FlatList basics (data, renderItem, keyExtractor) in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What will this FlatList display?

Consider this React Native FlatList component. What text will appear on the screen?

React Native

import React from 'react';
import { FlatList, Text, View } from 'react-native';

const data = [{id: '1', name: 'Apple'}, {id: '2', name: 'Banana'}];

export default function App() {
  return (
    <View>
      <FlatList
        data={data}
        keyExtractor={item => item.id}
        renderItem={({item}) => <Text>{item.name}</Text>}
      />
    </View>
  );
}

A The screen shows an error because renderItem is missing.

B The screen shows two lines: '1' and '2'.

C The screen is blank because keyExtractor is missing.

D The screen shows two lines: 'Apple' and 'Banana'.

💡 Hint

Look at what renderItem returns for each item.

FlatList uses the data array and for each item calls renderItem, which returns a Text component showing item.name. So it displays 'Apple' and 'Banana'.

📝 Syntax

intermediate

Which keyExtractor is correct?

You want to use FlatList with this data: [{key: 'a'}, {key: 'b'}]. Which keyExtractor function will NOT cause a warning or error?

A keyExtractor={item => item.key}

B keyExtractor={item => item.id}

C keyExtractor={item => item}

D keyExtractor={item => item.toString()}

💡 Hint

The keyExtractor must return a string that exists in each item.

Each item has a 'key' property, so keyExtractor should return item.key. Using item.id will be undefined and cause warnings.

❓ lifecycle

advanced

What happens if keyExtractor returns non-unique keys?

If FlatList's keyExtractor returns the same key for multiple items, what is the likely result?

A FlatList will render all items correctly with no issues.

B FlatList will automatically fix keys to be unique.

C FlatList will show a warning and may render items incorrectly or skip updates.

D FlatList will crash the app immediately.

💡 Hint

Keys must be unique to help React identify items.

Non-unique keys cause React to confuse items, leading to warnings and rendering bugs.

🔧 Debug

advanced

Why does this FlatList show blank screen?

This FlatList code shows a blank screen. What is the problem? const data = [{id: '1', title: 'One'}, {id: '2', title: 'Two'}]; item.id} renderItem={() => {item.title}} />

A data array is empty, so nothing renders.

B renderItem does not receive 'item' parameter, so item.title is undefined.

C keyExtractor returns wrong keys causing blank screen.

D FlatList requires a style prop to show content.

💡 Hint

Check how renderItem receives its argument.

renderItem must destructure or access the item from its argument. Here, renderItem has no parameter, so item is undefined.

🧠 Conceptual

expert

Why use keyExtractor instead of index as key?

Why is it better to use a unique id in keyExtractor instead of the item index as key in FlatList?

A Using unique ids prevents rendering bugs when list items change order or are added/removed.

B Using index as key improves performance and prevents bugs.

C Using index as key is required by FlatList and unique ids cause errors.

D Using unique ids makes FlatList slower and less efficient.

💡 Hint

Think about what happens when list items reorder.

Keys help React track items. Using stable unique ids avoids bugs when items reorder or change. Index keys cause wrong item reuse.

Practice

1. What is the main purpose of the keyExtractor prop in a React Native FlatList?

easy

A. To handle user taps on list items.

B. To define how each item should be displayed on the screen.

C. To specify the data source for the list.

D. To provide a unique key for each list item to help React identify items.

Solution

1. Step 1: Understand the role of keyExtractor

   The keyExtractor is used to give each item a unique key, which helps React track which items changed, added, or removed.

2. Step 2: Differentiate from other props

   renderItem defines how to show items, data is the list source, and tap handling is done separately.

3. Final Answer:

   To provide a unique key for each list item to help React identify items. -> Option D

4. Quick Check:

   keyExtractor = unique keys [OK]

Hint: Keys uniquely identify items in lists [OK]

Common Mistakes:

• Confusing keyExtractor with renderItem
• Using index as key without keyExtractor
• Thinking keyExtractor handles item display

2. Which of the following is the correct syntax to define a FlatList with data and renderItem props?

easy

A. {item}} />

B. {item}} />

C. {item}} />

D. {data}} />

Solution

1. Step 1: Check prop names

   The correct props are data for the list and renderItem for rendering each item.

