Next.js 15 Fullstack Web Apps · レッスン

Client Componentsとインタラクティブ性

クライアント側の状態を必要とするインタラクティブなUI要素に、Client Componentsをいつ、どのように使用するかを学びます。

「Client Componentsとインタラクティブ性」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2/4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。

Meet Client Components

Welcome to Client Components! In Next.js with the App Router, components are Server Components by default. But sometimes, you need interactivity directly in the browser.

Client Components are special components that run on the client-side, meaning in the user's web browser. They are essential for adding dynamic features to your application.

Server vs. Client: A Quick Look

Let's quickly recap: Server Components render on the server, ideal for static content, data fetching, and reducing client-side JavaScript.

Client Components, on the other hand, allow for:

User interaction (clicks, input changes)
Managing state with React Hooks (useState, useEffect)
Accessing browser-specific APIs (localStorage, window)

They bring your UI to life!

Why Use Client Components?

You should reach for a Client Component when your UI needs to be interactive. Think about common web features:

Counters & Toggles: Any UI element that changes based on user input.
Forms: Input fields, validation, and submission logic.
Animations: Client-side animations and transitions.
Real-time Updates: Components that respond to WebSocket events.
Browser-specific Logic: Anything that directly manipulates the DOM or uses browser APIs.

The 'use client' Directive

To tell Next.js that a component should be a Client Component, you simply add the "use client" directive at the very top of the file, before any imports.

This directive is crucial! It marks the boundary between server-side and client-side code. Everything below it (including imported modules) will be treated as part of the client bundle.

Building an Interactive Counter

Here's a classic example: a simple counter. Notice the "use client" directive and the use of React's useState Hook to manage the count.

The button's onClick handler updates the state, triggering a re-render in the browser.

    "use client";
    import React, { useState } from 'react';

    export default function Counter() {
      const [count, setCount] = useState(0);

      return (
        <div>
          <p>Count: {count}</p>
          <button onClick={() => setCount(count + 1)}>
            Increment
          </button>
        </div>
      );
    }

Handling More Interactions

Beyond simple clicks, Client Components excel at managing input from users. Here's an example using an input field to mirror text as you type.

The onChange event handler updates the component's state every time the input value changes.

    "use client";
    import React, { useState } from 'react';

    export default function InputMirror() {
      const [text, setText] = useState('');

      return (
        <div>
          <input
            type="text"
            value={text}
            onChange={(e) => setText(e.target.value)}
            placeholder="Type here..."
          />
          <p>You typed: {text}</p>
        </div>
      );
    }

Client-Side Effects with useEffect

The useEffect Hook is your go-to for side effects that need to run in the browser, like setting up timers, fetching data on component mount, or subscribing to external events.

This timer component uses useEffect to update its seconds state every second.

    "use client";
    import React, { useState, useEffect } from 'react';

    export default function Timer() {
      const [seconds, setSeconds] = useState(0);

      useEffect(() => {
        const interval = setInterval(() => {
          setSeconds(prev => prev + 1);
        }, 1000);
        return () => clearInterval(interval); // Cleanup function
      }, []); // Empty dependency array means run once on mount

      return (
        <p>Timer: {seconds} seconds</p>
      );
    }

Accessing Browser Features

Since Client Components run in the browser, they have full access to browser-specific APIs and global objects like window and localStorage.

This example shows how to read from and write to localStorage, which is unavailable in Server Components.

    "use client";
    import React, { useState, useEffect } from 'react';

    export default function LocalStorageDisplay() {
      const [storedValue, setStoredValue] = useState('');

      useEffect(() => {
        // This code runs only on the client after hydration
        const item = localStorage.getItem('myKey');
        if (item) {
          setStoredValue(item);
        }
      }, []);

      const saveValue = () => {
        localStorage.setItem('myKey', 'Hello from CoddyKit!');
        setStoredValue('Hello from CoddyKit!');
      };

      return (
        <div>
          <p>Stored: {storedValue || 'Nothing yet!'}</p>
          <button onClick={saveValue}>
            Save to LocalStorage
          </button>
        </div>
      );
    }

Nesting Client Components

You can nest Client Components within other Client Components, and even within Server Components. A Server Component can render a Client Component as a child.

When a Server Component renders a Client Component, the Client Component's JavaScript is sent to the browser, and it 'hydrates' (becomes interactive) once loaded.

This allows you to build complex UIs where static parts are server-rendered, and interactive parts are client-rendered.

Client Component Check

Time to test your understanding of Client Components!

Client Components: Key Takeaways

You've mastered the basics of Client Components!

They are marked with "use client" at the top of the file.
Essential for interactivity, state management with Hooks (useState, useEffect), and event handling.
They can access browser-specific APIs (e.g., localStorage, window).
Server Components can render Client Components, allowing for a mix of server and client rendering.

Next, we'll explore more advanced data fetching patterns, leveraging both Server and Client Components effectively!

よくある質問

「Client Componentsとインタラクティブ性」レッスンは無料ですか？

はい。「Client Componentsとインタラクティブ性」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。

「Client Componentsとインタラクティブ性」で何を学びますか？

クライアント側の状態を必要とするインタラクティブなUI要素に、Client Componentsをいつ、どのように使用するかを学びます。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2/4です。

「Client Componentsとインタラクティブ性」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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