Express.js Explained: What it is and How to Use it in Your JavaScript Project

What is Express.js in JavaScript?

Express.js is a fast, unopinionated, and minimalist web framework for Node.js. It simplifies the process of building server-side applications and APIs by providing a robust set of features for handling HTTP requests, routing, middleware, and more. Built on top of Node.js, Express allows developers to create scalable and maintainable web applications using JavaScript quickly.

In simple terms, Express acts as a layer between the core Node.js server and our application’s logic, helping manage routes, requests, and responses efficiently. Whether we’re building a single-page app, a RESTful API, or a full-stack web application, Express.js makes backend development easier and more organized.

Let’s explore why developers prefer it and what makes it so powerful.

Key features of Express.js

Express.js comes packed with features that streamline web application development. Here are some of its standout capabilities:

• Minimal and Flexible: Express provides just the essentials to build web apps, allowing us to structure our application as preferred. We can add only the features we need using middleware and third-party packages.

• Powerful Routing System: Express’s robust routing mechanism lets us define URL patterns and HTTP methods (like GET, POST, PUT, DELETE) cleanly and organized. This is especially useful for building RESTful APIs.

• Middleware Support: Middleware functions in Express are like building blocks for the app. They can execute code, modify requests and responses, and end request-response cycles. This makes adding authentication, error handling, logging, and more easy.

• Integration with Templating Engines: Express supports multiple templating engines, such as Pug, EJS, and Handlebars, allowing developers to render dynamic HTML pages directly from the server.

• Built-in Error Handling: Express provides default error-handling mechanisms and lets us customize them to improve our application’s reliability and debugging experience.

• Compatibility with REST and CRUD Operations: Express simplifies the creation of RESTful services by mapping HTTP methods to CRUD operations with clean syntax and organized route handling.

• Active Community and Rich Ecosystem: Backed by a large and active community, Express has a vast ecosystem of plugins and middleware, making extending and customizing functionality easy.

Now that we’ve seen what Express.js offers, let’s break down how it actually works behind the scenes.

How Express.js works

Express is a lightweight layer that simplifies how our app listens for requests, processes them, and sends back responses. Here’s a simplified overview of how Express processes a request:

1. Server Receives an HTTP Request: When a client (like a browser or mobile app) sends a request to our server—for example, visiting a URL or submitting a form, Express listens for that request using the appropriate HTTP method (GET, POST, etc.).

2. Request Matches a Route: Express checks the incoming request against the routes defined in our code. Each route corresponds to a specific path and method. If it finds a match, it continues to process the request.

3. Middleware Functions Kick In: Before the response is sent, the request passes through a series of middleware functions. These can perform tasks like:

• Logging the request
• Checking user authentication
• Parsing request data (like JSON or form inputs)

Middleware can either:

• End the request-response cycle (e.g., send a response),
• Or call next() to pass control to the next middleware.

4. Response is Sent to the Client: Once the middleware stack has completed its job, our application sends a response to the client. This response could be HTML, JSON, a redirect, or even an error message.

5. Optional Error Handling: If something goes wrong, Express allows us to handle errors using custom error-handling middleware, making debugging easier and more controlled.

Now that we understand how Express.js works, let’s see how to get it up and running in your Node.js application.

How to use Express.js in a Node.js application

Integrating Express into a Node.js project is an efficient process. Let’s walk through the key steps to install it and set up a basic server.

Step 1: Install Express.js

To use Express, we first need to install it via npm (Node Package Manager). Make sure you have Node.js installed, then open your terminal and run:

npm install express 

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This will add Express to our project and save it as a dependency in the package.json file.

Step 2: Create and use Express in the code

Here’s an example of how to set up a basic Express server:

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

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

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

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

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In this code:

• express() initializes the app.
• app.get() sets up a route to respond to GET requests on the home page (/).
• res.send() sends a plain text response.
• app.listen() starts the server and listens on port 3000.

This basic setup can be expanded by adding more routes, using middleware, and connecting to databases or frontend frameworks.