2. Step 2: Check renderItem function parameter

   renderItem receives an object with item property, so it should be ({item}) => ....

3. Final Answer:

   <FlatList data={items} renderItem={({item}) => <Text>{item}</Text>} /> -> Option A

4. Quick Check:

   Correct props and destructuring = C [OK]

Hint: Use data and renderItem with ({item}) parameter [OK]

Common Mistakes:

• Using wrong prop names like 'items' or 'render'
• Not destructuring item in renderItem
• Passing item directly without braces

3. Given this code snippet, what will be displayed on the screen?

const data = [{id: '1', name: 'Apple'}, {id: '2', name: 'Banana'}];

<FlatList
  data={data}
  keyExtractor={item => item.id}
  renderItem={({item}) => <Text>{item.name}</Text>}
/>

medium

A. A blank screen because keyExtractor is missing.

B. A list showing 'Apple' and 'Banana' as text items.

C. An error because renderItem is not a function.

D. A list showing '1' and '2' as text items.

Solution

1. Step 1: Analyze data and keyExtractor

   Data has two objects with ids and names. keyExtractor returns item.id, which is correct.

2. Step 2: Analyze renderItem output

   renderItem shows item.name, so it will display 'Apple' and 'Banana'.

3. Final Answer:

   A list showing 'Apple' and 'Banana' as text items. -> Option B

4. Quick Check:

   renderItem shows item.name = Apple, Banana [OK]

Hint: renderItem shows item.name, so names appear [OK]

Common Mistakes:

• Thinking keyExtractor missing causes blank screen
• Confusing item.id with item.name in renderItem
• Assuming renderItem is missing or wrong

4. What is wrong with this FlatList usage?

const data = [{id: 'a'}, {id: 'b'}];

<FlatList
  data={data}
  renderItem={({item}) => <Text>{item.id}</Text>}
/>

medium

A. FlatList requires a style prop to render.

B. renderItem should not use destructuring for item.

C. Missing keyExtractor prop, which may cause warnings or errors.

D. data should be an array of strings, not objects.

Solution

1. Step 1: Check for keyExtractor

   The code does not provide a keyExtractor, so React Native will warn because keys are needed for list items.

2. Step 2: Validate other props

   renderItem correctly destructures item, data can be objects, and style is optional.

3. Final Answer:

   Missing keyExtractor prop, which may cause warnings or errors. -> Option C

4. Quick Check:

   keyExtractor missing = warning [OK]

Hint: Always add keyExtractor for unique keys [OK]

Common Mistakes:

• Ignoring missing keyExtractor warnings
• Thinking renderItem destructuring is wrong
• Believing style prop is mandatory

5. You want to display a list of users with unique IDs, but your data array has duplicate IDs. How should you modify the keyExtractor to avoid key conflicts in FlatList?

const users = [
  {id: '1', name: 'Alice'},
  {id: '1', name: 'Bob'},
  {id: '2', name: 'Charlie'}
];

<FlatList
  data={users}
  keyExtractor={item => item.id}
  renderItem={({item}) => <Text>{item.name}</Text>}
/>

hard

A. Change keyExtractor to combine id and index: (item, index) => item.id + index

B. Remove keyExtractor and use default keys.

C. Use only index as keyExtractor: (item, index) => index.toString()

D. Keep keyExtractor as item.id because duplicates don't matter.

Solution

1. Step 1: Identify duplicate keys problem

   Using only item.id causes duplicate keys for '1', which React warns against.

2. Step 2: Fix keyExtractor to ensure uniqueness

   Combining item.id with index creates unique keys like '11', '12', '20'.

3. Final Answer:

   Change keyExtractor to combine id and index: (item, index) => item.id + index -> Option A

4. Quick Check:

   Unique keys prevent warnings = Change keyExtractor to combine id and index: (item, index) => item.id + index [OK]

Hint: Combine id and index for unique keys [OK]

Common Mistakes:

• Ignoring duplicate keys causing warnings
• Using only index loses stable keys
• Removing keyExtractor causes errors