Composition vs inheritance in NextJS - Performance Comparison

Performance: Composition vs inheritance

MEDIUM IMPACT

This concept affects how components are structured and rendered, impacting bundle size, rendering speed, and update efficiency.

Building reusable UI components in Next.js

NextJS

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

function IconButton(props) {
  return <Button {...props}><Icon />{props.label}</Button>;
}

Composition keeps components flat and focused, enabling React to optimize rendering and tree shaking to reduce bundle size.

📈 Performance Gainsingle reflow per render, smaller bundle size, faster interaction response

Building reusable UI components in Next.js

NextJS

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

class IconButton extends Button {
  render() {
    return <button><Icon />{this.props.label}</button>;
  }
}

Inheritance creates deep component trees and forces re-rendering of entire base classes, increasing render time and bundle size.

📉 Performance Costtriggers multiple reflows per inheritance level, adds extra JS to bundle, blocks rendering longer

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Inheritance with deep class hierarchy	High due to nested components	Multiple reflows per inheritance level	Higher paint cost due to complex tree	[X] Bad
Composition with functional components	Lower, flat component tree	Single reflow per update	Lower paint cost with optimized updates	[OK] Good

Rendering Pipeline

Composition allows React to efficiently reconcile component trees by minimizing unnecessary updates, while inheritance can cause larger component trees and more complex reconciliation.

→JavaScript Parsing

→Component Rendering

→Reconciliation

→Paint

⚠️ BottleneckComponent Rendering and Reconciliation

Core Web Vital Affected

INP

This concept affects how components are structured and rendered, impacting bundle size, rendering speed, and update efficiency.

Optimization Tips

1Use composition to keep component trees shallow and manageable.

2Avoid deep inheritance hierarchies to reduce reflows and rendering cost.

3Favor functional components with composition for better bundle size and update speed.

Performance Quiz - 3 Questions

Which pattern generally leads to smaller bundle sizes and faster rendering in Next.js?

A Composition with functional components

B Deep inheritance hierarchies

C Using class components with inheritance

D Mixing both inheritance and composition equally

DevTools: Performance

How to check: Record a performance profile while interacting with components; look for long scripting and rendering tasks caused by deep component trees.

What to look for: Look for long React reconciliation phases and multiple reflows indicating inefficient component structure.

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