Component composition in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to render the

component inside .

React

function Header() {
  return <h1>Welcome!</h1>;
}

function App() {
  return (
    <div>
      [1]
    </div>
  );
}

Drag options to blanks, or click blank then click option'

A<Header />

B<header />

CHeader()

D<Header></Header>

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase tags like

which is a HTML tag, not the React component.

Calling Header() directly instead of using JSX syntax.

Use

to include the Header component inside App. It is the correct JSX syntax for rendering a component.

2fill in blank

medium

Complete the code to pass a 'title' prop from to

and display it.

React

function Header(props) {
  return <h1>[1]</h1>;
}

function App() {
  return <Header title="Hello!" />;
}

Drag options to blanks, or click blank then click option'

Aprops.title

Btitle

Cthis.props.title

Dprops['title']

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'title' directly without 'props.' prefix.

Using 'this.props' in a functional component.

Use props.title to access the 'title' prop inside the Header component.

3fill in blank

hard

Fix the error in the code to correctly render children inside <Wrapper>.

React

function Wrapper(props) {
  return <div>[1]</div>;
}

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

Drag options to blanks, or click blank then click option'

Aprops.child

Bprops.children

Cchildren

Dthis.props.children

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'props.child' which does not exist.

Using 'this.props.children' in a functional component.

Use props.children to render the nested content inside the Wrapper component.

4fill in blank

hard

Fill both blanks to create a component that wraps content with a section and a class.

React

function SectionWrapper(props) {
  return <section className=[1]>[2]</section>;
}

function App() {
  return (
    <SectionWrapper className="highlight">
      <p>Important info</p>
    </SectionWrapper>
  );
}

Drag options to blanks, or click blank then click option'

Aprops.className

Bprops.children

CclassName

Dchildren

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'className' without 'props.' prefix.

Using 'children' without 'props.' prefix.

Use props.className for the class attribute and props.children to render nested content.

5fill in blank

hard

Fill all three blanks to create a component that renders a list of items passed as props.

React

function ItemList({ [1], [2] }) {
  return (
    <ul>
      {items.map((item, index) => (
        <li key=[3]>{item}</li>
      ))}
    </ul>
  );
}

function App() {
  const items = ['Apple', 'Banana', 'Cherry'];
  return <ItemList items={items} />;
}

Drag options to blanks, or click blank then click option'

Aitems

Bindex

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Not destructuring 'items' from props.

Using 'item' as key instead of 'index'.

Destructure 'items' and 'index' from props and use 'index' as the key in the list.

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