Array Declaration and Initialization in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array Initialization with Spread Operator

What is the output of the following JavaScript code after array initialization and modification?

DSA Javascript

const arr1 = [1, 2, 3];
const arr2 = [...arr1];
arr2.push(4);
console.log(arr1);
console.log(arr2);

A [1, 2, 3]\n[1, 2, 3, 4]

B [1, 2, 3, 4]\n[1, 2, 3, 4]

C [1, 2, 3]\n[1, 2, 3]

D [1, 2, 3, 4]\n[1, 2, 3]

💡 Hint

Think about whether arr2 is a copy or a reference to arr1.

The spread operator creates a shallow copy of arr1 into arr2. Modifying arr2 does not affect arr1. So arr1 remains [1, 2, 3], arr2 becomes [1, 2, 3, 4].

❓ Predict Output

intermediate

Output of Array.fill() Method

What is the output of the following JavaScript code using Array.fill()?

DSA Javascript

const arr = new Array(4).fill(0);
arr[1] = 5;
console.log(arr);

A [0, 0, 0, 0]

B [5, 5, 5, 5]

C [0, 5, 0, 0]

D [undefined, 5, undefined, undefined]

💡 Hint

Array.fill sets all elements to the same value initially.

Array.fill(0) sets all 4 elements to 0. Then arr[1] is changed to 5. So the array becomes [0, 5, 0, 0].

❓ Predict Output

advanced

Output of Nested Array Initialization and Modification

What is the output of this code involving nested arrays and initialization?

DSA Javascript

const arr = new Array(3).fill([0, 0]);
arr[0][0] = 1;
console.log(arr);

A [[1, 0], [1, 0], [0, 0]]

B [[1, 0], [0, 0], [0, 0]]

C [[0, 0], [0, 0], [0, 0]]

D [[1, 0], [1, 0], [1, 0]]

💡 Hint

Consider whether the inner arrays are shared references or separate copies.

Using fill with an array fills all positions with the same reference. Changing arr[0][0] changes all inner arrays at index 0.

🧠 Conceptual

advanced

Understanding Array Initialization with map()

Which code snippet correctly initializes an array of length 5 with values 0 to 4?

A Array(5).map((_, i) => i)

B Array.from({ length: 5 }, (_, i) => i)

C new Array(5).fill().map((_, i) => i)

D [...Array(5)].map((_, i) => i)

💡 Hint

Remember that map() on empty slots does not run.

Array.from with length and map function correctly creates [0,1,2,3,4]. map() on empty slots does not execute callback.

🔧 Debug

expert

Identify the Error in Array Initialization

What error does the following code produce?

DSA Javascript

const arr = Array(3).fill([]);
arr[0].push(1);
console.log(arr);

A No error; output: [[1], [1], [1]]

B TypeError: arr[0].push is not a function

C SyntaxError: Unexpected token

D ReferenceError: arr is not defined

💡 Hint

Think about what fill([]) does with references.

fill([]) fills all positions with the same array reference. Pushing to arr[0] affects all elements.

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