Composition vs inheritance in NextJS

Introduction

Composition and inheritance are two ways to build components by reusing code. Composition means putting smaller parts together. Inheritance means creating a new component based on an existing one.

When you want to build a UI by combining simple components like buttons and cards.

When you want to share common behavior but keep components flexible.

When you want to avoid complex component hierarchies that are hard to maintain.

When you want to customize parts of a component without changing its core.

When you want clearer and easier-to-understand code by using small building blocks.

Syntax

NextJS

/* Composition example */
function Button({ children }) {
  return <button>{children}</button>;
}

function Dialog() {
  return (
    <div>
      <h2>Title</h2>
      <Button>Click me</Button>
    </div>
  );
}

/* Inheritance example (not common in React/Next.js) */
import React from 'react';
class BaseButton extends React.Component {
  render() {
    return <button>{this.props.label}</button>;
  }
}

class SpecialButton extends BaseButton {
  render() {
    return <button style={{ color: 'red' }}>{this.props.label}</button>;
  }
}

In Next.js and React, composition is preferred over inheritance.

Composition uses props and children to combine components.

Examples

This shows composition by using the WelcomeMessage component inside Page.

NextJS

function WelcomeMessage({ name }) {
  return <p>Welcome, {name}!</p>;
}

function Page() {
  return (
    <div>
      <WelcomeMessage name="Alice" />
    </div>
  );
}

This shows inheritance by extending Button to create DangerButton with red background.

NextJS

import React from 'react';
class Button extends React.Component {
  render() {
    return <button>{this.props.label}</button>;
  }
}

class DangerButton extends Button {
  render() {
    return <button style={{ backgroundColor: 'red' }}>{this.props.label}</button>;
  }
}

Sample Program

This example shows composition by building UserInfo from Avatar and user data. It also shows inheritance by creating RedButton from Button. In Next.js, composition is the recommended way.

NextJS

import React from 'react';

// Composition: Small reusable components
function Avatar({ user }) {
  return <img src={user.avatarUrl} alt={`${user.name} avatar`} style={{ width: '50px', borderRadius: '50%' }} />;
}

function UserInfo({ user }) {
  return (
    <div style={{ display: 'flex', alignItems: 'center', gap: '1rem' }}>
      <Avatar user={user} />
      <div>{user.name}</div>
    </div>
  );
}

// Inheritance example (rare in React, shown for comparison)
class Button extends React.Component {
  render() {
    return <button>{this.props.label}</button>;
  }
}

class RedButton extends Button {
  render() {
    return <button style={{ color: 'white', backgroundColor: 'red' }}>{this.props.label}</button>;
  }
}

export default function App() {
  const user = { name: 'Jane Doe', avatarUrl: 'https://via.placeholder.com/50' };

  return (
    <div style={{ padding: '1rem', fontFamily: 'Arial, sans-serif' }}>
      <h1>User Profile</h1>
      <UserInfo user={user} />
      <div style={{ marginTop: '1rem' }}>
        <Button label="Normal Button" />
        <RedButton label="Red Button" />
      </div>
    </div>
  );
}

Important Notes

React and Next.js favor composition because it keeps components simple and flexible.

Inheritance can make code harder to follow and is rarely used in React.

Use composition to build UI like Lego blocks, combining small pieces into bigger ones.

Summary

Composition means building components by combining smaller parts.

Inheritance means creating new components by extending existing ones.

In Next.js, prefer composition for clearer and easier code.

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