Understanding JSX: Why React Uses It and How It Works
Posted on April 3, 2025 • 7 min read • 1,342 wordsLearn what JSX is, why React uses it, and how it works to improve UI development, readability, and performance in React applications.
React, a popular JavaScript library for building user interfaces, has transformed how developers approach front-end development. One of the key features of React that sets it apart from other libraries is JSX. JSX, or JavaScript XML, allows developers to write HTML-like code within JavaScript, making React code more readable and maintainable. However, many developers, especially beginners, often wonder what JSX is, why React uses it, and how it works.
JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows developers to write HTML-like code within their JavaScript files. JSX makes it easier to describe the structure of the UI in a format that closely resembles HTML, but it ultimately compiles down to JavaScript code that React can process.
While JSX is not a requirement in React, it is widely adopted because it allows developers to seamlessly mix HTML and JavaScript, which can improve productivity and maintainability. Instead of separating the logic and the markup, JSX enables developers to create components that encapsulate both the behavior (JavaScript) and the appearance (HTML) of the UI in one place.
JSX syntax looks quite similar to HTML, but there are some important differences:
<img /> or <input /> need to be self-closing, unlike HTML where they can be written as <img> or <input>.class in HTML becomes className in JSX because class is a reserved keyword in JavaScript.{}). For example, {2 + 2} will output 4 within the JSX code.const element = <h1>Hello, {user.name}!</h1>;Though JSX looks similar to HTML, it is not HTML. It is important to understand that JSX is a JavaScript syntax extension. Once the code is transpiled by a tool like Babel, it becomes standard JavaScript code, not raw HTML.
React introduced JSX as a way to make UI development more intuitive and streamlined. Here are several reasons why React uses JSX:
JSX allows developers to describe the structure of the user interface directly in their JavaScript code. By doing this, it makes it easier to visualize the UI while reading the code, improving both readability and maintainability. Developers can see the HTML structure alongside the logic, without needing to switch between multiple files or formats.
React applications are composed of reusable components. JSX allows developers to define the UI layout and behavior of components in one place. Without JSX, developers would need to manually manipulate the DOM and create elements using JavaScript code, which can quickly become cumbersome and harder to manage. With JSX, React takes care of this for developers, ensuring components are structured properly and are easy to reuse.
Because JSX allows the combination of markup and JavaScript, it simplifies the debugging process. When developers see an issue with their UI, they can directly trace it back to the corresponding component, view the state, and inspect how the JavaScript logic is influencing the HTML output. This direct coupling between markup and logic can significantly streamline the debugging process.
React promotes a declarative programming model, where developers describe what the UI should look like for a given state. JSX allows developers to write code that is more declarative than imperative. Instead of describing each individual step of how the UI should change, developers simply describe the end state of the UI. React and JSX then take care of how to efficiently update the UI when the state changes.
const element = <h1>Welcome, {username}</h1>;Although JSX looks similar to HTML, it doesn’t run in the browser directly. JSX needs to be transpiled into regular JavaScript code that the browser can understand. This process involves transforming JSX code into React createElement calls, which React uses to generate the virtual DOM.
Browsers do not natively understand JSX. As a result, tools like Babel are used to transpile JSX code into standard JavaScript. During the transpilation process, JSX is transformed into React.createElement calls. These calls are what React uses to create the virtual DOM.
For example, consider the following JSX code:
const element = <h1>Hello, World!</h1>;Babel would transform this JSX code into the following JavaScript:
const element = React.createElement('h1', null, 'Hello, World!');This transformation is what enables React to render the JSX elements in the virtual DOM.
React uses a virtual DOM to optimize rendering performance. Instead of directly modifying the real DOM (which can be slow), React updates a virtual representation of the DOM first. This virtual DOM is a lightweight copy of the actual DOM, and React uses it to determine what parts of the real DOM need to be updated when state or props change.
When JSX is rendered, React creates a virtual DOM tree, which is later compared with the previous version using a process called reconciliation. React then calculates the minimal number of changes required to update the real DOM efficiently.
JSX is used to define React components. A React component is a JavaScript function or class that returns JSX code. The JSX code is what defines the component’s structure and UI elements.
Here’s a simple example of a functional React component that uses JSX:
function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}In this example, the Greeting component accepts props and returns a JSX expression that displays a personalized greeting. When you use this component within a larger application, React will render it as part of the UI.
While functional components are increasingly popular in React development, class components are still widely used. JSX can also be used in class components to render the UI.
class Greeting extends React.Component {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}JSX offers several benefits that enhance the development process:
By combining HTML-like syntax with JavaScript, JSX makes code more concise and readable. Developers can focus on the structure of their UI without worrying about the details of manually creating DOM elements.
JSX integrates seamlessly with modern JavaScript tools and libraries, such as ES6 features, code editors, and linters. These tools often come with built-in support for JSX syntax, offering features like syntax highlighting, error checking, and auto-completion, all of which improve the overall developer experience.
React promotes a component-based architecture, and JSX plays a central role in this approach. With JSX, developers can create reusable components that can be easily shared across different parts of the application, ensuring consistent and modular code.
Using JSX enables React to optimize rendering performance through the virtual DOM. React’s reconciliation process ensures that only the necessary parts of the DOM are updated, which can improve performance, especially in large applications with frequent state changes.
JSX is a powerful and essential feature of React that simplifies the process of building user interfaces. By combining the benefits of JavaScript and HTML, JSX allows developers to create readable, maintainable, and performant code. Whether you’re a beginner or an experienced React developer, understanding JSX is crucial for building modern web applications. It enables you to write declarative, component-based code that seamlessly integrates with React’s rendering engine, ultimately enhancing the user experience.
In summary, React uses JSX to make it easier for developers to write, understand, and manage the code that drives user interfaces. JSX helps with readability, simplifies debugging, improves performance, and aligns perfectly with React’s declarative model. By understanding how JSX works and why it’s so important, you can harness its full potential in your React applications and become a more effective React developer.