SectionList for grouped data in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What will this SectionList render?

Consider this React Native code snippet using SectionList. What will the UI show?

React Native

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

const DATA = [
  { title: 'Fruits', data: ['Apple', 'Banana'] },
  { title: 'Vegetables', data: ['Carrot', 'Peas'] }
];

export default function App() {
  return (
    <SectionList
      sections={DATA}
      keyExtractor={(item, index) => item + index}
      renderItem={({ item }) => <Text>{item}</Text>}
      renderSectionHeader={({ section }) => <Text>{section.title}</Text>}
    />
  );
}

A A flat list showing all items without any headers.

B An empty screen because data is not correctly passed.

C Only the section headers 'Fruits' and 'Vegetables' are shown, no items.

D A list with two headers: 'Fruits' and 'Vegetables', each followed by their items.

💡 Hint

SectionList groups items by sections and shows headers for each group.

SectionList renders each section header and its items. Here, 'Fruits' and 'Vegetables' are headers, each followed by their respective items.

📝 Syntax

intermediate

Identify the syntax error in this SectionList usage

Which option contains a syntax error that will prevent this SectionList from rendering?

React Native

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

const DATA = [
  { title: 'A', data: ['Item1'] }
];

export default function App() {
  return (
    <SectionList
      sections={DATA}
      keyExtractor={(item) => item}
      renderItem={({ item }) => <Text>{item}</Text>}
      renderSectionHeader={({ section }) => <Text>{section.title}</Text>}
    />
  );
}

A renderSectionHeader={({ section => <Text>{section.title}</Text>}

B keyExtractor={(item) => item}

C renderItem={({ item }) => <Text>{item}</Text>}

D sections={DATA}

💡 Hint

Check for missing closing brackets or parentheses.

Option A is missing the closing parenthesis for the arrow function parameter destructuring, causing a syntax error.

❓ lifecycle

advanced

When does SectionList re-render its items?

Given a SectionList with static sections prop, which action will cause it to re-render its items?

A Updating the sections prop with a new array reference but same content.

B Changing the keyExtractor function to a new function instance.

C Updating the renderItem function to a new function instance.

D Changing the state unrelated to SectionList.

💡 Hint

SectionList uses shallow comparison on sections prop to decide re-render.

SectionList re-renders when the sections prop changes reference, even if content is same. Changing functions alone does not trigger re-render.

❓ navigation

advanced

How to scroll to a specific section in SectionList?

Which method and parameters correctly scroll SectionList to the 'Vegetables' section (index 1)?

A sectionListRef.current.scrollToIndex({ index: 1 })

B sectionListRef.current.scrollToLocation({ sectionIndex: 1, itemIndex: 0 })

C sectionListRef.current.scrollToOffset({ offset: 1 })

D sectionListRef.current.scrollToSection({ section: 'Vegetables' })

💡 Hint

Use scrollToLocation with sectionIndex and itemIndex to scroll to a section header.

scrollToLocation with sectionIndex and itemIndex scrolls to the start of a section. scrollToIndex and scrollToOffset do not support sections.

🔧 Debug

expert

Why does SectionList not display any items?

Given this code, why does the SectionList render only headers but no items?

React Native

const DATA = [
  { title: 'Group1', data: [] },
  { title: 'Group2', data: [] }
];

<SectionList
  sections={DATA}
  keyExtractor={(item, index) => item + index}
  renderItem={({ item }) => <Text>{item}</Text>}
  renderSectionHeader={({ section }) => <Text>{section.title}</Text>}
/>

A Because renderItem is missing a return statement.

B Because keyExtractor returns duplicate keys causing rendering issues.

C Because the first section's data array is empty, so no items show for it.

D Because sections prop is not an array.

💡 Hint

Check the data arrays inside each section.

The sections have empty data arrays, so no items render; only headers are shown.

Practice

1. What is the main purpose of using SectionList in React Native?

easy

A. To display a list of items grouped by sections with headers

B. To create a grid layout of images

C. To handle user input forms

D. To navigate between screens

Solution

1. Step 1: Understand SectionList usage

   SectionList is designed to show lists grouped by sections, each with a header and items.

2. Step 2: Compare options with SectionList purpose

   Options B, C, and D describe other UI or functionality unrelated to grouped lists.

