Array Declaration and Initialization in DSA Typescript - Build from Scratch

Array Declaration and Initialization

📖 Scenario: You are helping a small shop owner keep track of the prices of some fruits they sell. You will create an array to store these prices and then display them.

🎯 Goal: Create an array called fruitPrices with exact prices, then print the array to see all prices.

📋 What You'll Learn

Create an array called fruitPrices with the exact values: 10, 20, 30, 40, 50

Create a variable called numberOfFruits and set it to the length of fruitPrices

Use a for loop with variables index to iterate from 0 to numberOfFruits - 1

Inside the loop, print each price from fruitPrices using the index

Print the entire fruitPrices array at the end

💡 Why This Matters

🌍 Real World

Arrays are used everywhere to store lists of items like prices, names, or measurements. This helps shops, websites, and apps keep track of many things easily.

💼 Career

Knowing how to create and use arrays is a basic skill for any programming job, especially when working with collections of data.

1

Create the array of fruit prices

Create an array called fruitPrices with these exact values: 10, 20, 30, 40, 50

DSA Typescript

// Your code here

Hint

Use square brackets [] to create the array and separate values with commas.

2

Create a variable for the number of fruits

Create a variable called numberOfFruits and set it to the length of the fruitPrices array

DSA Typescript

const fruitPrices: number[] = [10, 20, 30, 40, 50];
// Your code here

Hint

Use the .length property to get the number of items in the array.

3

Loop through the array and print each price

Use a for loop with variable index to go from 0 to numberOfFruits - 1. Inside the loop, print each price from fruitPrices using index

DSA Typescript

const fruitPrices: number[] = [10, 20, 30, 40, 50];
const numberOfFruits: number = fruitPrices.length;
// Your code here

Hint

Use a for loop starting at 0 and ending before numberOfFruits. Use console.log to print each price.

4

Print the entire array

Print the entire fruitPrices array using console.log(fruitPrices)

DSA Typescript

const fruitPrices: number[] = [10, 20, 30, 40, 50];
const numberOfFruits: number = fruitPrices.length;
for (let index = 0; index < numberOfFruits; index++) {
    console.log(fruitPrices[index]);
}
// Your code here

Hint

Use console.log(fruitPrices) to print the whole array after the loop.

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