Array Declaration and Initialization in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of numbers.

DSA Typescript

let numbers: number[] = [1];

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

A{1, 2, 3}

B[1, 2, 3]

C(1, 2, 3)

D<1, 2, 3>

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which is for objects, not arrays.

Using parentheses () which is for tuples or function calls.

Using angle brackets <> which is for generics, not arrays.

In TypeScript, arrays are declared using square brackets []. So, [1, 2, 3] is the correct way to initialize an array of numbers.

2fill in blank

medium

Complete the code to declare an empty array of strings.

DSA Typescript

let fruits: string[] = [1];

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

Anull

B{}

C[]

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using {} which creates an object, not an array.

Using null or empty string which are not arrays.

An empty array is declared with empty square brackets []. This creates an array ready to hold string elements.

3fill in blank

hard

Fix the error in the array declaration to hold boolean values.

DSA Typescript

let flags: [1] = [true, false, true];

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

Aboolean[]

Bboolean

CBoolean[]

Dbool[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Boolean[]' which refers to the object wrapper, not the primitive type.

Using 'bool[]' which is not a valid TypeScript type.

In TypeScript, the correct way to declare an array of booleans is boolean[]. 'Boolean[]' is incorrect because Boolean is an object type, not the primitive type.

4fill in blank

hard

Fill both blanks to declare and initialize an array of numbers with values 10, 20, 30.

DSA Typescript

let scores: [1] = [2];

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

Anumber[]

B[10, 20, 30]

CArray<number>

D{10, 20, 30}

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} to initialize the array which is incorrect.

Mixing declaration styles like 'number[]' with object initialization.

The variable 'scores' is declared as an array of numbers using 'number[]'. The array is initialized with values using square brackets [10, 20, 30].

5fill in blank

hard

Fill all three blanks to declare a constant array of strings and add a new element.

DSA Typescript

const colors: [1] = ['red', 'green'];
colors.[2]('blue');
console.log(colors); // Output: [3]

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

Astring[]

Bpush

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

DArray<string>

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' and then trying to reassign the array variable instead of modifying it.

Using incorrect method names like 'add' instead of 'push'.

Expecting the output to be a string instead of an array.

The array 'colors' is declared as a string array using 'string[]'. The 'push' method adds 'blue' to the array. The final output shows all three colors in the array.

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