An Express JS Introduction

Moin moin and hello,

Express.js is a popular and versatile web application framework for Node.js, providing a simple yet powerful API for handling HTTP requests and responses, implementing middleware, serving static files, and more.

An Introduction to Express.js: Key Features and Benefits

Express.js is a popular web application framework for Node.js that provides a powerful and flexible set of features for building web applications and APIs. Express.js is known for its minimalist approach to web development, allowing developers to build scalable and robust applications with minimal overhead. In this article, we’ll explore some of the essential features and benefits of Express.js.

Setup

Before we can get started we have to install Express.js and create a new project. To install Express.js, we’ll use the npm package manager, which comes bundled with Node.js. To create a new project, we’ll use the npm init command, which will walk us through the process of creating a new package.json file for our project.

First, make sure you have Node.js and npm installed on your machine. You can check this by running node -v and npm -v in your terminal or command prompt.

To create a new Express.js project, open your terminal or command prompt and navigate to the directory where you want to create your project. Then run the following command:

npm init

This will create a new package.json file in your directory, which will track the dependencies of your project.

Next, install Express.js as a dependency of your project by running the following command:

npm install express

This will download and install the latest version of Express.js and add it to your package.json file.

Once Express.js is installed, you can create a new file called app.js (or any other name you prefer) in your project directory. In this file, you can define your Express.js application using the following code:

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

// Define routes and middleware here

app.listen(3000, () => console.log('Server started on port 3000'));

This code creates a new Express.js application instance, defines routes and middleware as needed, and starts the server on port 3000.

Finally, to start your new Express.js project, simply run the following command in your terminal or command prompt:

node app.js

This will start your Express.js application and make it available at http://localhost:3000.

Routing

One of the key features of Express.js is its powerful routing system. Express.js makes it easy to define custom routes for your application and map them to specific URLs and HTTP methods. With Express.js, you can define routes that handle GET, POST, PUT, DELETE, and other HTTP methods, making it easy to build APIs and web applications that respond to different types of requests.

Here’s an example of how to define a route in Express.js:

app.get('/users', (req, res) => {
  // handle GET request for /users
});

In this example, we define a GET route for /users. When a user visits /users in their web browser, Express.js will invoke the callback function defined in the second argument, allowing us to handle the request and return a response.

Middleware

Another key feature of Express.js is its middleware system. Middleware functions in Express.js are functions that execute before the final request handler, giving you the ability to modify the request and response objects, perform authentication and authorization checks, and more.

Middleware functions can be defined globally or on a per-route basis, and can be chained together to create complex request processing pipelines. Express.js provides a number of built-in middleware functions, such as body-parser for parsing JSON and URL-encoded request bodies, and cookie-parser for parsing cookies.

Here’s an example of how to define a middleware function in Express.js:

app.use((req, res, next) => {
  console.log('Received request:', req.url);
  next();
});

In this example, we define a middleware function that logs the URL of every request that our application receives. This function is defined using the use() method, which registers it to be called for every incoming request.

It is imoprtant to note that middleware functions must call the next() function to pass control to the next middleware function in the pipeline. If you forget to call next(), your application will hang and never respond to the request.

Views

Express.js also provides a simple and flexible way to render views and templates in response to incoming requests. Views in Express.js are typically implemented using a templating engine such as Handlebars, EJS, or Pug.

Here’s an example of how to render a view in Express.js using the Handlebars templating engine:

const express = require('express');
const exphbs  = require('express-handlebars');

const app = express();

app.engine('handlebars', exphbs());
app.set('view engine', 'handlebars');

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

In this example, we configure Express.js to use the Handlebars templating engine by calling app.engine() and app.set(). We then define a route that renders a view called home, passing in some data to be used by the template.

Create a new directory call views in your project directory and create a new file called home.handlebars. Then add the following code to the file:

<h1>{{ title }}</h1>
<p>{{ message }}</p>

You can now test your application by visiting the home page at http://localhost:3000. Express.js should render the home.handlebars view and display the title and message that we passed in.

Error Handling

Finally, Express.js provides a flexible and robust system for handling errors in your application. By default, Express.js provides a simple error handling middleware that can be used to catch and log errors that occur during request processing.

In addition to the built-in error handling middleware, Express.js also provides the ability to define custom error handling middleware functions that can be used to handle specific types of errors. This allows you to create error handling pipelines that can handle different types of errors in different ways.

Error-handling middleware functions take four arguments instead of three:** (err, req, res, next)**:

// Example error-handling middleware function
app.use(function(err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

In this query, we’re asking for the title, author, reviews, and author fields of the Book object with an id of “123”. Note how we’re fetching data from nested objects (reviews and author) in a single request.

Static Files

Express can serve static files, such as images, CSS, and JavaScript files, using the express.static() middleware function.

To use the express.static() middleware function, call it with the path to the directory that contains the static assets, and pass the resulting function to the app.use() function.

// Serve static files in a public directory
app.use(express.static('public'));

Conclusion

Express is a powerful and flexible framework for building web applications in Node.js. Its simple yet powerful API allows developers to quickly build robust applications that handle HTTP requests and responses, implement middleware, and serve static files. With its modular design and vast ecosystem of middleware and plugins, Express is an excellent choice for building web applications of any size or complexity.

That’s all for now, bye!