Beginner’s Guide to Express JS: A Comprehensive Course

Express JS is a popular and lightweight web application framework for Node.js. It is designed to make building web applications and APIs easier and faster. In this tutorial, we will cover the basics of Express JS and show you how to get started with building your own web applications.

Before we begin, make sure you have Node.js installed on your machine. You can download and install Node.js from the official website (https://nodejs.org/). Once you have Node.js installed, you can install Express JS using npm, which is the Node.js package manager. Open your terminal or command prompt and type the following command:

npm install express

This will install Express JS and its dependencies in your project directory. Now that you have Express JS installed, let’s create a simple web server using Express.

Create a new file called app.js and open it in your code editor. In this file, we will initialize an Express application, define some routes, and start the server.

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 on http://localhost:${port}`);
});

In this code snippet, we first import the Express module and create a new Express application using express(). We then define a route using app.get() that handles requests to the root URL (/) and sends a simple text response (Hello World!). Finally, we start the server listening on port 3000.

To run this server, save the app.js file and type the following command in your terminal:

node app.js

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

Now that you have a basic Express server up and running, let’s dive into some more advanced topics.

Routing

Routing in Express refers to determining how an application responds to a client request. You can define routes for different URLs and HTTP methods using Express’s routing methods. Let’s create a simple example with multiple routes:

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

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

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

In this example, we have defined three routes for the root URL, /about, and /contact. When a client makes a GET request to any of these URLs, Express will send the corresponding response.

Middleware

Middleware functions are functions that have access to the request and response objects in Express. They can perform actions on the request object, modify the response object, or end the request-response cycle. Middleware functions can be used to perform tasks such as logging, authentication, error handling, etc.

Let’s create a simple middleware function that logs the current date and time for every request:

app.use((req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  next();
});

This middleware function logs the current date and time, HTTP method, and URL for every incoming request. The next() function is used to pass control to the next middleware function in the stack.

Static Files

Express allows you to serve static files such as images, CSS, and JavaScript files by using the express.static middleware. Let’s create a public directory in your project folder and serve static files from it:

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

Now you can place your static files in the public directory, and Express will serve them at the specified URL.

Error Handling

Express provides error handling middleware to handle runtime errors in your application. You can define error handling middleware functions that take four arguments (err, req, res, next) to process errors. Let’s create a simple error handling middleware function:

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

This error handling middleware function logs the error stack trace and sends a generic error message to the client with a 500 status code.

This is just a brief introduction to Express JS and its features. There are many more advanced topics and functionalities that you can explore, such as template engines, database integration, middleware libraries, and more. Express JS is a powerful tool for building web applications and APIs, and learning how to use it effectively can greatly enhance your development skills. I hope this tutorial has provided you with a good foundation to start building your own Express applications. Happy coding!