Why Arrays Exist and What Problem They Solve in DSA Javascript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an array with three numbers.

DSA Javascript

const numbers = [[1]];

Drag options to blanks, or click blank then click option'

A1; 2; 3

B1 2 3

C'1', '2', '3'

D1, 2, 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using spaces instead of commas

Using semicolons instead of commas

Using quotes around numbers unnecessarily

The correct way to create an array with three numbers is to separate them by commas inside square brackets.

2fill in blank

medium

Complete the code to access the first element of the array.

DSA Javascript

const first = numbers[1];

Drag options to blanks, or click blank then click option'

A(0)

B{0}

C[0]

D<0>

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of brackets

Using curly braces instead of brackets

Using angle brackets instead of brackets

Arrays are accessed using square brackets with the index inside. Indexing starts at 0.

3fill in blank

hard

Fix the error in the code to add a new element to the end of the array.

DSA Javascript

numbers.[1](4);

Drag options to blanks, or click blank then click option'

Apush

Badd

Cappend

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' which is not a method

Using 'append' which is not a JavaScript array method

Using 'insert' which is not a method

The correct method to add an element at the end of an array in JavaScript is push.

4fill in blank

hard

Fill both blanks to create an array of length 5 filled with zeros.

DSA Javascript

const zeros = new Array([1]).[2](0);

Drag options to blanks, or click blank then click option'

A5

Bfill

Cpush

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push' instead of 'fill'

Using 'length' as a method

Using wrong size for the array

We create a new array of length 5 and fill it with zeros using the fill method.

5fill in blank

hard

Fill all three blanks to create an array of squares of numbers from 0 to 4.

DSA Javascript

const squares = Array.from({length: [1], (_, [2]) => [2] [3] 2);

Drag options to blanks, or click blank then click option'

A5

Bx

C**

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of '**' for exponent

Using wrong variable name

Wrong length value

We create an array of length 5, mapping each index x from 0 to 4 to its square using the exponent operator **.

Practice

1. Why do arrays exist in programming languages like JavaScript?

easy

A. To store multiple values in a single variable in an ordered way

B. To perform mathematical calculations faster

C. To replace all other data structures

D. To store only one value at a time

Solution

1. Step 1: Understand the purpose of arrays

   Arrays allow storing many values together instead of separate variables for each value.

2. Step 2: Recognize the order and grouping feature

   Arrays keep values in order and allow easy access by position (index).

3. Final Answer:

   To store multiple values in a single variable in an ordered way -> Option A

4. Quick Check:

   Arrays group values in order = A [OK]

Hint: Arrays group many values in order inside one variable [OK]

Common Mistakes:

• Thinking arrays store only one value
• Confusing arrays with math operations
• Believing arrays replace all data structures

2. Which of the following is the correct way to create an empty array in JavaScript?

easy

A. let arr = {}

B. let arr = ()

C. let arr = []

D. let arr = ''

Solution

1. Step 1: Recall JavaScript array syntax

   Arrays are created using square brackets [].

2. Step 2: Identify the correct empty array syntax

   Using [] creates an empty array, while {} creates an object, () is invalid, and '' is a string.

3. Final Answer:

   let arr = [] -> Option C

4. Quick Check:

   Empty array uses [] = C [OK]

Hint: Use square brackets [] to create arrays in JavaScript [OK]

Common Mistakes:

• Using curly braces {} which create objects
• Using parentheses () which are invalid for arrays
• Using empty quotes '' which create strings

3. What will be the output of the following code?

const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits[1]);

medium

A. banana

B. apple

C. cherry

D. undefined

Solution

1. Step 1: Understand array indexing

   Arrays start at index 0, so fruits[0] = 'apple', fruits[1] = 'banana'.

2. Step 2: Identify the value at index 1

   fruits[1] accesses the second element, which is 'banana'.

3. Final Answer:

   banana -> Option A

4. Quick Check:

   fruits[1] = banana [OK]

Hint: Remember array indexes start at 0 in JavaScript [OK]

Common Mistakes:

• Thinking index starts at 1
• Confusing element positions
• Expecting undefined for valid index

4. Find the error in this code that tries to add an element to an array:

const numbers = [1, 2, 3];
numbers.add(4);

medium

A. Numbers array should be a string

B. Missing semicolon after array declaration

C. Array should be declared with curly braces

D. Using add() method which does not exist for arrays

Solution

1. Step 1: Check array methods for adding elements

   JavaScript arrays use push() to add elements, not add().

2. Step 2: Identify the incorrect method usage

   Calling numbers.add(4) causes an error because add() is undefined for arrays.

3. Final Answer:

   Using add() method which does not exist for arrays -> Option D

4. Quick Check:

   Use push() to add elements, not add() [OK]

Hint: Use push() to add elements to arrays, not add() [OK]

Common Mistakes:

• Using add() instead of push()
• Thinking arrays use curly braces
• Confusing arrays with strings

5. You want to store a list of student names and quickly access any name by its position. Which data structure solves this problem best and why?

hard

A. Object, because it stores key-value pairs without order

B. Array, because it stores ordered elements accessible by index

C. Set, because it stores unique values without order

D. Map, because it stores key-value pairs with keys as indexes

Solution

1. Step 1: Understand the need for ordered storage with index access

   We want to store names in order and access by position number.

2. Step 2: Compare data structures for ordered access

   Arrays keep order and allow access by index, objects and sets do not guarantee order, maps use keys but not numeric indexes by default.

3. Final Answer:

   Array, because it stores ordered elements accessible by index -> Option B

4. Quick Check:

   Ordered access by position = Array [OK]

Hint: Use arrays for ordered lists with index access [OK]

Common Mistakes:

• Choosing objects or sets which lack order
• Thinking maps use numeric indexes by default
• Ignoring the need for order and position access