Rendering lists in React - Mini Project: Build & Apply

Rendering lists in React

📖 Scenario: You are building a simple React app that shows a list of fruits. You want to display each fruit name on the page.

🎯 Goal: Build a React functional component that renders a list of fruit names using the map() method.

📋 What You'll Learn

Create an array of fruit names

Create a variable to hold the list items

Use map() to transform the array into JSX list elements

Render the list inside a <ul> element

💡 Why This Matters

🌍 Real World

Rendering lists is a common task in React apps to display collections like menus, user comments, or product lists.

💼 Career

Understanding how to render lists with keys is essential for React developers to build efficient and bug-free user interfaces.

1

Create the fruits array

Create a constant array called fruits with these exact string values: 'Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'.

React

// Create the fruits array with the exact values
// Your code here

Hint

Use const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];

2

Create the listItems variable

Create a constant called listItems and set it to null for now.

React

const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];

// Create listItems variable and set it to null
// Your code here

Hint

Write const listItems = null;

3

Use map() to create list items

Change the listItems constant to use fruits.map() to create an array of <li> elements. Use fruit as the iterator variable. Each <li> should have a key attribute set to the fruit name and display the fruit name inside.

React

const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];

// Use map() to create listItems with  elements and keys
// Your code here

Hint

Use fruits.map(fruit => <li key={fruit}>{fruit}</li>)

4

Render the list inside a component

Create a React functional component called FruitList that returns a <ul> element containing the listItems. Export the component as default.

React

const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];
const listItems = fruits.map(fruit => {fruit});

// Create FruitList component that returns  with listItems
// Your code here

Hint

Write a function FruitList that returns <ul>{listItems}</ul> and export it.

Practice

1. What is the main purpose of using the map() method when rendering lists in React?

easy

A. To filter out unwanted items from the list

B. To sort the list items alphabetically

C. To transform an array of data into an array of React elements

D. To add styles to each list item

Solution

1. Step 1: Understand the role of map() in React

   The map() method is used to take an array of data and create a new array by transforming each item. In React, this means turning data into React elements.

2. Step 2: Identify what map() does in rendering lists

   When rendering lists, map() creates a React element for each item, which React can then display on the screen.

3. Final Answer:

   To transform an array of data into an array of React elements -> Option C

4. Quick Check:

   map() creates elements from data [OK]

Hint: Remember: map() turns data into elements [OK]

Common Mistakes:

• Thinking map() sorts or filters data
• Using map() without returning elements
• Confusing map() with forEach()

2. Which of the following is the correct way to add a unique key to list items in React?

easy

A. {items.map(item => <li key={item.id}>{item.name}</li>)}

B. {items.map(item => <li key={Math.random()}>{item.name}</li>)}

C. {items.map(item => <li key="index">{item.name}</li>)}

D. {items.map(item => <li>{item.name}</li>)}

Solution

1. Step 1: Understand the importance of keys in React lists

   Keys help React identify which items changed, added, or removed. They must be unique and stable for each list item.

2. Step 2: Evaluate each option for key usage

   {items.map(item => <li>{item.name}</li>)} has no key, so React will warn. {items.map(item => <li key={item.id}>{item.name}</li>)} uses a unique id from the item, which is correct. {items.map(item => <li key="index">{item.name}</li>)} uses a string 'index' for all keys, which is not unique. {items.map(item => <li key={Math.random()}>{item.name}</li>)} uses Math.random(), which changes every render and is unstable.

3. Final Answer:

   {items.map(item => <li key={item.id}>{item.name}</li>)} -> Option A

4. Quick Check:

   Unique stable keys = {items.map(item => <li key={item.id}>{item.name}</li>)} [OK]

Hint: Use unique stable keys from data, not random or fixed strings [OK]

Common Mistakes:

• Omitting the key prop entirely
• Using index or random values as keys
• Using non-unique keys causing rendering bugs

3. What will be the output of this React code snippet?

const fruits = ['Apple', 'Banana', 'Cherry'];

function FruitList() {
  return (
    <ul>
      {fruits.map((fruit, index) => <li key={index}>{fruit}</li>)}
    </ul>
  );
}

medium

A. SyntaxError due to missing keys

B. AppleBananaCherry

C. 012

D. Empty list rendered

Solution

1. Step 1: Understand the map function output

   The map() creates a list item for each fruit with the fruit name as content.

2. Step 2: Check the key usage and rendering

   Using index as key is allowed (though not ideal), so React renders the list with fruit names inside <li> tags.

3. Final Answer:

   <ul><li>Apple</li><li>Banana</li><li>Cherry</li></ul> -> Option B

4. Quick Check:

   map() renders fruit names as list items [OK]

Hint: map() returns elements with content, keys don't affect output text [OK]

Common Mistakes:

• Confusing keys with displayed content
• Expecting errors from using index keys
• Thinking map returns indexes instead of values

4. Identify the error in this React list rendering code:

const numbers = [1, 2, 3];

function NumberList() {
  return (
    <ul>
      {numbers.map(number => <li>number</li>)}
    </ul>
  );
}

medium

A. Missing key prop on list items

B. numbers array is empty, so no output

C. Syntax error due to arrow function

D. The word 'number' is rendered literally, not the variable

Solution

1. Step 1: Analyze the JSX inside map

   The code writes <li>number</li> which renders the literal text 'number', not the variable value.

2. Step 2: Identify how to fix variable rendering

   To render the variable, it should be inside curly braces: <li>{number}</li>.

3. Final Answer:

   The word 'number' is rendered literally, not the variable -> Option D

4. Quick Check:

   JSX needs curly braces for variables [OK]

Hint: Use curly braces {} to render variables in JSX [OK]

Common Mistakes:

• Forgetting curly braces around variables
• Ignoring missing key warnings
• Assuming array is empty without checking

5. You have a list of user objects with possible duplicate IDs:

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

function UserList() {
  return (
    <ul>
      {users.map((user, index) => <li key={`${user.id}-${index}`}>{user.name}</li>)}
    </ul>
  );
}

What problem can occur and how to fix it?

hard

A. Duplicate keys cause rendering issues; use unique keys like index or combined values

B. No problem; React handles duplicate keys automatically

C. Syntax error due to duplicate IDs in objects

D. List will render only first two users ignoring duplicates

Solution

1. Step 1: Understand React's key requirement

   Keys must be unique to help React track items. Duplicate keys cause rendering bugs or warnings.

2. Step 2: Identify the duplicate key issue in code

   Two users share id=1, so keys are not unique. This can confuse React during updates.

3. Step 3: Suggest a fix for unique keys

   Use a unique key like combining id and index: key={`${user.id}-${index}`} or a unique identifier.

4. Final Answer:

   Duplicate keys cause rendering issues; use unique keys like index or combined values -> Option A

5. Quick Check:

   Unique keys prevent rendering bugs [OK]

Hint: Keys must be unique; combine id with index if duplicates exist [OK]

Common Mistakes:

• Ignoring duplicate keys causing UI bugs
• Assuming React auto-fixes duplicate keys
• Using only index keys without data context