3. Final Answer:

   To display a list of items grouped by sections with headers -> Option A

4. Quick Check:

   SectionList = grouped list display [OK]

Hint: SectionList groups items with headers, unlike FlatList [OK]

Common Mistakes:

• Confusing SectionList with FlatList
• Thinking SectionList is for navigation
• Assuming SectionList creates grids

2. Which of the following is the correct way to define the sections prop for a SectionList?

easy

A. [{ title: 'Fruits', data: ['Apple', 'Banana'] }, { title: 'Veggies', data: ['Carrot'] }]

B. ['Fruits', 'Apple', 'Banana', 'Veggies', 'Carrot']

C. { title: 'Fruits', items: ['Apple', 'Banana'] }

D. [{ header: 'Fruits', items: ['Apple', 'Banana'] }]

Solution

1. Step 1: Recall SectionList sections structure

   Each section is an object with a title and a data array of items.

2. Step 2: Check options for correct keys and structure

   [{ title: 'Fruits', data: ['Apple', 'Banana'] }, { title: 'Veggies', data: ['Carrot'] }] matches the required format. Options B, C, and D use wrong keys or structure.

3. Final Answer:

   [{ title: 'Fruits', data: ['Apple', 'Banana'] }, { title: 'Veggies', data: ['Carrot'] }] -> Option A

4. Quick Check:

   sections = array of {title, data} objects [OK]

Hint: Sections need title and data keys exactly [OK]

Common Mistakes:

• Using wrong keys like 'items' or 'header'
• Passing a flat array instead of array of objects
• Confusing data array with section title

3. Given this code snippet, what will be displayed as section headers?

const sections = [
  { title: 'A', data: ['Apple', 'Apricot'] },
  { title: 'B', data: ['Banana'] }
];

<SectionList
  sections={sections}
  renderItem={({ item }) => <Text>{item}</Text>}
  renderSectionHeader={({ section }) => <Text>{section.title}</Text>}
/>

medium

A. No headers will be shown

B. Apple and Banana as headers

C. Only A as header, B is ignored

D. A and B as headers above their items

Solution

1. Step 1: Understand renderSectionHeader usage

   The renderSectionHeader renders the title of each section as header.

2. Step 2: Identify section titles in data

   Sections have titles 'A' and 'B', so these will be shown as headers above their items.

3. Final Answer:

   A and B as headers above their items -> Option D

4. Quick Check:

   Section titles = headers [OK]

Hint: Section headers come from section.title, not items [OK]

Common Mistakes:

• Thinking items become headers
• Assuming only first section shows header
• Forgetting to use renderSectionHeader

4. What is wrong with this SectionList usage?

const sections = [
  { title: 'Numbers', data: 12345 }
];

<SectionList
  sections={sections}
  renderItem={({ item }) => <Text>{item}</Text>}
  renderSectionHeader={({ section }) => <Text>{section.title}</Text>}
/>

medium

A. The title property must be an array

B. The data property should be an array, not a number

C. renderItem should be renderSectionFooter

D. sections should be a flat array, not array of objects

Solution

1. Step 1: Check the type of data property

   The data property must be an array of items, but here it is a number (12345).

2. Step 2: Understand consequences of wrong data type

   This will cause runtime errors because SectionList expects an array to iterate over.

3. Final Answer:

   The data property should be an array, not a number -> Option B

4. Quick Check:

   data must be array [OK]

Hint: data must always be an array, never a single value [OK]

Common Mistakes:

• Passing a single value instead of array to data
• Confusing title type with data type
• Using wrong render method names

5. You want to display a SectionList with sections for weekdays and weekend, but only show days that start with 'S'. How should you prepare the sections data?

hard

A. [{ title: 'Weekdays', data: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'] }]

B. [{ title: 'Weekdays', data: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'] }, { title: 'Weekend', data: ['Saturday', 'Sunday'] }]

C. [{ title: 'Weekdays', data: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'].filter(day => day.startsWith('S')) }, { title: 'Weekend', data: ['Saturday', 'Sunday'].filter(day => day.startsWith('S')) }]

D. [{ title: 'Days', data: ['Saturday', 'Sunday'] }]

Solution

1. Step 1: Filter days starting with 'S' in each group

   Use filter(day => day.startsWith('S')) on weekdays and weekend arrays to keep only days starting with 'S'.

2. Step 2: Assign filtered arrays to correct section titles

   Weekdays filtered will be empty, weekend filtered will have 'Saturday' and 'Sunday'. This matches the requirement.

3. Final Answer:

   [{ title: 'Weekdays', data: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'].filter(day => day.startsWith('S')) }, { title: 'Weekend', data: ['Saturday', 'Sunday'].filter(day => day.startsWith('S')) }] -> Option C

4. Quick Check:

   Filter each section's data by startsWith('S') [OK]

Hint: Filter each section's data array before passing to SectionList [OK]

Common Mistakes:

• Not filtering weekdays, showing all days
• Combining all days into one section
• Ignoring filtering and passing raw arrays