Demystifying JSX: Understanding How it Works Behind the Scenes

3 min readOct 20, 2023

Introduction

If you’re stepping into the world of web development, you’ve likely encountered JSX — a powerful tool that simplifies the creation of interactive web applications. In this article, we’ll take a deep dive into JSX, unravel its inner workings, and shed light on its role in crafting dynamic user interfaces.

The Building Blocks of JSX

Before we venture into the inner workings of JSX, let’s first refresh our understanding of what JSX looks like. JSX allows you to write code that closely resembles HTML, but it’s, in fact, a syntax extension of JavaScript. Consider this simple example:

const element = <h1>Hello, JSX!</h1>;

At first glance, it may seem like you’re writing HTML, but it’s important to note that this code is pure JavaScript. JSX elements can include attributes, event handlers, and can be nested within each other, making it a fantastic tool for UI creation.

Compilation to JavaScript

One crucial point to understand about JSX is that it doesn’t run directly in web browsers. It needs to be translated into standard JavaScript to be understood by browsers. This is where Babel, a popular JavaScript compiler, comes into play. Babel takes your JSX code and compiles it into JavaScript that browsers can execute.

Let’s take a look at how the previous example gets transformed by Babel:

const element = React.createElement("h1", null, "Hello, JSX!");

Babel replaces JSX tags with a React.createElement function. This function takes three arguments: the type of element, the element's attributes (which can be null), and the element's children (in this case, the text "Hello, JSX!").

React and the Virtual DOM

React, a JavaScript library for building user interfaces, plays a pivotal role in rendering JSX. When you use JSX in a React application, it generates a virtual representation of the Document Object Model (DOM), known as the Virtual DOM. The Virtual DOM serves as a lightweight copy of the actual DOM and enables React to efficiently update and render components.

Consider this simple React component:

function Greeting() {
  return <h1>Hello, JSX!</h1>;
}

Behind the scenes, React creates a Virtual DOM representation of the Greeting component and its JSX. Whenever there are updates to the component or its props, React calculates the differences between the old and new Virtual DOM, making the process of rendering and updating highly efficient.

Reconciliation and Rendering

Once React has the Virtual DOM representation, it initiates a process known as reconciliation. During reconciliation, React identifies the differences between the old Virtual DOM and the new one and updates only the parts that have changed. This process is critical for optimizing the performance of web applications.

Finally, React updates the actual DOM to reflect the changes in the Virtual DOM. This is what users see and interact with in their web browsers.

Wrapping Up

In this article, we’ve dived into the magic behind JSX, a powerful tool for building web applications. We’ve explored how JSX is compiled into JavaScript, its role in React’s Virtual DOM, and the process of reconciliation and rendering.

Understanding how JSX works behind the scenes is essential for web developers. It enables you to create efficient, interactive, and responsive web applications while enjoying the simplicity and familiarity of HTML-like syntax. So, go ahead, keep experimenting with JSX, and unveil the wonders it can bring to your web development projects. Happy coding!