React Components and JSX
Understand the building blocks of React applications and how JSX simplifies UI creation.
React Components and JSX is a free Next.js 15 Fullstack Web Apps lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Next.js 15 Fullstack Web Apps learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What are React Components?
Welcome to the world of React! At its heart, React is all about components.
- Think of components as reusable, independent building blocks for your user interface (UI).
- They let you break down complex UIs into smaller, manageable pieces.
- Each component is like a JavaScript function that returns what should be displayed on the screen.
Meet JSX: JavaScript XML
Writing UI in pure JavaScript can get complicated. That's where JSX comes in!
- JSX is a syntax extension for JavaScript that lets you write HTML-like code directly within your JavaScript files.
- It makes defining your UI intuitive and familiar, blending HTML structure with JavaScript's power.
- Browsers don't understand JSX directly, so it gets transformed into regular JavaScript.
JSX: Behind the Scenes
While JSX looks like HTML, it's not. When your code is compiled (or 'transpiled'), JSX is converted into plain JavaScript function calls.
For example, <h1>Hello</h1> becomes something like React.createElement('h1', null, 'Hello').
This conversion is handled automatically by tools like Babel, allowing you to write cleaner, more readable UI code.
Your First Functional Component
In modern React, components are usually written as JavaScript functions. These are called Functional Components.
They take 'props' (short for properties) as input and return JSX, which describes what the UI should look like.
function WelcomeMessage() {
return (
<h1>Hello, CoddyKit!</h1>
);
}
// In a real app, this component would be rendered
// by ReactDOM or a framework like Next.js.
// For now, think of it as a blueprint for UI.JSX Rule 1: Single Root Element
A crucial rule in JSX is that every JSX expression must return a single root element.
This means you can't have two top-level elements directly next to each other. If you need to return multiple elements, wrap them in a single parent element, like a <div>, or use a Fragment (<></>) for a cleaner approach that doesn't add extra nodes to the DOM.
function UserCard() {
return (
<>
<h2>John Doe</h2>
<p>Software Developer</p>
</>
);
}
// Incorrect: return (
// <h2>John Doe</h2>
// <p>Software Developer</p>
// );JSX Rule 2: camelCase Props
While JSX looks like HTML, it's still JavaScript. This means some HTML attributes are renamed to follow JavaScript's camelCase convention.
classbecomesclassNameforbecomeshtmlFor- Event handlers like
onclickbecomeonClick
This helps avoid conflicts with JavaScript reserved keywords.
<div className="container">
<label htmlFor="username">Name:</label>
<input type="text" id="username" />
</div>JavaScript Expressions in Action
One of the most powerful features of JSX is the ability to embed JavaScript expressions directly within your UI using curly braces {}.
You can use variables, perform calculations, or call functions inside these braces. Let's see a simple runnable example of a variable:
function main() {
const appName = "CoddyKit";
console.log(`Welcome to ${appName}!`);
}
main();Self-Closing Elements
In HTML, some tags can be self-closing (like <img> or <input>). In JSX, all elements must either have a closing tag or be self-closing.
If an element has no children, you must close it with a slash before the closing angle bracket, like <img /> or <br />.
<div>
<input type="text" placeholder="Enter your name" />
<br />
<img src="/logo.png" alt="App Logo" />
</div>Composing Components
The real magic of React components is their ability to be composed. You can use one component inside another, building complex UIs from simple, focused pieces.
This promotes reusability and makes your application's structure clear and maintainable. It's like building with LEGO bricks!
function Button() {
return <button>Click Me</button>;
}
function Header() {
return (
<header>
<h1>My Awesome App</h1>
<Button /> {/* Using the Button component */}
</header>
);
}Component & JSX Check
Time for a quick check! Which of the following is a key rule when writing JSX?
Recap: Components & JSX
Great job! In this lesson, you've learned the foundational concepts of React:
- Components are the reusable building blocks of your UI, typically written as functions.
- JSX is a syntax extension for JavaScript that lets you write HTML-like code for your UI.
- Key JSX rules include returning a single root element, using camelCase for attributes (like
className), and self-closing empty tags. - You can embed JavaScript expressions within JSX using curly braces
{}.
Understanding components and JSX is crucial for building any React application, including those with Next.js!
Frequently asked questions
Is the “React Components and JSX” lesson free?
Yes — the full text of “React Components and JSX” is free to read here on the web, and the Next.js 15 Fullstack Web Apps course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Next.js 15 Fullstack Web Apps course, upgrade to CoddyKit PRO.
What will I learn in “React Components and JSX”?
Understand the building blocks of React applications and how JSX simplifies UI creation. You practise Next.js 15 Fullstack Web Apps with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Next.js 15 Fullstack Web Apps?
No prior experience is required. Next.js 15 Fullstack Web Apps on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “React Components and JSX” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Next.js 15 Fullstack Web Apps lesson?
Yes. Every Next.js 15 Fullstack Web Apps lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.