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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array Initialization with Spread Operator

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

DSA Typescript

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

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

B [1, 2, 3]\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 new array or a reference to arr1.

The spread operator creates a shallow copy of arr1 into arr2. Modifying arr2 does not affect arr1.

❓ Predict Output

intermediate

Array Initialization with Fixed Size and Default Values

What will be the output of this TypeScript code that initializes an array with a fixed size and default values?

DSA Typescript

const arr = new Array(3).fill(0);
arr[1] = 5;
console.log(arr);

A [0, 0, 5]

B [5, 0, 0]

C [0, 5, 0]

D [undefined, 5, undefined]

💡 Hint

Remember that fill sets all elements to the same value initially.

The array is initialized with three zeros. Then the element at index 1 is changed to 5.

🔧 Debug

advanced

Identify the Error in Array Initialization

What error will this TypeScript code produce when run?

DSA Typescript

const arr: number[] = new Array(-1);

A TypeError: Cannot assign to read only property

B No error, creates empty array

C SyntaxError: Unexpected token

D RangeError: Invalid array length

💡 Hint

Array size cannot be negative.

Array constructor throws RangeError if length is negative.

❓ Predict Output

advanced

Output of Nested Array Initialization and Modification

What is the output of this TypeScript code involving nested arrays?

DSA Typescript

const arr = Array(3).fill([0]);
arr[0][0] = 5;
console.log(arr);

A [[5], [5], [5]]

B [[5], [0], [0]]

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

D [[5], [5], [0]]

💡 Hint

Consider whether fill creates separate inner arrays or references the same one.

fill uses the same reference for each element, so changing one changes all.

🧠 Conceptual

expert

Understanding Array Initialization with map vs fill

Which statement correctly explains the difference between initializing an array with fill and map in TypeScript?

A fill creates multiple references to the same object, map creates new objects for each element

B fill creates new objects for each element, map creates multiple references to the same object

C Both fill and map create new objects for each element

D Both fill and map create multiple references to the same object

💡 Hint

Think about how fill and map assign values to array elements.

fill assigns the same reference to all elements; map calls a function to create new objects.

Practice

1. Which of the following is the correct way to declare an array of numbers in TypeScript?

easy

A. const numbers: number[] = [1, 2, 3];

B. const numbers: number = [1, 2, 3];

C. const numbers: array = (1, 2, 3);

D. const numbers = {1, 2, 3};

Solution

1. Step 1: Understand TypeScript array syntax

   In TypeScript, arrays are declared using the type followed by square brackets, e.g., number[].

2. Step 2: Check each option's syntax

   const numbers: number[] = [1, 2, 3]; correctly declares an array of numbers. Options A, B, and D use incorrect syntax for arrays.

3. Final Answer:

   const numbers: number[] = [1, 2, 3]; -> Option A

4. Quick Check:

   Array declaration with type[] = correct syntax [OK]

Hint: Use type[] to declare arrays in TypeScript [OK]

Common Mistakes:

• Using type without [] for arrays
• Using parentheses instead of square brackets
• Using curly braces instead of square brackets

2. Which of the following is the correct syntax to initialize an empty array of strings in TypeScript?

easy

A. let names = string[];

B. let names: string = [];

C. let names: string[] = [];

D. let names: [] = string;

Solution

1. Step 1: Identify correct type annotation for empty string array

   To declare an empty array of strings, use string[] as the type and assign an empty array [].

2. Step 2: Validate each option

   let names: string[] = []; correctly uses let names: string[] = []. Others have syntax errors or wrong type placement.

3. Final Answer:

   let names: string[] = []; -> Option C

4. Quick Check:

   Empty array of strings = string[] = correct [OK]

Hint: Use type[] with empty brackets to declare empty arrays [OK]

Common Mistakes:

• Omitting [] after type for arrays
• Assigning type incorrectly after =
• Using invalid syntax like let names = string[];

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

const arr: number[] = [10, 20, 30];
arr[1] = 50;
console.log(arr);

medium

A. [10, 20, 30]

B. [10, 50, 30]

C. [50, 20, 30]

D. Error: Cannot assign to arr[1]

Solution

1. Step 1: Understand array element assignment

   Arrays allow changing elements by index. arr[1] changes the second element.

2. Step 2: Apply the assignment and print

   arr[1] = 50 changes the second element from 20 to 50. Printing arr shows [10, 50, 30].

3. Final Answer:

   [10, 50, 30] -> Option B

4. Quick Check:

   Index assignment updates array element [OK]

Hint: Array index starts at 0; arr[1] is second element [OK]

Common Mistakes:

• Thinking arrays are immutable
• Confusing index 1 with index 0
• Expecting syntax error on assignment

4. Identify the error in the following TypeScript array declaration:

let nums: number[] = (1, 2, 3);

medium

A. Array elements must be strings

B. Missing type annotation for nums

C. Using let instead of const

D. Using parentheses instead of square brackets for array initialization

Solution

1. Step 1: Check array initialization syntax

   Arrays must be initialized with square brackets []. Parentheses () create a comma expression, not an array.

2. Step 2: Identify the error

   Using (1, 2, 3) does not create an array but evaluates to 3. This is incorrect for array initialization.

3. Final Answer:

   Using parentheses instead of square brackets for array initialization -> Option D

4. Quick Check:

   Arrays use [] not () for initialization [OK]

Hint: Use [] for arrays, () is for grouping expressions [OK]

Common Mistakes:

• Confusing parentheses with brackets
• Ignoring type annotations
• Thinking let vs const causes error here

5. You want to declare and initialize a 2D array of numbers with 3 rows and 2 columns, all initialized to zero. Which is the correct TypeScript code?

hard

A. const matrix: number[][] = [[0, 0], [0, 0], [0, 0]];

B. const matrix: number[3][2] = [[0, 0], [0, 0], [0, 0]];

C. const matrix: number[] = [0, 0, 0, 0, 0, 0];

D. const matrix = new Array(3, 2).fill(0);

Solution

1. Step 1: Understand 2D array declaration in TypeScript

   2D arrays are arrays of arrays, so type is number[][].

2. Step 2: Check initialization for 3 rows and 2 columns

   const matrix: number[][] = [[0, 0], [0, 0], [0, 0]]; correctly initializes 3 arrays each with 2 zeros. const matrix: number[3][2] = [[0, 0], [0, 0], [0, 0]]; uses invalid syntax. const matrix: number[] = [0, 0, 0, 0, 0, 0]; is 1D array. const matrix = new Array(3, 2).fill(0); creates array with 2 elements, not 2D.

3. Final Answer:

   const matrix: number[][] = [[0, 0], [0, 0], [0, 0]]; -> Option A

4. Quick Check:

   2D array = array of arrays with type number[][] [OK]

Hint: 2D arrays use type number[][] and nested brackets [OK]

Common Mistakes:

• Using number[3][2] which is invalid in TypeScript
• Confusing 1D and 2D array initialization
• Using new Array with multiple arguments incorrectly