Default props in React - Mini Project: Build & Apply

Using Default Props in React Functional Components

📖 Scenario: You are building a simple React app that shows a greeting message. Sometimes the user name is not provided, so you want to show a default name instead.

🎯 Goal: Create a React functional component called Greeting that shows a message with a name prop. Use default props to show Guest if no name is given.

📋 What You'll Learn

Create a functional component named Greeting that accepts a name prop

Set default props so that name is Guest if not provided

Render a <div> with the text Hello, {name}!

Use the component twice: once with a name prop and once without

💡 Why This Matters

🌍 Real World

Default props help keep React components flexible and user-friendly by providing fallback values when some data is missing.

💼 Career

Knowing how to use default props is essential for building robust React components that handle missing or optional data gracefully.

1

Create the Greeting component with a name prop

Create a React functional component called Greeting that accepts a name prop and returns a <div> with the text Hello, {name}!.

React

function Greeting({ name }) {
  // Your code here
}

Hint

Use a function that takes { name } as parameter and returns JSX with Hello, {name}!.

2

Add default props for the name

Add default props to the Greeting component so that name is Guest if no prop is passed.

React

function Greeting({ name }) {
  return Hello, {name}!;
}

// Your code here to add default props

Hint

Use Greeting.defaultProps = { name: 'Guest' } after the function.

3

Use the Greeting component with and without the name prop

Render the Greeting component twice inside a parent App component: once with name="Alice" and once without any name prop.

React

function Greeting({ name }) {
  return Hello, {name}!;
}

Greeting.defaultProps = {
  name: 'Guest'
};

function App() {
  // Your code here
}

Hint

Use a fragment <></> to wrap two Greeting components inside App.

4

Export the App component as default

Add a default export for the App component at the end of the file.

React

function Greeting({ name }) {
  return Hello, {name}!;
}

Greeting.defaultProps = {
  name: 'Guest'
};

function App() {
  return (
    
      
      
    
  );
}

// Your code here to export App

Hint

Use export default App; at the end of the file.

Practice

1. What is the main purpose of default props in a React functional component?

easy

A. To enforce type checking on props

B. To optimize component rendering speed

C. To make the component stateful

D. To provide fallback values for props that are not passed

Solution

1. Step 1: Understand what props are

   Props are inputs to React components that can be passed from parent components.

2. Step 2: Identify the role of default props

   Default props provide fallback values when no value is passed for a prop, preventing undefined values.

3. Final Answer:

   To provide fallback values for props that are not passed -> Option D

4. Quick Check:

   Default props = fallback values [OK]

Hint: Default props fill in missing props automatically [OK]

Common Mistakes:

• Confusing default props with state
• Thinking default props enforce types
• Assuming default props improve performance

2. Which of the following is the correct way to set default props in a React functional component using ES6 syntax?

easy

A. function MyComponent(props = { name: 'Guest' }) { return {props.name}; }

B. function MyComponent({ name = 'Guest' }) { return {name}; }

C. MyComponent.defaultProps = { name: 'Guest' }; function MyComponent(props) { return {props.name}; }

D. function MyComponent(props) { props.name = props.name || 'Guest'; return {props.name}; }

Solution

1. Step 1: Review ES6 default parameter syntax

   Default values can be set directly in the function parameter list using destructuring with defaults.

2. Step 2: Check each option

   function MyComponent({ name = 'Guest' }) { return <div>{name}</div>; } correctly uses destructuring with a default value: { name = 'Guest' }.

3. Final Answer:

   function MyComponent({ name = 'Guest' }) { return <div>{name}</div>; } -> Option B

4. Quick Check:

   Default props in parameters = function MyComponent({ name = 'Guest' }) { return <div>{name}</div>; } [OK]

Hint: Use destructuring with default values in parameters [OK]

Common Mistakes:

• Trying to set default props inside the function body
• Using defaultProps with functional components incorrectly
• Assigning default props to the whole props object

3. What will be rendered by this component?

function Greeting({ name = 'Friend' }) {
  return <h1>Hello, {name}!

medium

A. <h1>Hello, Friend!</h1>

B. <h1>Hello, undefined!</h1>

C. Error: Missing required prop 'name'

D. <h1>Hello, !</h1>

Solution

1. Step 1: Check the default prop value

   The component uses destructuring with a default value: name = 'Friend'.

2. Step 2: Analyze the usage without passing name

   No prop is passed, so the default 'Friend' is used.

3. Final Answer:

   <h1>Hello, Friend!</h1> -> Option A

4. Quick Check:

   Default prop used = 'Friend' [OK]

Hint: No prop passed? Default value is used [OK]

Common Mistakes:

• Assuming missing props cause errors
• Expecting undefined instead of default
• Confusing default props with required props

4. Identify the error in this component that tries to set default props:

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

Button.defaultProps = { label: 'Click me' };

// Usage:
<Button />

medium

A. defaultProps is not supported on functional components

B. The component should use destructuring with default values instead

C. No error; this code works correctly

D. The defaultProps assignment should be inside the function

Solution

1. Step 1: Understand defaultProps on functional components

   React supports defaultProps on functional components, so this syntax is valid.

2. Step 2: Check usage and output

   Since no label prop is passed, defaultProps provides 'Click me', so the button shows that text.

3. Final Answer:

   No error; this code works correctly -> Option C

4. Quick Check:

   defaultProps works on functions = No error; this code works correctly [OK]

Hint: defaultProps still works on functions but prefer defaults in parameters [OK]

Common Mistakes:

• Thinking defaultProps is deprecated for functional components
• Placing defaultProps inside the function body
• Confusing defaultProps with state initialization

5. You want a component to have a default prop that is an object with nested values. Which approach correctly sets default props to avoid errors if the prop is missing?

function Profile({ user = { name: 'Guest', age: 0 } }) {
  return <div>Name: {user.name}, Age: {user.age}</div>;
}

hard

A. Use destructuring with a default object in the parameter as shown

B. Set defaultProps = { user: { name: 'Guest', age: 0 } } after the function

C. Initialize user inside the function body with if (!user) user = {...}

D. Pass user as a required prop and do not set defaults

Solution

1. Step 1: Understand default props for nested objects

   Setting default nested objects in parameters prevents errors when accessing properties.

2. Step 2: Compare approaches

   Use destructuring with a default object in the parameter as shown uses destructuring with a default object, which safely provides nested defaults.

3. Final Answer:

   Use destructuring with a default object in the parameter as shown -> Option A

4. Quick Check:

   Default nested object in parameters = Use destructuring with a default object in the parameter as shown [OK]

Hint: Set nested defaults in parameter destructuring to avoid undefined errors [OK]

Common Mistakes:

• Setting defaultProps with nested objects can cause shallow merge issues
• Initializing defaults inside function body can be verbose and error-prone
• Not providing defaults leads to runtime errors accessing nested props