Next.js 15 Fullstack Web Apps · Lesson

Client Components and Interactivity

Learn when and how to use Client Components for interactive UI elements that require client-side state.

Client Components and Interactivity is a free Next.js 15 Fullstack Web Apps lesson on CoddyKit — lesson 2 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.

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!

Frequently asked questions

Is the “Client Components and Interactivity” lesson free?

Yes — the full text of “Client Components and Interactivity” 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 “Client Components and Interactivity”?

Learn when and how to use Client Components for interactive UI elements that require client-side state. 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 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Client Components and Interactivity” 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.

All lessons in this course

← Back to Next.js 15 Fullstack Web Apps