Component composition in React - Mini Project: Build & Apply

Component Composition in React

📖 Scenario: You are building a simple user profile page. The page shows a user's name and a short bio. To keep your code clean and reusable, you want to split the page into smaller pieces called components.

🎯 Goal: Create a React app with two components: UserName and UserBio. Then compose them inside a parent component called UserProfile to display the user's name and bio together.

📋 What You'll Learn

Create a UserName component that shows the user's name.

Create a UserBio component that shows the user's bio.

Create a UserProfile component that uses UserName and UserBio inside it.

Use React functional components and hooks if needed.

Use JSX syntax correctly.

💡 Why This Matters

🌍 Real World

Component composition is how React apps build complex user interfaces by combining small, reusable pieces. This makes apps easier to build and maintain.

💼 Career

Understanding component composition is essential for React developers. It is a core skill used in almost every React project in the industry.

1

Create the UserName component

Create a React functional component called UserName that returns an <h1> element with the text "Alice Johnson".

React

// Create the UserName component here
// Your code here

Hint

Remember, a React functional component is a function that returns JSX. Use function UserName() and return <h1>Alice Johnson</h1>.

2

Create the UserBio component

Create a React functional component called UserBio that returns a <p> element with the text "Frontend developer and coffee lover.".

React

function UserName() {
  return Alice Johnson;
}

// Create the UserBio component here
// Your code here

Hint

Use the same pattern as UserName. Create a function UserBio that returns a paragraph with the bio text.

3

Create the UserProfile component

Create a React functional component called UserProfile that returns a <div> containing the UserName and UserBio components inside it.

React

function UserName() {
  return Alice Johnson;
}

function UserBio() {
  return Frontend developer and coffee lover.;
}

// Create the UserProfile component here
// Your code here

Hint

Inside UserProfile, return a <div> that includes the UserName and UserBio components as JSX tags.

4

Render the UserProfile component

Add a React root render call that renders the UserProfile component inside an HTML element with the id root. Use ReactDOM.createRoot and root.render.

React

function UserName() {
  return Alice Johnson;
}

function UserBio() {
  return Frontend developer and coffee lover.;
}

function UserProfile() {
  return (
    
      
      
    
  );
}

// Render UserProfile inside the element with id 'root'
// Your code here

Hint

Use ReactDOM.createRoot(document.getElementById('root')) to create the root, then call root.render(<UserProfile />) to show the component.

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