Default props in React Native - Mini App: Build & Ship

Build: Greeting Screen

A simple screen that shows a greeting message. If no name is given, it uses a default name.

Target UI

---------------------
| Greeting Screen   |
|-------------------|
| Hello, [Name]!    |
|                   |
| [Change Name] Btn |
---------------------

Create a Greeting component that accepts a 'name' prop

If 'name' is not provided, use 'Friend' as the default

Display the greeting message: 'Hello, [Name]!'

Add a button labeled 'Change Name' that changes the name to 'React Native Learner' when pressed

Starter Code

React Native

import React, { useState } from 'react';
import { View, Text, Button, StyleSheet } from 'react-native';

const Greeting = ({ name }) => {
  // TODO: Use default prop for name
  return (
    <Text style={styles.greetingText}>Hello, {name}!</Text>
  );
};

const App = () => {
  const [name, setName] = useState();

  return (
    <View style={styles.container}>
      <Greeting name={name} />
      <Button title="Change Name" onPress={() => setName('React Native Learner')} />
    </View>
  );
};

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    padding: 16
  },
  greetingText: {
    fontSize: 24,
    marginBottom: 20
  }
});

export default App;

Task 1

Solution

React Native

import React, { useState } from 'react';
import { View, Text, Button, StyleSheet } from 'react-native';

const Greeting = ({ name = 'Friend' }) => {
  return (
    <Text style={styles.greetingText}>Hello, {name}!</Text>
  );
};

const App = () => {
  const [name, setName] = useState();

  return (
    <View style={styles.container}>
      <Greeting name={name} />
      <Button title="Change Name" onPress={() => setName('React Native Learner')} />
    </View>
  );
};

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    padding: 16
  },
  greetingText: {
    fontSize: 24,
    marginBottom: 20
  }
});

export default App;

We set a default value for the name prop directly in the function parameter using = 'Friend'. This means if no name is passed, it will show 'Friend'. When the button is pressed, the name changes to 'React Native Learner' and the greeting updates.

This is a simple way to provide default props in React Native functional components without extra code.

Final Result

Completed Screen

---------------------
| Greeting Screen   |
|-------------------|
| Hello, Friend!    |
|                   |
| [Change Name] Btn |
---------------------

Initially, the greeting shows 'Hello, Friend!' because no name is set.

When the user taps 'Change Name', the greeting updates to 'Hello, React Native Learner!'.

Stretch Goal

Add a TextInput to allow the user to type a custom name that updates the greeting in real-time.

💡 Hint

Use React Native's TextInput component and update the state on text change.

Practice

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

easy

A. To provide fallback values when no props are passed

B. To force the user to always pass props

C. To style the component automatically

D. To make the component stateful

Solution

1. Step 1: Understand what happens when props are missing

   If a prop is not passed, the component might break or behave unexpectedly.

2. Step 2: Role of default props

   Default props provide a fallback value so the component still works smoothly without explicit props.

3. Final Answer:

   To provide fallback values when no props are passed -> Option A

4. Quick Check:

   Default props = fallback values [OK]

Hint: Default props fill missing values automatically [OK]

Common Mistakes:

• Thinking default props force prop passing
• Confusing default props with styling
• Assuming default props make components stateful

2. Which of the following is the correct way to set a default prop value in a React Native functional component?

easy

A. MyComponent.propTypes = { title: 'Hello' }

B. function MyComponent(props = { title: 'Hello' }) { ... }

C. function MyComponent({ title = 'Hello' }) { ... }

D. const MyComponent = (props) => { props.title = 'Hello' }

Solution

1. Step 1: Identify default prop syntax in function parameters

   Using destructuring with default values like { title = 'Hello' } is the modern and correct way.

2. Step 2: Check other options

   function MyComponent(props = { title: 'Hello' }) { ... } wrongly assigns default to whole props object, C uses propTypes which is for validation not defaults, D mutates props which is bad practice.

3. Final Answer:

   function MyComponent({ title = 'Hello' }) { ... } -> Option C

4. Quick Check:

   Destructure with default = correct syntax [OK]

Hint: Use destructuring with = for default props [OK]

Common Mistakes:

• Assigning default to entire props object
• Confusing propTypes with defaultProps
• Mutating props inside component

3. What will be displayed by this component if used as <Greeting /> without passing any props?

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

medium

A. Hello, !

B. Hello, Guest!

C. Hello, undefined!

D. Error: name is required

Solution

1. Step 1: Check default prop value

   The parameter name has a default value 'Guest' if no prop is passed.

2. Step 2: Analyze component output

   Since no name prop is passed, it uses 'Guest' and renders 'Hello, Guest!'.

3. Final Answer:

   Hello, Guest! -> Option B

4. Quick Check:

   Default prop used = 'Guest' [OK]

Hint: Default prop shows when no prop passed [OK]

Common Mistakes:

• Expecting empty or undefined name
• Thinking component throws error without prop
• Ignoring default parameter syntax

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

function Button(props) {
  return {props.label};
}

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

medium

A. defaultProps is not supported on functional components

B. defaultProps should be set inside the function parameters

C. The component should use destructuring for props

D. There is no error; this is correct

Solution

1. Step 1: Check if defaultProps works on functional components

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

2. Step 2: Verify syntax correctness

   The syntax used is correct and will provide default label if none passed.

3. Final Answer:

   There is no error; this is correct -> Option D

4. Quick Check:

   defaultProps on function = valid [OK]

Hint: defaultProps works on functions too [OK]

Common Mistakes:

• Assuming defaultProps only works on classes
• Thinking destructuring is mandatory
• Confusing defaultProps with state

5. You want a component to show a default color 'blue' but only if the passed color prop is a non-empty string. Which is the best way to set this default in a React Native functional component?

hard

A. function Box({ color }) { color = color ? color : 'blue'; ... }

B. function Box({ color = 'blue' }) { ... }

C. function Box({ color }) { color &&= 'blue'; ... }

D. function Box({ color = '' }) { if (color) color = 'blue'; ... }

Solution

1. Step 1: Understand default parameter limits

   Default parameters only apply if the prop is undefined, not empty string.

2. Step 2: Choose a method that checks for non-empty string

   Using a conditional (color ? color : 'blue') ensures empty strings get replaced by 'blue'.

3. Step 3: Compare options

   A fails if color is empty string (uses empty). C's color &&= 'blue' sets 'blue' only if color is truthy (opposite). D's if (color) sets 'blue' only if truthy (wrong).

4. Final Answer:

   function Box({ color }) { color = color ? color : 'blue'; ... } -> Option A

5. Quick Check:

   Conditional check handles empty string default [OK]

Hint: Use conditional to handle empty strings as default [OK]

Common Mistakes:

• Relying only on default parameters for empty strings
• Using &&= which sets value when truthy
• Using if (color) which triggers on truthy