Component composition in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is component composition in React?

Component composition means building complex UI by combining smaller, reusable components together, like putting Lego blocks to make a bigger model.

beginner

How do you pass content from a parent component to a child component in React?

You pass content using props. For example, you can pass JSX or data as props to child components to customize what they show.

intermediate

What is the purpose of children prop in React component composition?

The children prop lets a component receive and render nested elements inside it, making it flexible to wrap or decorate other components.

intermediate

Why is component composition preferred over inheritance in React?

React favors composition because it is simpler and more flexible. Components can be combined in many ways without complex class hierarchies, making code easier to understand and reuse.

beginner

Show a simple example of component composition using a Card component wrapping a Button component.

Example:<br><pre>function Card({ children }) {
  return &lt;div className="card"&gt;{children}&lt;/div&gt;;
}

function Button() {
  return &lt;button&gt;Click me&lt;/button&gt;;
}

function App() {
  return &lt;Card&gt;&lt;Button /&gt;&lt;/Card&gt;;
}</pre>

What does the children prop represent in React?

A A method to update props

B The nested elements inside a component

C A special state variable

D A lifecycle hook

The children prop contains whatever elements are nested inside a component when it is used.

Which of these is a benefit of component composition?

A It allows building UI from small reusable parts

B It forces use of class inheritance

C It prevents passing props

D It disables nesting components

Component composition helps build complex UI by combining small reusable components.

How do you pass a button element as content to a Card component?

A <Card><Button /></Card>

B <Card button="Click" />

C <Button><Card /></Button>

D You cannot pass elements as content

You can nest the <Button /> inside <Card> tags to pass it as children.

Why does React prefer composition over inheritance?

A Composition disables props

B Inheritance is faster

C Inheritance is required for hooks

D Composition is simpler and more flexible

React favors composition because it is easier to manage and reuse components without complex class hierarchies.

Which prop is commonly used to pass nested JSX to a component?

A state

B key

C children

D ref

The children prop is the standard way to pass nested JSX elements to a component.

Explain in your own words what component composition is and why it is useful in React.

Think about how Lego blocks build bigger models.

Describe how the children prop works and give an example of how you would use it in a React component.

Imagine a box component that can hold anything inside.

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