Step 3: Run the application

To start this application, run the following command on your terminal:

node app.js 

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Here, app.js is the name of the file. The output will appear as follows:

So, can we use Express.js for every kind of backend project? Not always! Let’s explore when it’s the right fit.

When and why to use Express.js

Express.js is an excellent choice when you want complete control over your application’s backend without the constraints of a rigid framework. You should consider using Express if:

• You’re building a RESTful API: Express’s robust routing and middleware system make it ideal for creating APIs that connect to frontend apps or mobile clients.

• You need a lightweight and fast solution: Since it’s minimal by default, Express doesn’t bloat your application with unnecessary features, making it perfect for projects where performance matters.

• You prefer flexibility over convention: Express allows you to structure your project as you like, which is great for experienced developers or small teams.

• You want to stick to JavaScript across the stack: If you already use JavaScript on the frontend, Express allows you to use the same language on the server, reducing context switching.

• You’re prototyping or building MVPs: It’s quick setup and simple syntax make Express a favorite for hackathons, early-stage products, or any project that needs to get up and running fast.

That said, for complex applications that require strict architecture, built-in security, or advanced features out of the box, you may want to explore more opinionated frameworks like Django or Rails.

Next, let’s look at its strengths and where it might fall short, so you can decide if it’s the right fit for your project.

Advantages and limitations of Express.js

Like any technology, Express.js has its benefits and limitations. Understanding both will help us make informed decisions when building web applications.

Advantages:

• Fast and Lightweight: Express is minimal by design, meaning it doesn’t come with unnecessary features, making it fast and easy to customize.

• Flexible Structure: It doesn’t enforce a specific project structure or architecture, giving developers the freedom to organize code as they see fit.

• Large Ecosystem and Community Support: With thousands of middleware options and strong community backing, we can quickly extend functionality without reinventing the wheel.

• Full JavaScript Stack: Express is built on Node.js, which allows for using JavaScript on both the front end and the back end, streamlining development.

• Perfect for APIs: Express is particularly well-suited for building RESTful APIs due to its clean routing system and middleware support.

Disadvantages:

• Lack of Built-in Features: Its minimalism means many features (like authentication, validation, etc.) must be added manually via middleware or third-party libraries.

• No Defined Project Structure: While flexible, this can lead to inconsistent code organization across projects or teams, especially for beginners.

• Callback Hell (in older code): Though modern JavaScript (with Promises and async/await) helps, poorly managed middleware can still lead to deeply nested callbacks.

• Less Opinionated: Unlike frameworks like Django or Rails, Express offers fewer conventions and built-in tools, which can lead to more boilerplate for complex apps.

Knowing these pros and cons can help us decide whether Express.js is the right fit for our project or if another framework might better suit our needs.

Conclusion

In this article, we explored what Express.js is, how it works, its key features, and how to use it in our Node.js projects. We also discussed when using Express to streamline back-end development is best. Express simplifies building robust and scalable web applications, particularly when paired with Node.js.

If you’re ready to go further, Learn Node.js on Codecademy covers everything from setting up servers with HTTP to mastering Node.js essentials, perfect for taking your Express skills to the next level.

Frequently asked questions

1. Is Express.js a frontend or backend?

Express.js is a backend framework that runs on Node.js. It helps build server-side applications and APIs by handling routing, requests, and responses.

2. Is Express.js better than Node.js?

Express.js isn’t better or worse—it’s built on top of Node.js. Think of Node.js as the engine, and Express.js as a tool that makes building web apps with that engine easier and faster.

3. Is Express.js an API?

No, Express.js is not an API—it’s a web application framework used to easily build APIs (among other things). It provides methods and tools that simplify routing, middleware, and HTTP handling.

4. What is the difference between React and Express?

React is a front-end library for building user interfaces, while Express is a backend framework for handling server-side logic. They serve different purposes in a full-stack application.

5. Can I use React and Express together?

Yes! Express and React are often used together in full-stack JavaScript applications. Express handles the backend (API, server logic), and React manages the frontend (user interface).