Array Declaration and Initialization in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an empty array named numbers.

DSA Javascript

const numbers = [1];

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

A()

Bnull

C{}

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which create objects, not arrays.

Using parentheses () which are for function calls.

Assigning null instead of an empty array.

In JavaScript, arrays are declared using square brackets []. So, const numbers = []; creates an empty array.

2fill in blank

medium

Complete the code to create an array fruits with three elements: 'apple', 'banana', and 'cherry'.

DSA Javascript

const fruits = [1];

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

A['apple', 'banana', 'cherry']

B('apple', 'banana', 'cherry')

C{'apple', 'banana', 'cherry'}

D'apple', 'banana', 'cherry'

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or curly braces instead of square brackets.

Forgetting commas between elements.

Not putting quotes around string elements.

Arrays in JavaScript are created using square brackets with elements separated by commas.

3fill in blank

hard

Fix the error in the code to correctly declare an array colors with 'red', 'green', and 'blue'.

DSA Javascript

const colors = [1];

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

A['red'; 'green'; 'blue']

B['red', 'green', 'blue']

C['red' 'green' 'blue']

D('red', 'green', 'blue')

Attempts:

3 left

💡 Hint

Common Mistakes

Using semicolons instead of commas between elements.

Omitting commas between elements.

Using parentheses instead of square brackets.

Array elements must be separated by commas inside square brackets. Semicolons or missing commas cause errors.

4fill in blank

hard

Fill both blanks to create an array numbers with numbers 1 to 5.

DSA Javascript

const numbers = [1];
console.log(numbers[2]);

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

A[1, 2, 3, 4, 5]

Blength

Csize

D(1, 2, 3, 4, 5)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets for array declaration.

Using .size which is not valid for arrays in JavaScript.

Arrays are declared with square brackets. To get the number of elements, use the length property.

5fill in blank

hard

Fill all three blanks to declare an array letters, add 'd', and print the element at index 2 of the updated array.

DSA Javascript

const letters = [1];
letters[2]('d');
console.log(letters[3]);

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

A['a', 'b', 'c']

Bpush

C[2]

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push to add elements.

Trying to access elements with parentheses instead of square brackets.

Forgetting to declare the array with square brackets.

Declare the array with square brackets. Use push to add an element. Access element at index 2 with [2].

Practice

1. Which of the following is the correct way to declare an empty array in JavaScript?

easy

A. const arr = ();

B. const arr = [];

C. const arr = {};

D. const arr = array();

Solution

1. Step 1: Understand array declaration syntax

   In JavaScript, arrays are declared using square brackets [].

2. Step 2: Check each option

   const arr = []; uses square brackets which is correct. Options B and C use parentheses and curly braces which are incorrect for arrays. const arr = array(); uses a function call which is not valid.

3. Final Answer:

   const arr = []; -> Option B

4. Quick Check:

   Array declaration uses [] = const arr = []; [OK]

Hint: Arrays use square brackets [] for declaration [OK]

Common Mistakes:

• Using parentheses () instead of square brackets
• Using curly braces {} which declare objects
• Trying to call array() like a function

2. Which of the following is the correct syntax to declare and initialize an array with values 1, 2, and 3?

easy

A. const arr = array[1, 2, 3];

B. const arr = (1, 2, 3);

C. const arr = {1, 2, 3};

D. const arr = [1, 2, 3];

Solution

1. Step 1: Recall array initialization syntax

   Arrays are initialized with square brackets containing comma-separated values.

2. Step 2: Evaluate each option

   const arr = [1, 2, 3]; correctly uses square brackets with values. Options B and C use parentheses and curly braces which are invalid. const arr = array[1, 2, 3]; uses incorrect syntax with 'array' keyword.

3. Final Answer:

   const arr = [1, 2, 3]; -> Option D

4. Quick Check:

   Array with values uses [] and commas = const arr = [1, 2, 3]; [OK]

Hint: Use square brackets with commas to list values [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using curly braces which create objects
• Trying to use 'array' keyword which doesn't exist

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

const arr = new Array(3);
console.log(arr);

medium

A. [undefined, undefined, undefined]

B. [0, 0, 0]

C. [empty x 3]

D. SyntaxError

Solution

1. Step 1: Understand new Array(n) behavior

   Using new Array(3) creates an array with length 3 but no defined values, called a sparse array.

2. Step 2: Check console output for sparse arrays

   Console shows sparse arrays as [empty x 3], not as undefined or zeros.

3. Final Answer:

   [empty x 3] -> Option C

4. Quick Check:

   new Array(3) creates sparse array = [empty x 3] [OK]

Hint: new Array(n) creates empty slots, not undefined values [OK]

Common Mistakes:

• Assuming new Array(n) fills with zeros
• Thinking undefined values appear explicitly
• Expecting a syntax error

4. Identify the error in the following array declaration and fix it:

const arr = new Array[1, 2, 3];

medium

A. Replace square brackets with parentheses: new Array(1, 2, 3);

B. Remove 'new' keyword: Array[1, 2, 3];

C. Use curly braces instead: new Array{1, 2, 3};

D. No error, code is correct

Solution

1. Step 1: Identify correct syntax for new Array()

   The Array constructor requires parentheses, not square brackets, to pass elements.

2. Step 2: Fix the syntax

   Change new Array[1, 2, 3] to new Array(1, 2, 3) to correctly create the array.

3. Final Answer:

   Replace square brackets with parentheses: new Array(1, 2, 3); -> Option A

4. Quick Check:

   Array constructor uses () not [] = Replace square brackets with parentheses: new Array(1, 2, 3); [OK]

Hint: Array constructor needs parentheses, not square brackets [OK]

Common Mistakes:

• Using square brackets with new Array
• Removing 'new' keyword incorrectly
• Using curly braces which are invalid here

5. How can you programmatically create a 2D array (array of arrays) with 3 rows and 2 columns, all initialized to zero in JavaScript?

hard

A. const arr = Array.from({length: 3}, () => Array(2).fill(0));

B. const arr = Array(3).fill(Array(2).fill(0));

C. const arr = [[0,0],[0,0],[0,0]];

D. All of the above

Solution

1. Step 1: Understand 2D array initialization

   Using Array.from with a function creates separate inner arrays to avoid shared references.

2. Step 2: Analyze each option

   const arr = Array(3).fill(Array(2).fill(0)); fills outer array with the same inner array reference, causing changes in one row to affect all. const arr = Array.from({length: 3}, () => Array(2).fill(0)); correctly creates separate inner arrays. const arr = [[0,0],[0,0],[0,0]]; manually creates the array but is less flexible. All of the above is incorrect because only A is fully correct for dynamic initialization.

3. Final Answer:

   const arr = Array.from({length: 3}, () => Array(2).fill(0)); -> Option A

4. Quick Check:

   Use Array.from with function for separate inner arrays = const arr = Array.from({length: 3}, () => Array(2).fill(0)); [OK]

Hint: Use Array.from with function to avoid shared inner arrays [OK]

Common Mistakes:

• Using fill with array causes shared references
• Manually typing arrays is inflexible
• Assuming all options are correct