Map function usage in React - Mini Project: Build & Apply

Map Function Usage in React

📖 Scenario: You are building a simple React component that shows a list of fruits on a webpage. Each fruit name should appear as a separate item in a list.

🎯 Goal: Create a React functional component that uses the map function to display each fruit from an array as a list item inside an unordered list.

📋 What You'll Learn

Create an array of fruits with exact values: 'Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'.

Create a variable called fruitList that uses map to convert each fruit into a <li> element with a unique key.

Render the fruitList inside a <ul> element in the component return.

Use a functional component named FruitList.

💡 Why This Matters

🌍 Real World

Displaying lists of items dynamically is common in web apps, such as showing products, messages, or user profiles.

💼 Career

Understanding how to use map in React to render lists is a fundamental skill for frontend developers working with React.

1

Create the fruits array

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

React

import React from 'react';

export default function FruitList() {
  // Create the fruits array here
}

Hint

Use const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry']; inside the component.

2

Create the fruitList variable using map

Inside the FruitList component, create a constant called fruitList that uses fruits.map to return a list of <li> elements. Each <li> should have a key set to the fruit name and display the fruit name as text.

React

import React from 'react';

export default function FruitList() {
  const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];
  // Create fruitList using map here
}

Hint

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

3

Render the fruitList inside a <ul>

In the return statement of the FruitList component, return a <ul> element that contains the fruitList variable.

React

import React from 'react';

export default function FruitList() {
  const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];
  const fruitList = fruits.map(fruit => {fruit});
  // Return the fruitList inside a  here
}

Hint

Use return <ul>{fruitList}</ul>; to render the list.

4

Complete the FruitList component

Ensure the FruitList component is a default export and uses a functional component syntax with the fruits array, fruitList variable, and returns the <ul> with the mapped list items.

React

import React from 'react';

// Final check: ensure the component is complete and exported

export default function FruitList() {
  const fruits = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry'];
  const fruitList = fruits.map(fruit => {fruit});
  return {fruitList};
}

Hint

Make sure the component is exported as default and returns the mapped list inside a <ul>.

Practice

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

easy

A. To mutate the original array directly

B. To remove duplicate elements from the array

C. To sort the array elements alphabetically

D. To transform an array into a list of React elements

Solution

1. Step 1: Understand what map does in JavaScript

   The map function creates a new array by applying a function to each element of the original array without changing it.

2. Step 2: Apply this to React rendering

   In React, map is used to convert each item in an array into a React element, which can then be rendered as a list.

3. Final Answer:

   To transform an array into a list of React elements -> Option D

4. Quick Check:

   map creates React elements from arrays = B [OK]

Hint: Remember: map creates new arrays, perfect for rendering lists [OK]

Common Mistakes:

• Thinking map changes the original array
• Confusing map with sorting or filtering
• Not realizing map returns a new array

2. Which of the following is the correct syntax to use map inside a React component to render a list of names stored in const names = ['Anna', 'Bob', 'Cara']?

easy

A. {names.map(name => <li key={name}>{name}</li>)}

B. {names.map(name => <li>name</li>)}

C. {names.forEach(name => <li key={name}>{name}</li>)}

D. {names.map(name => <li>{name}</li> key={name})}

Solution

1. Step 1: Check the correct use of map and JSX

   Using map with an arrow function returning JSX elements is correct. The element must have a unique key prop inside the returned JSX.

2. Step 2: Identify the correct syntax

   {names.map(name => <li key={name}>{name}</li>)} correctly uses map, returns a list item with curly braces around name, and includes a unique key prop.

3. Final Answer:

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

4. Quick Check:

   map + JSX + key prop = A [OK]

Hint: Always add a unique key prop inside map JSX elements [OK]

Common Mistakes:

• Using forEach instead of map
• Missing curly braces around variables in JSX
• Placing key prop outside the JSX element

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

const numbers = [1, 2, 3];
const listItems = numbers.map(n => <li key={n}>Number: {n * 2}</li>);
return <ul>{listItems}</ul>;

medium

A. Number: 2Number: 4Number: 6

B. SyntaxError due to missing parentheses

C. Number: n * 2Number: n * 2Number: n * 2

D. Number: 1Number: 2Number: 3

Solution

1. Step 1: Understand the map transformation

   Each number in the array is multiplied by 2 inside the JSX expression: {n * 2}.

2. Step 2: Predict the rendered list items

   The list items will show "Number: 2", "Number: 4", and "Number: 6" respectively.

3. Final Answer:

   <ul><li>Number: 2</li><li>Number: 4</li><li>Number: 6</li></ul> -> Option A

4. Quick Check:

   n * 2 applied in map = D [OK]

Hint: Check the expression inside map carefully for output values [OK]

Common Mistakes:

• Assuming original numbers are shown without multiplication
• Confusing JSX expression syntax
• Thinking map returns strings instead of elements

4. Identify the error in this React code using map:

const items = ['a', 'b', 'c'];
return <ul>{items.map(item => <li>{item}</li>)};

medium

A. map function cannot be used inside JSX

B. Missing unique key prop on <li> elements

C. Incorrect use of curly braces around item

D. Array items must be numbers, not strings

Solution

1. Step 1: Check JSX inside map

   The map function returns
2. elements but none have a key prop, which React requires for lists.

3. Step 2: Confirm other syntax is correct

   Curly braces around item are correct, map is valid inside JSX, and array can contain strings.

4. Final Answer:

   Missing unique key prop on <li> elements -> Option B

5. Quick Check:

   React needs keys in lists = C [OK]

Hint: Always add key prop to elements inside map [OK]

Common Mistakes:

• Ignoring missing key warnings
• Thinking keys are optional
• Confusing key prop placement

5. Given this nested array of user objects:

const users = [
  { id: 1, name: 'Alice', hobbies: ['Reading', 'Hiking'] },
  { id: 2, name: 'Bob', hobbies: ['Cooking'] }
];

// Which code correctly renders a list of users with their hobbies?

hard

A. {users.map(user => <div key={user.id}><h3>{user.name}</h3><ul>{user.hobbies.map(hobby => <li key={user.id}>{hobby}</li>)}

B. {users.map(user => <div><h3>{user.name}</h3><ul>{user.hobbies.map(hobby => <li>{hobby}</li>)}

C. {users.map(user => <div key={user.id}><h3>{user.name}</h3><ul>{user.hobbies.map(hobby => <li key={hobby}>{hobby}</li>)}

D. {users.map(user => <div key={user.name}><h3>{user.name}</h3><ul>{user.hobbies.map(hobby => <li key={user.id}>{hobby}</li>)}

Solution

1. Step 1: Check keys on outer and inner lists

   The outer div uses user.id as key, which is unique. The inner li uses hobby as key, which is unique per hobby.

2. Step 2: Verify correct JSX structure and keys

   {users.map(user => <div key={user.id}><h3>{user.name}</h3><ul>{user.hobbies.map(hobby => <li key={hobby}>{hobby}</li>)} correctly nests the hobbies list inside each user and assigns proper keys. Other options miss keys or reuse keys incorrectly.

3. Final Answer:

   {users.map(user => <div key={user.id}><h3>{user.name}</h3><ul>{user.hobbies.map(hobby => <li key={hobby}>{hobby}</li>)} -> Option C

4. Quick Check:

   Unique keys on nested lists = A [OK]

Hint: Use unique keys for both outer and inner mapped elements [OK]

Common Mistakes:

• Reusing same key for multiple elements
• Missing key on inner map elements
• Using non-unique keys like user name or id incorrectly