Array Declaration and Initialization in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an array in simple terms?

An array is like a row of boxes where each box can hold a value. You can find any box by its position number.

beginner

How do you declare an empty array of numbers in TypeScript?

You write: <code>let arr: number[] = [];</code> This means arr is an array that will hold numbers, and it starts empty.

beginner

What does this code do? <br><code>let fruits: string[] = ['apple', 'banana', 'cherry'];</code>

It creates an array named fruits that holds three strings: 'apple', 'banana', and 'cherry'.

intermediate

How can you declare an array with a fixed size but no values yet in TypeScript?

You can write: <code>let arr: number[] = new Array(5);</code> This creates an array with 5 empty slots.

beginner

What happens if you try to put a string in a number array in TypeScript?

TypeScript will give an error because the array is only for numbers. It helps catch mistakes early.

How do you declare an array of strings named 'colors' with values 'red' and 'blue' in TypeScript?

A let colors = (string) ['red', 'blue'];

B let colors: array = ['red', 'blue'];

C let colors: string[] = ['red', 'blue'];

D let colors: string = ['red', 'blue'];

Option C correctly declares a string array with two values.

What is the initial length of let nums = new Array(4);?

A 1

B 0

C undefined

D 4

The array has 4 empty slots, so its length is 4.

Which of these is NOT a valid way to declare an empty number array in TypeScript?

A let arr: number = [];

B let arr: number[] = new Array();

C let arr: number[] = [];

D let arr = [];

Option A is wrong because arr is declared as a number, not an array.

What type of error will TypeScript show if you add a string to a number array?

A Type error

B Syntax error

C Runtime error

D No error

TypeScript shows a type error because the value does not match the array type.

How do you access the first element of an array named 'items'?

A items(0)

B items[0]

C items.first()

D items{0}

Square brackets with index 0 access the first element.

Explain how to declare and initialize an array of numbers with values 1, 2, and 3 in TypeScript.

Think about the syntax for arrays and how to specify types.

Describe what happens when you create an array with new Array(5) in TypeScript.

Consider the difference between size and content.

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