Map method in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the map() method do in JavaScript?

The map() method creates a new array by applying a function to each element of the original array.

beginner

How does map() differ from forEach()?

map() returns a new array with transformed elements, while forEach() just runs a function on each element without returning anything.

intermediate

What arguments does the callback function inside map() receive?

The callback receives three arguments: the current element, the index of the element, and the original array.

intermediate

Can map() change the size of the array it returns?

No, map() always returns a new array of the same length as the original array.

beginner

Give an example of using map() to double numbers in an array.

Example: <br><code>const numbers = [1, 2, 3];<br>const doubled = numbers.map(n => n * 2);<br>console.log(doubled); // [2, 4, 6]</code>

What does the map() method return?

A A single value

B Undefined

C The original array

D A new array with transformed elements

map() always returns a new array where each element is the result of the callback function.

Which of these is a valid use of map()?

A To filter out elements from an array

B To create a new array by modifying each element

C To sort an array

D To remove duplicates

map() transforms each element and returns a new array of the same length.

What arguments does the callback function in map() receive?

A Element, index, array

B Element only

C Index only

D Array only

The callback receives the current element, its index, and the original array.

If you want to change the size of an array, is map() the right method?

A No, it keeps the same length

B Yes, but only to add elements

C Yes, it can add or remove elements

D No, it deletes elements only

map() always returns an array with the same number of elements as the original.

What will this code output?
[1, 2, 3].map(x => x + 1)

A [1, 3, 5]

B [1, 2, 3]

C [2, 3, 4]

D [0, 1, 2]

Each element is increased by 1, so the new array is [2, 3, 4].

Explain how the map() method works and when you would use it.

Think about turning a list of raw items into a list of changed items.

Describe the difference between map() and forEach() in JavaScript.

One returns a new list, the other just runs code on each item.

Practice

1.

What does the map method do in JavaScript?

easy

A. Sorts the elements of the array in place.

B. Changes the original array by removing elements.

C. Creates a new array by applying a function to each element of the original array.

D. Filters elements from the array based on a condition.

Solution

1. Step 1: Understand the purpose of map

   The map method applies a function to each element and creates a new array with the results.

2. Step 2: Compare with other array methods

   Unlike filter or sort, map does not remove or reorder elements but transforms them.

3. Final Answer:

   Creates a new array by applying a function to each element of the original array. -> Option C

4. Quick Check:

   map creates new transformed array = A [OK]

Hint: Map always returns a new array with transformed items [OK]

Common Mistakes:

• Thinking map changes the original array
• Confusing map with filter or sort
• Believing map removes elements

2.

Which of the following is the correct syntax to double each number in the array nums using map?

const nums = [1, 2, 3];

easy

A. const doubled = nums.map(x => x * 2);

B. const doubled = nums.map(x * 2);

C. const doubled = nums.map(x => { x * 2 });

D. const doubled = nums.map(x => return x * 2);

Solution

1. Step 1: Check arrow function syntax inside map

   The function must be x => x * 2 to return the doubled value directly.

2. Step 2: Identify syntax errors in other options

   const doubled = nums.map(x * 2); misses the arrow function syntax. const doubled = nums.map(x => { x * 2 }); uses braces but no return statement, so returns undefined. const doubled = nums.map(x => return x * 2); incorrectly uses return without braces.

3. Final Answer:

   const doubled = nums.map(x => x * 2); -> Option A

4. Quick Check:

   Correct arrow function syntax = B [OK]

Hint: Use arrow function with implicit return inside map [OK]

Common Mistakes:

• Missing arrow (=>) in the function
• Using braces without return statement
• Incorrect return syntax inside arrow function

3.

What is the output of the following code?

const arr = [1, 2, 3];
const result = arr.map(x => x + 1);
console.log(result);

medium

A. [1, 2, 3]

B. [2, 3, 4]

C. [1, 3, 5]

D. Error

Solution

1. Step 1: Understand the map function applied

   Each element x in arr is increased by 1, so 1 becomes 2, 2 becomes 3, and 3 becomes 4.

2. Step 2: Check the output array

   The new array is [2, 3, 4], which is logged to the console.

3. Final Answer:

   [2, 3, 4] -> Option B

4. Quick Check:

   Each element +1 = [2, 3, 4] [OK]

Hint: Add 1 to each element inside map to get output [OK]

Common Mistakes:

• Thinking original array changes
• Confusing output with original array
• Expecting error due to syntax

4.

Find the error in this code snippet:

const numbers = [1, 2, 3];
const newNumbers = numbers.map(num => { num * 3 });
console.log(newNumbers);

medium

A. The arrow function syntax is incorrect.

B. The array numbers is not defined.

C. map method cannot be used on arrays.

D. Missing return statement inside the arrow function block.

Solution

1. Step 1: Analyze arrow function with braces

   When using braces in arrow functions, an explicit return is needed to return a value.

2. Step 2: Identify missing return

   The function body { num * 3 } does not return anything, so newNumbers will be an array of undefined.

3. Final Answer:

   Missing return statement inside the arrow function block. -> Option D

4. Quick Check:

   Arrow function with braces needs return [OK]

Hint: Add return inside braces or remove braces for implicit return [OK]

Common Mistakes:

• Forgetting return inside arrow function with braces
• Thinking map changes original array
• Assuming map works only with simple functions

5.

You have an array of objects representing people:

const people = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 30 },
  { name: 'Carol', age: 35 }
];

// Use map to create an array of greetings like "Hello, Alice!"

Which code correctly creates this greetings array?

hard

A. const greetings = people.map(person => `Hello, ${person.name}!`);

B. const greetings = people.map(person => { `Hello, ${person.name}!` });

C. const greetings = people.map(person => { return `Hello, ${person.age}!` });

D. const greetings = people.map(person => person.name + 'Hello!');

Solution

1. Step 1: Use map to transform each object to a greeting string

   The function should return a string like "Hello, Alice!" using template literals.

2. Step 2: Check arrow function syntax for returning string

   const greetings = people.map(person => `Hello, ${person.name}!`); uses implicit return with template literal correctly. const greetings = people.map(person => { `Hello, ${person.name}!` }); misses return inside braces. const greetings = people.map(person => { return `Hello, ${person.age}!` }); incorrectly uses age instead of name. const greetings = people.map(person => person.name + 'Hello!'); concatenates incorrectly.

3. Final Answer:

   const greetings = people.map(person => `Hello, ${person.name}!`); -> Option A

4. Quick Check:

   Implicit return with template literal = C [OK]

Hint: Use template literals with implicit return in map [OK]

Common Mistakes:

• Missing return inside braces
• Incorrect string concatenation
• Confusing implicit and explicit return