Default props in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are default props in React?

Default props are values given to a React component's props that are used if no value is provided by the parent component. They help ensure the component works with sensible defaults.

beginner

How do you set default props in a React functional component?

You can set default props by assigning default values in the function parameter list, like <code>function MyComponent({ title = 'Hello' }) { ... }</code>. This way, if <code>title</code> is not passed, it defaults to 'Hello'.

beginner

Why are default props useful in React components?

They prevent errors by providing fallback values, improve component reusability, and make components easier to use without always specifying every prop.

beginner

Can you override default props in React? How?

Yes, you override default props by passing a prop value when using the component. The passed value replaces the default.

beginner

Show an example of a React functional component with default props using ES6 syntax.

Example:<br><pre>function Greeting({ name = 'Friend' }) {<br>  return &lt;h1&gt;Hello, {name}!&lt;/h1&gt;<br>}</pre>

What happens if you don't pass a prop that has a default value in a React component?

A The component does not render.

B The component throws an error.

C The component uses the default value for that prop.

D The prop value becomes undefined.

If a prop is not passed, React uses the default value defined for that prop.

How do you define default props in a React functional component?

A By setting default values in the function parameters.

B By using <code>this.defaultProps</code> inside the function.

C By calling <code>setDefaultProps()</code> method.

D By wrapping the component in a <code>DefaultProps</code> tag.

In functional components, default props are set by giving default values in the parameter list.

Can default props be overridden by passing a prop value?

A Yes, passed props always override default props.

B No, default props cannot be changed.

C Only if the prop is a string.

D Only if the component is a class component.

Props passed to a component override any default props set.

Which of these is a benefit of using default props?

A Prevents components from rendering.

B Makes components slower.

C Requires more code to write.

D Avoids errors when props are missing.

Default props provide fallback values, preventing errors if props are missing.

What is the default prop value for name in this component?

function Welcome({ name = 'Guest' }) { return <p>Hi, {name}!</p> }

A undefined

B 'Guest'

C null

D Empty string

The default value for name is set to 'Guest' in the function parameter.

Explain what default props are in React and why they are useful.

Think about what happens when a prop is missing.

Describe how to set default props in a React functional component with an example.

Use ES6 syntax for function parameters.

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