Composition vs inheritance in NextJS - Hands-On Comparison

Composition vs Inheritance in Next.js Components

📖 Scenario: You are building a simple Next.js app that shows user profiles. You want to understand how to reuse UI parts by using composition instead of inheritance.

🎯 Goal: Create two components: a ProfileCard that shows user info, and a UserList that uses ProfileCard to display multiple users. This will demonstrate composition in Next.js.

📋 What You'll Learn

Create a list of user objects with name and email

Create a configuration variable for the title of the user list

Use composition by creating a ProfileCard component that receives user data as props

Create a UserList component that renders multiple ProfileCard components

Use functional components and hooks as needed

Follow Next.js 14+ App Router conventions with React 19+

💡 Why This Matters

🌍 Real World

Building user profile lists or any repeated UI elements in modern web apps using React and Next.js.

💼 Career

Understanding composition is key for React developers to write clean, reusable components and avoid complex inheritance hierarchies.

1

Create the user data array

Create a constant array called users with these exact objects: { id: 1, name: 'Alice', email: '[e-mail]' }, { id: 2, name: 'Bob', email: '[e-mail]' }, and { id: 3, name: 'Charlie', email: '[e-mail]' }.

NextJS

// Create the users array with the specified objects
// Your code here

Hint

Use const users = [ ... ] with objects inside the array.

2

Add a title configuration variable

Create a constant string called listTitle and set it to 'User Profiles'.

NextJS

const users = [
  { id: 1, name: 'Alice', email: '[e-mail]' },
  { id: 2, name: 'Bob', email: '[e-mail]' },
  { id: 3, name: 'Charlie', email: '[e-mail]' }
];

// Create a constant listTitle with value 'User Profiles'
// Your code here

Hint

Use const listTitle = 'User Profiles'; to create the title variable.

3

Create the ProfileCard component

Create a React functional component called ProfileCard that takes a user prop. It should return a <section> with a <h2> showing user.name and a <p> showing user.email.

NextJS

const users = [
  { id: 1, name: 'Alice', email: '[e-mail]' },
  { id: 2, name: 'Bob', email: '[e-mail]' },
  { id: 3, name: 'Charlie', email: '[e-mail]' }
];

const listTitle = 'User Profiles';

// Create ProfileCard component here
// Your code here

Hint

Use function ProfileCard({ user }) { return ( ... ) } and JSX tags inside.

4

Create the UserList component using ProfileCard

Create a React functional component called UserList. It should return a <main> containing an <h1> with listTitle and map over users to render a <ProfileCard> for each user. Use user.id as the key.

NextJS

const users = [
  { id: 1, name: 'Alice', email: '[e-mail]' },
  { id: 2, name: 'Bob', email: '[e-mail]' },
  { id: 3, name: 'Charlie', email: '[e-mail]' }
];

const listTitle = 'User Profiles';

function ProfileCard({ user }) {
  return (
    
      {user.name}
      {user.email}
    
  );
}

// Create UserList component here
// Your code here

Hint

Use users.map(user => <ProfileCard key={user.id} user={user} />) inside the UserList component.

Practice

1. In Next.js, what is the main advantage of using composition over inheritance when building components?

easy

A. It allows combining small components for better reuse and clarity.

B. It requires less code to extend existing components.

C. It automatically manages component lifecycle methods.

D. It enforces strict type checking at compile time.

Solution

1. Step 1: Understand composition in Next.js

   Composition means building components by combining smaller, reusable parts to create complex UI.

2. Step 2: Compare with inheritance

   Inheritance extends existing components but can lead to complex and less clear code structures.

3. Final Answer:

   It allows combining small components for better reuse and clarity. -> Option A

4. Quick Check:

   Composition = combining parts for clarity [OK]

Hint: Composition combines parts; inheritance extends one component [OK]

Common Mistakes:

• Thinking inheritance is simpler for UI reuse
• Confusing lifecycle management with composition
• Assuming inheritance is preferred in Next.js

2. Which of the following is the correct way to use composition in a Next.js functional component?

easy

A. function Parent() { return <Child>; }

B. function Parent() { return <Child></Child>; }

C. function Parent() { return Child(); }

D. class Parent extends Child {}

Solution

1. Step 1: Identify JSX syntax for composition

   Composition uses JSX tags to include child components inside parent components.

