Component composition in React

Introduction

Component composition helps you build complex UIs by combining small, simple pieces. It makes your code easier to understand and reuse.

You want to build a page by putting together smaller parts like header, footer, and content.

You want to reuse a button style in many places but with different text or actions.

You want to create a layout that wraps other components inside it.

You want to pass content from one component to another to customize what shows up.

You want to keep your code organized by splitting big components into smaller ones.

Syntax

React

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

function Child({ children }) {
  return <div>{children}</div>;
}

Use children prop to pass nested components inside another component.

Components can be functions that return JSX elements.

Examples

This example shows a Card component wrapping some content passed as children.

React

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

function App() {
  return (
    <Card>
      <h2>Title</h2>
      <p>This is inside the card.</p>
    </Card>
  );
}

Here, Toolbar composes two Button components with different labels and actions.

React

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

function Toolbar() {
  return (
    <div>
      <Button label="Save" onClick={() => alert('Saved!')} />
      <Button label="Cancel" onClick={() => alert('Canceled!')} />
    </div>
  );
}

This example shows passing components as props to build a page layout.

React

function Layout({ header, content, footer }) {
  return (
    <div>
      <header>{header}</header>
      <main>{content}</main>
      <footer>{footer}</footer>
    </div>
  );
}

function Page() {
  return (
    <Layout
      header={<h1>Welcome</h1>}
      content={<p>This is the main content.</p>}
      footer={<small>© 2024</small>}
    />
  );
}

Sample Program

This React app shows how Container wraps other components and content. The Greeting component is composed inside Container along with a paragraph.

React

import React from 'react';

function Container({ children }) {
  return <section style={{ border: '2px solid #4a90e2', padding: '1rem', borderRadius: '0.5rem' }}>{children}</section>;
}

function Greeting({ name }) {
  return <h2>Hello, {name}!</h2>;
}

function App() {
  return (
    <Container>
      <Greeting name="Alice" />
      <p>Welcome to the component composition example.</p>
    </Container>
  );
}

export default App;

Important Notes

Always use semantic HTML elements inside components for better accessibility.

Use the children prop to pass nested content easily.

Component composition helps keep your UI code clean and reusable.

Summary

Component composition means building UI by combining smaller components.

Use children or props to pass components inside others.

This approach makes your code easier to read, reuse, and maintain.

Practice

1. What is the main purpose of component composition in React?

easy

A. To make components run faster by skipping rendering

B. To write all UI code in a single component

C. To avoid using props between components

D. To build UI by combining smaller components

Solution

1. Step 1: Understand component composition

   Component composition means creating complex UI by combining smaller, reusable components.

2. Step 2: Compare options

   Only To build UI by combining smaller components correctly describes this purpose. Others are incorrect or unrelated.

3. Final Answer:

   To build UI by combining smaller components -> Option D

4. Quick Check:

   Component composition = combining smaller components [OK]

Hint: Think: How do you build complex UI simply? [OK]

Common Mistakes:

• Thinking composition means one big component
• Confusing composition with performance optimization
• Ignoring the use of smaller components

2. Which of the following is the correct way to pass child components in React?

easy

A. Child

B. {Child}

C.

D. Child()

Solution

1. Step 1: Identify correct JSX syntax for children

   In React, child components are passed inside the parent tags as JSX elements, like <Parent><Child /></Parent>.

2. Step 2: Evaluate options

   <Parent><Child /></Parent> uses correct JSX syntax. Options A, C, and D are invalid or incorrect ways to pass components.

3. Final Answer:

   <Parent><Child /></Parent> -> Option C

4. Quick Check:

   Children passed as JSX elements = <Parent><Child /></Parent> [OK]

Hint: Children go inside parent tags as JSX elements [OK]

Common Mistakes:

• Passing children as plain text instead of components
• Using curly braces incorrectly around components
• Calling components like functions inside JSX

3. What will be rendered by this React code?

function Wrapper({ children }) {
  return <div className="wrapper">{children}</div>;
}

function App() {
  return (
    <Wrapper>
      <p>Hello</p>
      <p>World</p>
    </Wrapper>
  );
}

medium

A. HelloWorld

B. HelloWorld

C. HelloWorld

D. Error: children must be a single element

Solution

1. Step 1: Understand Wrapper component

   Wrapper returns a div with class "wrapper" that renders its children inside.

2. Step 2: Check children passed to Wrapper

   App passes two <p> elements as children, so both appear inside the div.

3. Final Answer:

   <div class="wrapper"><p>Hello</p><p>World</p></div> -> Option A

4. Quick Check:

   Children rendered inside wrapper div = <div class="wrapper"><p>Hello</p><p>World</p></div> [OK]

Hint: Children inside Wrapper appear inside its div container [OK]

Common Mistakes:

• Thinking children must be a single element
• Expecting children to concatenate as text
• Ignoring the wrapper div output

4. Identify the error in this React component composition:

function Container(props) {
  return <div>{props.children}</div>;
}

function App() {
  return (
    <Container>
      <h1>Title</h1>
      <h2>Subtitle</h2>
    </Container>
  );
}

// Usage
ReactDOM.render(<App />, document.getElementById('root'));

medium

A. No error; this is valid composition

B. props.children must be wrapped in a fragment

C. Container must return a single child element

D. Cannot pass multiple children without keys

Solution

1. Step 1: Review Container component

   Container returns a div wrapping props.children, which can be multiple elements.

2. Step 2: Check usage in App

   App passes two headings as children, which is valid JSX inside a div.

3. Final Answer:

   No error; this is valid composition -> Option A

4. Quick Check:

   Multiple children inside a div is valid JSX [OK]

Hint: Multiple children inside a div are allowed without extra wrappers [OK]

Common Mistakes:

• Thinking multiple children need a fragment wrapper
• Confusing React rules about keys with composition errors
• Assuming props.children must be a single element

5. You want to create a reusable Card component that accepts a header and footer as props and displays children content in the middle. Which is the best way to compose this component?

hard

A. Render header and footer inside children and pass content as a separate prop

B. Use props.header and props.footer to render header and footer, and props.children for content inside a container div

C. Pass header and footer as children and ignore props.children

D. Use only props.children and expect header/footer inside it

Solution

1. Step 1: Understand Card component needs

   Card should render header, children content, and footer separately for reuse and clarity.

2. Step 2: Evaluate composition options

   Use props.header and props.footer to render header and footer, and props.children for content inside a container div correctly uses props.header and props.footer for fixed parts and props.children for flexible content.

3. Final Answer:

   Use props.header and props.footer to render header and footer, and props.children for content inside a container div -> Option B

4. Quick Check:

   Separate header/footer props + children content = Use props.header and props.footer to render header and footer, and props.children for content inside a container div [OK]

Hint: Use props for fixed parts, children for flexible content [OK]

Common Mistakes:

• Passing header/footer as children causing confusion
• Ignoring children prop for main content
• Mixing content and layout in children prop