Express.js Overview

Express.js, commonly known as Express, is a fast, free, and open-source Node.js framework. It offers a rich collection of features and tools, making it a popular choice for developing web applications and APIs in Node.js. By extending Node.js with additional capabilities such as middleware and routing, Express has become the most widely used standard server framework for Node.js.

Request Flow in an Express.js Application

In an Express.js application, the HTTP request flow works as follows:

Client Request: The client sends a request using the HTTP protocol, the primary communication method for most web applications.
HTTP Server: The HTTP server receives the request and passes it to Express for processing. Node.js, being the entry point for the server, handles the initial reception of the request.
Express Processing: Express adds functionalities to the native request and response objects, making them easier to handle or manipulate. It then passes the request to the middleware.
Middleware Processing: Middleware functions process the request, determine which function to call, and handle the request accordingly.
Client Response: Finally, the HTTP server sends the processed response back to the client.

Why Choose Express?

Express significantly simplifies and enhances the back-end development of web applications. Compared to standard JavaScript, it offers a more organized and efficient approach. Here are some key advantages of using Express:

Efficient
Fast
Organized
Easy to Learn

Features of Express

Express provides several features that facilitate web application development:

Routing: Defines how an application responds to client requests for specific endpoints.
Middleware: Functions executed between receiving raw requests and processing them through the final route.
Templating: Dynamically renders HTML pages using data from a database or other sources.

Routing

Routing in Express involves defining how an application responds to different client-side requests for specific endpoints. These requests can be made using various HTTP methods such as GET, POST, PUT, and DELETE.

Middleware

Middleware in Express.js consists of functions that execute between receiving raw requests and processing them through the final intended route. Middleware can be used for various purposes, such as authentication, request validation, and modifying request/response objects. Types of middleware include:

Application Middleware: Runs for all endpoints and is bound to the application using the use() function.
Router Middleware: Similar to application middleware but bound to a router object and used for specific routes.
Error-Handling Middleware: Detects and handles errors in the application, with additional arguments in its function signature.
Third-Party Middleware: Integrates middleware from third-party packages to enhance functionality.

Templating

Templating in Express.js involves dynamically rendering HTML pages using data from databases or other data sources. Popular templating engines used with Express include Pug, Handlebars, EJS, and Mustache.

Code Explanation

Here is a brief explanation of a simple Express.js application:

javascript

const express = require('express');  // Line 2: Import the Express.js module.

const app = express();               // Line 5: Create an instance of the Express application.
app.get('/', (req, res) => {         // Line 8-10: Define a route for the root URL.

  res.send('Hello, World!');         // Send the response "Hello, World!" back to the client.

});

app.listen(3000, () => { // Line 13-15: Start the Express server on port 3000. console.log('Server is running on port 3000'); // Log a message indicating the server is running. });

Line 2: The require function imports the Express.js module.
Line 5: An instance of the Express application is created.
Lines 8-10: A route is defined using the app.get() method. When a GET request is made to the root URL (/), the callback function sends “Hello, World!” as the response.
Lines 13-15: The app.listen() method starts the Express server on port 3000. Upon successful startup, it logs a message to the console.

What is Express.js?