Next.js 15 Fullstack Web Apps · レッスン

ReactコンポーネントとJSX

Reactアプリケーションの構成要素と、JSXによってUI作成が簡潔になる仕組みを理解します。

「ReactコンポーネントとJSX」はCoddyKit上の無料Next.js 15 Fullstack Web Appsレッスンです。これはレッスン1/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはNext.js 15 Fullstack Web Apps学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Next.js 15 Fullstack Web Appsコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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.

class becomes className
for becomes htmlFor
Event handlers like onclick become onClick

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!

よくある質問

「ReactコンポーネントとJSX」レッスンは無料ですか？

はい。「ReactコンポーネントとJSX」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Next.js 15 Fullstack Web Appsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Next.js 15 Fullstack Web Appsコースには全4レッスンが含まれています。

「ReactコンポーネントとJSX」で何を学びますか？

Reactアプリケーションの構成要素と、JSXによってUI作成が簡潔になる仕組みを理解します。ブラウザで直接実行するハンズオンコードでNext.js 15 Fullstack Web Appsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Next.js 15 Fullstack Web Appsを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのNext.js 15 Fullstack Web Appsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン1/4です。

「ReactコンポーネントとJSX」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このNext.js 15 Fullstack Web Appsレッスンでコードを書いて実行できますか？

はい。すべてのNext.js 15 Fullstack Web Appsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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