Map function usage in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the rendered output of this React component?

Consider this React component that uses map to render a list of items:

function FruitList() {
  const fruits = ['Apple', 'Banana', 'Cherry'];
  return (
    <ul>
      {fruits.map((fruit, index) => <li key={index}>{fruit}</li>)}
    </ul>
  );
}

What will this component render in the browser?

React

function FruitList() {
  const fruits = ['Apple', 'Banana', 'Cherry'];
  return (
    <ul>
      {fruits.map((fruit, index) => <li key={index}>{fruit}</li>)}
    </ul>
  );
}

A <ul>AppleBananaCherry</ul>

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

C <ul><li>0</li><li>1</li><li>2</li></ul>

D <ul><li>fruit</li><li>fruit</li><li>fruit</li></ul>

💡 Hint

Remember that map returns a new array of elements, and React renders each element inside the list.

The map function creates a new array of <li> elements, each containing the fruit name. The key prop helps React identify each item uniquely.

📝 Syntax

intermediate

Which option correctly uses map to double numbers in React JSX?

You want to display a list of doubled numbers from an array [1, 2, 3]. Which code snippet correctly uses map inside JSX?

A {[1, 2, 3].map(num => &lt;div key={num}&gt;{num * 2}&lt;/div&gt;)}

B {[1, 2, 3].map(num =&gt; &lt;div key&gt;{num * 2}&lt;/div&gt;)}

C {[1, 2, 3].map(num =&gt; &lt;div&gt;num * 2&lt;/div&gt;)}

D {[1, 2, 3].map(num) =&gt; &lt;div&gt;{num * 2}&lt;/div&gt;}

💡 Hint

Check for correct arrow function syntax and proper use of key attribute.

Option A correctly uses an arrow function and includes a unique key prop. Option A has an invalid key attribute (missing value), Option A has a syntax error, and Option A renders the string 'num * 2' literally.

🔧 Debug

advanced

Why does this React component throw a warning about keys?

Look at this component:

function Colors() {
  const colors = ['red', 'green', 'blue'];
  return (
    <div>
      {colors.map(color => <span>{color}</span>)}
    </div>
  );
}

What is the cause of the warning in the browser console?

React

function Colors() {
  const colors = ['red', 'green', 'blue'];
  return (
    <div>
      {colors.map(color => <span>{color}</span>)}
    </div>
  );
}

A The map function is missing a return statement.

B The 'color' variable is not defined inside the map function.

C React components cannot return multiple elements.

D Each child in a list should have a unique 'key' prop.

💡 Hint

React requires a special prop when rendering lists to track items.

React needs a unique key prop on each element created in a list to optimize rendering. Without it, React shows a warning.

❓ state_output

advanced

What is the output after clicking the button in this React component?

Consider this component that uses map and state:

import { useState } from 'react';

function CounterList() {
  const [counts, setCounts] = useState([0, 0, 0]);

  function incrementFirst() {
    const newCounts = counts.map((count, index) => index === 0 ? count + 1 : count);
    setCounts(newCounts);
  }

  return (
    <div>
      {counts.map((count, i) => <p key={i}>Count {i}: {count}</p>)}
      <button onClick={incrementFirst}>Increment First</button>
    </div>
  );
}

What will be displayed after clicking the button once?

React

import { useState } from 'react';

function CounterList() {
  const [counts, setCounts] = useState([0, 0, 0]);

  function incrementFirst() {
    const newCounts = counts.map((count, index) => index === 0 ? count + 1 : count);
    setCounts(newCounts);
  }

  return (
    <div>
      {counts.map((count, i) => <p key={i}>Count {i}: {count}</p>)}
      <button onClick={incrementFirst}>Increment First</button>
    </div>
  );
}

A

Count 0: 0
Count 1: 0
Count 2: 1

B

Count 0: 0
Count 1: 1
Count 2: 0

C

Count 0: 1
Count 1: 0
Count 2: 0

D

Count 0: 1
Count 1: 1
Count 2: 1

💡 Hint

Check which index is incremented in the map function.

The map function adds 1 only to the first element (index 0). The others remain unchanged.

🧠 Conceptual

expert

Why is using array index as key in map sometimes problematic?

In React, when rendering lists with map, developers often use the array index as the key prop. Why can this cause issues?

A Using index as key can cause incorrect component reuse when list items change order or are added/removed.

B React requires keys to be strings, so numbers like index cause errors.

C Keys must be unique across the entire app, so index keys cause conflicts.

D Using index as key slows down rendering because React re-renders all items every time.

💡 Hint

Think about what happens when the list changes dynamically.

When list items reorder or change, using index as key can cause React to reuse components incorrectly, leading to bugs or wrong UI updates.

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