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Introduction to Express.js

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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 is designed to build single-page, multi-page, and hybrid web applications efficiently. Express.js simplifies the process of handling HTTP requests, routing, middleware, and more.

Key Features of Express.js

  • Minimal and Flexible: Express.js is unopinionated, meaning it provides the basic tools to build web applications without enforcing a specific structure.
  • Middleware Support: Middleware functions can be used to handle requests, responses, and errors.
  • Routing: Express.js provides a powerful routing mechanism to handle different HTTP methods and URL paths.
  • Template Engines: It supports various template engines like Pug, EJS, and Handlebars for rendering dynamic content.
  • Extensible: Easily extendable with numerous third-party packages available via NPM.

Setting Up Express.js

Step 1: Install Node.js and NPM

Ensure you have Node.js and NPM installed on your machine. You can download and install them from Node.js official website.

Step 2: Create a New Project Directory

Create a new directory for your Express.js project and navigate into it:

mkdir my-express-app
cd my-express-app

Step 3: Initialize a New Node.js Project

Initialize a new Node.js project using NPM:

npm init -y

This command creates a package.json file with default settings.

Step 4: Install Express.js

Install Express.js as a dependency:

npm install express

Your First Express.js Application

Let's create a simple Express.js application that responds with "Hello, World!" when accessed via a web browser.

Step 1: Create the Application File

Create a new file named app.js in your project directory and add the following code:

// Import the Express module
const express = require('express');

// Create an instance of an Express application
const app = express();

// Define a route for the root URL ("/")
app.get('/', (req, res) => {
  res.send('Hello, World!');
});

// Start the server and listen on port 3000
const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server is running on http://localhost:${PORT}`);
});

Step 2: Run the Application

Run your Express.js application using Node.js:

node app.js

You should see the message Server is running on http://localhost:3000 in your terminal.

Step 3: Access the Application

Open your web browser and navigate to http://localhost:3000. You should see the message "Hello, World!" displayed on the page.

Explanation of the Code

  • Import the Express module: The require('express') statement imports the Express module.
  • Create an instance of an Express application: The express() function creates an instance of an Express application.
  • Define a route: The app.get('/', (req, res) => { ... }) method defines a route for the root URL ("/"). When a GET request is made to the root URL, the callback function is executed, sending the response "Hello, World!".
  • Start the server: The app.listen(PORT, () => { ... }) method starts the server and listens on the specified port (3000 in this case). The callback function logs a message indicating that the server is running.

Practical Exercise

Exercise: Create a Simple Express.js Application

  1. Create a new directory named exercise-express-app.
  2. Initialize a new Node.js project in the directory.
  3. Install Express.js.
  4. Create an app.js file with the following requirements:
    • Define a route for the root URL ("/") that responds with "Welcome to Express.js!".
    • Define a route for "/about" that responds with "About Page".
    • Start the server on port 4000.

Solution

mkdir exercise-express-app
cd exercise-express-app
npm init -y
npm install express

Create an app.js file with the following content:

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

app.get('/', (req, res) => {
  res.send('Welcome to Express.js!');
});

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

const PORT = 4000;
app.listen(PORT, () => {
  console.log(`Server is running on http://localhost:${PORT}`);
});

Run the application:

node app.js

Access the application in your web browser:

  • http://localhost:4000 should display "Welcome to Express.js!".
  • http://localhost:4000/about should display "About Page".

Summary

In this section, you learned about the basics of Express.js, including its key features and how to set up a simple Express.js application. You also created a basic application that responds to different routes. In the next module, we will dive deeper into setting up an Express application and explore middleware, routing, and error handling.

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Node.js Course

Module 1: Introduction to Node.js

Module 2: Core Concepts

Module 3: File System and I/O

Module 4: HTTP and Web Servers

Module 5: NPM and Package Management

Module 6: Express.js Framework

Module 7: Databases and ORMs

Module 8: Authentication and Authorization

Module 9: Testing and Debugging

Module 10: Advanced Topics

Module 11: Deployment and DevOps

Module 12: Real-World Projects

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