Composition vs inheritance in NextJS - When to Use Which

The Big Idea

Discover why mixing small parts beats copying big chunks in your code!

The Scenario

Imagine building a website where you copy and paste code to reuse features, then try to fix bugs in many places.

The Problem

Copying code everywhere makes updates slow and causes mistakes. Inheritance chains can get tangled and hard to follow.

The Solution

Composition lets you build small pieces and combine them flexibly, avoiding complex inheritance and making code easier to manage.

Before vs After

✗ Before

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

✓ After

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

What It Enables

Composition enables building UI from simple, reusable parts that fit together cleanly and adapt easily.

Real Life Example

Think of a car made from parts like wheels and engine instead of inheriting from a generic vehicle; you can swap parts easily.

Key Takeaways

Inheritance can create rigid, complex code structures.

Composition builds flexible, reusable components.

Next.js apps benefit from composition for clearer, maintainable UI.

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