Introduction to Express

Introduction to Express

In this section, we’ll introduce Express, a fast and minimalist web framework for Node.js. Express simplifies the process of building robust and scalable web applications and APIs. We’ll cover the basics of setting up an Express server, routing, and understanding middleware.

Installing and Setting Up Express

To get started with Express, you need to install it using npm.

1. Create a new Project Directory:

   bash Copy

   	mkdir express-app
   	cd express-app

2. Initialize a Node.js Project:

   bash Copy

   	npm init -y

3. Install Express:

   bash Copy

   	npm install express

4. Create the Entry Point: Create a file named app.js:

   javascript Copy

   	// Importing the Express module
   	const express = require('express');
   	
   	// Creating an instance of an Express application
   	const app = express();
   	
   	// Defining the port number where the server will listen for requests
   	const port = 3000;
   	
   	// Setting up a route for the root URL
   	app.get('/', (req, res) => {
   	  res.send('Hello, World!'); // Sending a response back to the client
   	});
   	
   	// Starting the server and logging a message to the console
   	app.listen(port, () => {
   	  console.log(`Server running at http://localhost:${port}/`);
   	});

5. Run Your Express Server:

   bash Copy

   	node app.js

   Open a browser and go to http://localhost:3000/ to see your “Hello, World!” message.

   Note: You will need to restart the server manually whenever you make changes to your code. We will cover automatic server restarts in a later section.

Basic Express Server Setup

In the example above, we set up a basic Express server that responds with “Hello, World!” to requests at the root URL. Let’s break down the components:

• Importing Express: We require the express module.
• Creating an Application Instance: const app = express(); creates an instance of an Express application.
• Defining a Route: app.get('/', ...) sets up a route to handle GET requests at the root URL (/).
• Starting the Server: app.listen(port, ...) starts the server and listens for incoming requests on the specified port.

Understanding Middleware

Middleware functions are the backbone of Express applications. They are functions that execute during the request-response cycle.

Example: Using middleware to log requests:

	// Middleware function to log request details
	app.use((req, res, next) => {
	  console.log(`${req.method} request for '${req.url}'`);
	  next(); // Passing control to the next middleware function
	});
	
	// Route to handle GET requests at the root URL
	app.get('/', (req, res) => {
	  res.send('Hello, World!');
	});

• Middleware Function: A function that takes three arguments: req, res, and next. It processes the request, performs some action, and then calls next() to pass control to the next middleware function.

Routing Basics

Routing in Express involves defining endpoints (URLs) and handling HTTP requests for those endpoints.

Example: Setting up multiple routes:

	// Route to handle GET requests at the root URL
	app.get('/', (req, res) => {
	  res.send('Home Page');
	});
	
	// Route to handle GET requests at the '/about' URL
	app.get('/about', (req, res) => {
	  res.send('About Page');
	});
	
	// Route to handle GET requests at the '/contact' URL
	app.get('/contact', (req, res) => {
	  res.send('Contact Page');
	});

• Defining Routes: app.get('/about', ...) defines a route for GET requests at the /about URL.
• Handling Requests: Each route specifies a callback function to handle the request and send a response.

Summary

In this section, we’ve introduced Express, set up a basic Express server, and explored middleware and routing. You’ve learned how to install and configure Express, create routes, and use middleware to enhance your application.

Lesson task

We will create a simple Express server that responds with a custom message. You will set up the server, define a route, and use middleware to log requests.

Goal

The goal is for you to demonstrate that you know the basics of Node.js and Express by setting up a simple server and understanding middleware and routing.

Brief

Create a new Express server that has the following end-points:

1. /: Responds with a custom message.

2. /about: Respond with a custom message.

3. /contact: Respond with a custom message.

Add a middleware function to log the request details for each incoming request.

NOTE: Lesson tasks do not get submitted on Moodle and are not assessed by tutors. They are mainly there for you to practise what you have learned in the lesson.

Additional resources

nodemon