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JSX is a syntax extension for JavaScript that lets you write HTML-like markup inside JavaScript files. While not required for React, JSX makes component code more readable and expressive.

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What is JSX?

JSX looks like HTML but it’s actually JavaScript. Each JSX element is transformed into a JavaScript function call. 
const element = <h1>Hello, world!</h1>;
This JSX is transformed by a compiler (like Babel) into: 
const element = React.createElement('h1', null, 'Hello, world!');

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How JSX Transforms to JavaScript

Under the hood, JSX elements are transformed into ReactElement objects using React’s JSX runtime. The modern JSX transform (introduced in React 17) uses jsx() and jsxs() functions: 
<div className="container">
<h1>Hello</h1>
<p>Welcome to React</p>
</div>
The modern JSX transform (using jsx() and jsxs()) was introduced in React 17 and has several advantages:
  • You don’t need to import React to use JSX
  • Better performance in some cases
  • Future compatibility improvements

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The ReactElement Structure

When JSX is compiled, it creates a ReactElement - a plain JavaScript object that describes what you want to see on the screen: 
// Simplified ReactElement structure
const element = {
  // Identifies this as a React element
  $$typeof: Symbol.for('react.transitional.element'),
  
  // The type of element (string for DOM, function/class for components)
  type: 'h1',
  
  // The key for reconciliation (if provided)
  key: null,
  
  // The props including children
  props: {
    children: 'Hello, world!'
  }
};
React elements are immutable. Once created, you can’t change their props or children. To update the UI, you create a new element and pass it to the renderer.

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Embedding Expressions

You can embed any JavaScript expression in JSX by wrapping it in curly braces: 
const name = 'Alice';
const element = <h1>Hello, {name}!</h1>;

function formatName(user) {
  return user.firstName + ' ' + user.lastName;
}

const user = {
  firstName: 'Harper',
  lastName: 'Perez'
};

const greeting = (
  <h1>
    Hello, {formatName(user)}!
  </h1>
);
You can use any valid JavaScript expression: 
<div>
  {/* Arithmetic */}
  <p>2 + 2 = {2 + 2}</p>
  
  {/* Function calls */}
  <p>{getUserName()}</p>
  
  {/* Conditional expressions */}
  <p>{isLoggedIn ? 'Welcome back!' : 'Please sign in'}</p>
  
  {/* Array methods */}
  <ul>
    {items.map(item => <li key={item.id}>{item.name}</li>)}
  </ul>
</div>

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JSX Attributes

You can specify attributes using either quotes for string literals or curly braces for expressions: 
// String literals
const element = <img src="avatar.png" alt="User avatar" />;

// Expressions
const element = <img src={user.avatarUrl} alt={user.name} />;

// Multiple attributes
const element = (
  <button
    className="btn btn-primary"
    onClick={handleClick}
    disabled={isLoading}
  >
    Submit
  </button>
);

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Attribute Naming

Since JSX is JavaScript, attribute names use camelCase:
  • className instead of class
  • htmlFor instead of for
  • onClick instead of onclick
  • tabIndex instead of tabindex
<label htmlFor="email" className="form-label">
  Email Address
</label>
<input
  id="email"
  className="form-input"
  tabIndex={0}
  onChange={handleChange}
/>

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Special Props: key and ref

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The key Prop

When creating lists of elements, you should provide a unique key prop to help React identify which items have changed: 
const items = [
  { id: 1, text: 'First' },
  { id: 2, text: 'Second' },
  { id: 3, text: 'Third' }
];

const list = (
  <ul>
    {items.map(item => (
      <li key={item.id}>{item.text}</li>
    ))}
  </ul>
);
The key prop is special and is not passed to your component as a prop. If you need the same value in your component, pass it explicitly with a different name:
<Item key={item.id} itemId={item.id} />

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The ref Prop

In React 19+, ref is now a regular prop (though it’s still special in how React handles it): 
function MyInput({ ref }) {
  return <input ref={ref} />;
}

function Parent() {
  const inputRef = useRef(null);
  
  return <MyInput ref={inputRef} />;
}

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JSX Represents Objects

Because JSX compiles to function calls, it can be used anywhere JavaScript expressions can be used: 
// Return from functions
function getGreeting(user) {
  if (user) {
    return <h1>Hello, {user.name}!</h1>;
  }
  return <h1>Hello, Stranger.</h1>;
}

// Assign to variables
const element = <h1>Hello!</h1>;

// Store in arrays
const elements = [
  <h1 key="1">First</h1>,
  <h2 key="2">Second</h2>
];

// Pass as arguments
function render(element) {
  root.render(element);
}
render(<App />);

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JSX Children

JSX tags can contain children: 
// Self-closing tags for empty elements
<img src="photo.jpg" />

// Opening and closing tags for elements with children
<div>
  <h1>Title</h1>
  <p>Description</p>
</div>
Children can be: 
// Text
<p>Hello, world!</p>

// Other JSX elements
<div>
  <Header />
  <Content />
</div>

// Expressions
<p>{calculateValue()}</p>

// Mixed
<div>
  Hello, {name}!
  <br />
  Welcome to React.
</div>

// Arrays (with keys)
<ul>
  {items.map(item => <li key={item.id}>{item.text}</li>)}
</ul>

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JSX Prevents Injection Attacks

React DOM escapes any values embedded in JSX before rendering them, preventing XSS (cross-site-scripting) attacks: 
const userInput = '<script>alert("XSS")</script>';

// This is safe - the string will be escaped
const element = <div>{userInput}</div>;
// Renders: <div>&lt;script&gt;alert("XSS")&lt;/script&gt;</div>
Everything in JSX is converted to a string before being rendered, so you can never inject anything that’s not explicitly written in your application.

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Fragments

Fragments let you group multiple children without adding extra DOM nodes: 
function Table() {
  return (
    <table>
      <tbody>
        <tr>
          <Columns />
        </tr>
      </tbody>
    </table>
  );
}

function Columns() {
  return (
    <>
      <td>Column 1</td>
      <td>Column 2</td>
    </>
  );
}
The short syntax <>...</> is equivalent to <React.Fragment>...</React.Fragment>. Use the full syntax when you need to pass a key: 
<React.Fragment key={item.id}>
  <dt>{item.term}</dt>
  <dd>{item.description}</dd>
</React.Fragment>

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Next Steps

Props and State

Learn how to pass data and manage state

Rendering

Understand how React renders elements to the DOM