When to use client components in NextJS - Mini Project: Build & Apply

When to Use Client Components in Next.js

📖 Scenario: You are building a simple Next.js app that shows a greeting message and a button to toggle the greeting text color. The greeting text color changes only when the user clicks the button.

🎯 Goal: Build a Next.js component that uses a client component to handle user interaction (button click) and state (color toggle) while the rest of the page remains server-rendered.

📋 What You'll Learn

Create a server component that renders a greeting message

Create a client component that toggles the greeting text color on button click

Use React useState hook inside the client component

Use the client component inside the server component

Add a button with an accessible aria-label for toggling color

💡 Why This Matters

🌍 Real World

Many Next.js apps use server components for fast loading and client components for interactive parts like buttons, forms, and toggles.

💼 Career

Understanding when to use client components is essential for building efficient, accessible, and maintainable Next.js applications.

1

Create the Server Component with Greeting Text

Create a server component called GreetingServer that returns a <div> with the text Hello from Server Component! inside a <p> tag.

NextJS

# Create the GreetingServer component

function GreetingServer() {
  // Your code here
}

Hint

Server components do not use hooks or client-side state.

2

Create the Client Component with Color Toggle State

Create a client component called ColorToggle that uses useState to toggle a boolean state called isRed starting as false. The component should return a <p> with text Color toggled! styled with red color if isRed is true, otherwise black. Also add a <button> with aria-label="Toggle color" that toggles isRed when clicked.

NextJS

function GreetingServer() {
  return (
    
      Hello from Server Component!
    
  );
}

export default GreetingServer;

// Create the ColorToggle client component below

// Your code here

Hint

Client components must start with "use client"; at the top.

3

Import and Use Client Component inside Server Component

Update GreetingServer to render <ColorToggle /> below the greeting text inside the <div>.

NextJS

"use client";

import { useState } from "react";

function ColorToggle() {
  const [isRed, setIsRed] = useState(false);

  return (
    
      Color toggled!
       setIsRed(!isRed)}>
        Toggle Color
      
    
  );
}

// Use  inside GreetingServer

function GreetingServer() {
  return (
    
      Hello from Server Component!
      {/* Your code here */}
    
  );
}

export default GreetingServer;

Hint

Client components can be imported and used inside server components like normal React components.

4

Add Accessibility and Final Touches

Ensure the button in ColorToggle has an aria-label="Toggle color" attribute for accessibility. Also, add a tabIndex="0" to the button to allow keyboard navigation.

NextJS

"use client";

import { useState } from "react";

function ColorToggle() {
  const [isRed, setIsRed] = useState(false);

  return (
    
      Color toggled!
       setIsRed(!isRed)}>
        Toggle Color
      
    
  );
}

function GreetingServer() {
  return (
    
      Hello from Server Component!
      
    
  );
}

export default GreetingServer;

// Add tabIndex="0" to the button for keyboard navigation
// Your code here

Hint

Adding tabIndex={0} makes the button reachable by keyboard users.

Practice

1. Which of the following is the main reason to use a client component in Next.js?

easy

A. To make parts of the app interactive and respond to user actions

B. To improve server-side rendering speed

C. To reduce the size of the JavaScript bundle sent to the browser

D. To fetch data from the database on the server

Solution

1. Step 1: Understand client components purpose

   Client components run in the browser and handle interactivity like clicks and animations.

2. Step 2: Compare options with this purpose

   Only To make parts of the app interactive and respond to user actions matches making the app interactive and responding to user actions.

3. Final Answer:

   To make parts of the app interactive and respond to user actions -> Option A

4. Quick Check:

   Client components = interactivity [OK]

Hint: Client components run in browser for interactivity [OK]

Common Mistakes:

• Thinking client components improve server rendering
• Confusing client components with server data fetching
• Believing client components reduce bundle size

2. Which is the correct way to mark a Next.js component as a client component?

easy

A. Wrap the component in <Client></Client> tags

B. Name the file with a .client.js extension

C. Add "use client"; at the top of the file

D. Add client: true in the component props

Solution

1. Step 1: Recall Next.js client component syntax

   Next.js requires "use client"; at the top of the file to mark it as a client component.

2. Step 2: Check other options

   Options A, C, and D are not valid Next.js syntax for client components.

3. Final Answer:

   Add "use client"; at the top of the file -> Option C

4. Quick Check:

   Client component marker = "use client"; [OK]

Hint: Remember: "use client"; at file top marks client component [OK]

Common Mistakes:

• Trying to use special tags to mark client components
• Using file extensions to indicate client components
• Passing props to mark client components

3. What will happen if you try to use useState in a Next.js server component without marking it as a client component?

medium

A. You will get a runtime error because hooks like useState only work in client components

B. The state will be shared across all users

C. The app will render normally without errors

D. The component will automatically become a client component

Solution

1. Step 1: Understand hook usage rules

   Hooks like useState only work in client components because they run in the browser.

2. Step 2: Check behavior in server components

   If you use useState in a server component without "use client" directive, Next.js throws a runtime error.

3. Final Answer:

   You will get a runtime error because hooks like useState only work in client components -> Option A

4. Quick Check:

   useState in server component = runtime error [OK]

Hint: Hooks need client components or error occurs [OK]

Common Mistakes:

• Assuming hooks work in server components
• Thinking state is shared globally without errors
• Believing components auto-switch to client

4. You wrote a Next.js component with "use client"; but forgot to import useState from React. What will happen?

medium

A. The component will render but state updates won't work

B. The component will work fine without errors

C. Next.js will automatically import useState for you

D. You will get a ReferenceError because useState is not defined

Solution

1. Step 1: Check import requirements for hooks

   Hooks like useState must be explicitly imported from React to be used.

2. Step 2: Analyze missing import effect

   Without importing, JavaScript throws a ReferenceError because useState is undefined.

3. Final Answer:

   You will get a ReferenceError because useState is not defined -> Option D

4. Quick Check:

   Missing import useState = ReferenceError [OK]

Hint: Always import hooks before using them [OK]

Common Mistakes:

• Assuming Next.js auto-imports React hooks
• Ignoring import errors thinking component still works
• Confusing runtime errors with silent failures

5. You have a Next.js app with a server component that fetches data and a button that updates UI state on click. How should you organize components?

hard

A. Make the whole component a client component to handle data fetching and button state

B. Keep data fetching in a server component and create a separate client component for the button with state

C. Use only server components and handle button clicks with server actions

D. Fetch data in the client component and handle button state in the server component

Solution

1. Step 1: Separate concerns for performance and interactivity

   Server components are best for data fetching; client components handle interactivity like button clicks.

2. Step 2: Organize components accordingly

   Keep data fetching in server component and create a small client component for the button with state and event handlers.

3. Final Answer:

   Keep data fetching in a server component and create a separate client component for the button with state -> Option B

4. Quick Check:

   Server fetch + client interactivity = separate components [OK]

Hint: Use server for data, client for interactivity [OK]

Common Mistakes:

• Making entire component client causing slower loads
• Trying to handle UI state in server components
• Fetching data in client causing unnecessary delays