templating:

Templating engines allow you to create dynamic HTML pages by embedding JavaScript variables and logic directly into the HTML.