jsdom vs happy-dom: Navigating the Nuances of JavaScript Testing
Tags: javascript, testing, jest
Published: 2023-12-08 21:03:17

---
title: jsdom vs happy-dom: Navigating the Nuances of JavaScript Testing
published: true
date: 2023-11-30 04:16:16 UTC
tags: #javascript #testing #jest
canonical_url: https://blog.seancoughlin.me/jsdom-vs-happy-dom-navigating-the-nuances-of-javascript-testing
---

## Introduction

JavaScript testing is a crucial part of ensuring that web applications work correctly across different environments. In this context, tools like [jsdom](https://github.com/jsdom/jsdom) and [happy-dom](https://github.com/capricorn86/happy-dom) are incredibly valuable. They both provide ways to simulate a browser-like environment for testing purposes. Let's dive into what each of these tools is and how they compare.

## Understanding Server-Side DOM in Web Development

Before we can dive fully into these tools we need to briefly discuss the DOM.

When we talk about the [DOM](https://developer.mozilla.org/en-US/docs/Web/API/Document_Object_Model) or Document Object Model, the immediate association is with client-side JavaScript in a web browser. It's a critical concept for web developers, as it represents the page's structure, style, and content, and allows scripts to interact with and modify it.

However, there's another side to this: the server-side DOM. Lets explore what this means and why it's important in modern web development.

### The Idea of a DOM Outside a Client (Server-Side DOM)

The [Document Object Model (DOM)](https://en.wikipedia.org/wiki/Document_Object_Model) is typically thought of as the model that the browser uses to turn an HTML document into a visible website. However, the [DOM standard](https://dom.spec.whatwg.org) can be implemented in pure JavaScript to run outside of a browser on a server.

One of the primary uses of a server-side DOM is in the context of testing web applications. In traditional scenarios, testing a web application's functionality requires rendering pages in a browser. However, with a server-side DOM, developers can simulate a browser environment on the server.

This approach is extremely useful for automated testing, allowing scripts to interact with a simulated DOM without the overhead of a real browser. The GUI of a browser is very nice for human usability but has terrible performance. Stripping out the browser overhead enables fast JavaScript unit testing.

### Tools for Server-Side DOM

In the JavaScript world, tools like jsdom and happy-dom are prime examples of server-side DOM implementations. They create a simulated browser-like DOM environment within a server context.

This means JavaScript code, which typically runs in a client-side browser, can be executed on the server. This simulation can include creating and manipulating DOM elements, handling events, and sometimes even emulating browser-specific behaviors and CSS support.

## What is happy-dom?

happy-dom is a relatively new entrant in the world of JavaScript testing. It's a lightweight, high-performance server-side DOM implementation. happy-Dom is designed to emulate a web browser's environment, allowing you to run and test your JavaScript code in a simulated browser-like environment on the server.

One of the key features of happy-dom is its speed and efficiency. It's built to be fast, which makes it an excellent choice for high-speed DOM operations and tests that require quick execution. This can be particularly beneficial in a [continuous integration/continuous deployment (CI/CD)](https://blog.seancoughlin.me/series/ci-cd) pipeline where speed is crucial.

## What is jsdom?

jsdom, on the other hand, is a more established tool in the JavaScript world. It offers a robust simulation of a web browser's environment. JSdom allows you to create a virtual DOM on the server-side, enabling the testing of JavaScript intended for client-side execution in a Node.js environment.

jsdom is known for its comprehensive emulation of the web browser's environment. It includes a wide range of browser features, making it a go-to choice for tests that require detailed emulation, including CSS support, layout, and more complex DOM manipulations.

## When to Use jsdom or happy-dom

Choosing between jsdom and happy-dom often depends on your specific testing requirements:

The rough rule of thumb is happy-dom might be faster (depending on your use case) and jsdom might be more complete (simulates more of what a browser would).

### Example Use Case

[Vitest](https://vitest.dev) supports the use of [different DOM rendering tools](https://vitest.dev/guide/environment#test-environment). You could choose to use either happy-dom or jsdom depending on your preference.

```
export default defineConfig({
    plugins: [svelte()],
    test: {
        environment: 'jsdom', // could pass in jsdom or happy-dom
    },
})

```

Feel free to [learn more about Vitest in one of my earlier posts](https://blog.seancoughlin.me/vitest-vs-jest-the-new-javascript-testing-framework).

## Conclusion

In conclusion, both JSdom and Happy-Dom offer valuable solutions for testing JavaScript in a non-browser environment. Your choice between the two should be guided by your project's specific needs. If you require detailed browser emulation, jsdom is your go-to tool. For rapid testing, particularly focused on DOM operations, happy-dom is an excellent choice.

As JavaScript continues to evolve, tools like these play a crucial role in ensuring the reliability and performance of web applications. As a software engineer, staying informed about these tools and understanding their strengths and weaknesses can significantly enhance your testing strategy and overall development workflow.

---

Originally published at https://blog.seancoughlin.me.