2. Step 2: Check options for correct JSX usage

   function Parent() { return <Child></Child>; } correctly uses <Child></Child> tags, which is valid JSX. function Parent() { return <Child>; } uses opening tag but missing closing tag. class Parent extends Child {} uses inheritance syntax, not composition. function Parent() { return Child(); } calls Child as a function, which is not standard JSX.

3. Final Answer:

   <Child></Child> JSX tags for composition -> Option B

4. Quick Check:

   JSX tags = composition [OK]

Hint: Use JSX tags to compose components, not class inheritance [OK]

Common Mistakes:

• Using class extends syntax for composition
• Calling components as plain functions without JSX
• Missing closing tag for JSX element

3. Given the code below, what will be rendered on the screen?

function Button({ label }) {
  return <button>{label}</button>;
}

function Toolbar() {
  return <div><Button label="Save" /></div>;
}

medium

A. Save

B. Save

C. Toolbar

D. Nothing renders due to syntax error

Solution

1. Step 1: Analyze Button component output

   The Button component returns a button element with the label text inside it.

2. Step 2: Analyze Toolbar component output

   Toolbar renders a div containing the Button component with label "Save", so the button with text "Save" appears inside the div.

3. Final Answer:

   <button>Save</button> -> Option A

4. Quick Check:

   Button label = Save [OK]

Hint: Look inside child component's return for rendered output [OK]

Common Mistakes:

• Confusing parent div content with button content
• Assuming Toolbar text appears inside button
• Thinking syntax error exists due to JSX

4. Identify the error in this Next.js component using inheritance:

class BaseButton extends React.Component {
  render() {
    return <button>Base</button>;
  }
}

function SubmitButton() {
  return <BaseButton label="Submit" />;
}

medium

A. SubmitButton should extend BaseButton instead of being a function.

B. BaseButton cannot be used inside a function component.

C. React.Component cannot be extended in Next.js.

D. BaseButton does not use the label prop in render method.

Solution

1. Step 1: Check BaseButton usage of props

   BaseButton receives a label prop but does not use it in the render method, so label is ignored.

2. Step 2: Check SubmitButton usage

   SubmitButton returns BaseButton with label prop, but since BaseButton ignores label, the button always shows "Base".

3. Final Answer:

   BaseButton does not use the label prop in render method. -> Option D

4. Quick Check:

   Unused prop = bug [OK]

Hint: Check if props passed are actually used in render [OK]

Common Mistakes:

• Thinking functional components can't use class components
• Assuming inheritance is disallowed in Next.js
• Confusing extending with composing

5. You want to create a reusable card component in Next.js that can display different content inside. Which approach best uses composition to achieve this?

function Card({ children }) {
  return <div className="card">{children}</div>;
}

function Profile() {
  return (
    <Card>
      <h2>User Name</h2>
      <p>User details here</p>
    </Card>
  );
}

hard

A. Pass content as a string prop to Card and render inside.

B. Extend Card component with Profile using class inheritance.

C. Use children prop to pass nested components inside Card.

D. Create separate CardProfile component that duplicates Card code.

Solution

1. Step 1: Understand composition with children prop

   The children prop allows passing any nested JSX inside Card, making it reusable for different content.

2. Step 2: Compare other options

   Extending with inheritance is less flexible and not recommended. Passing content as string limits flexibility. Duplicating code reduces reuse.

3. Final Answer:

   Use children prop to pass nested components inside Card. -> Option C

4. Quick Check:

   children prop = composition [OK]

Hint: Use children prop to nest content inside reusable components [OK]

Common Mistakes:

• Trying to extend components with class inheritance
• Passing content as plain strings instead of JSX
• Duplicating component code instead of composing