Number type behavior in Typescript - Mini Project: Build & Apply

Number type behavior

📖 Scenario: Imagine you are working with numbers in a simple calculator app. You want to understand how numbers behave in TypeScript, including integers, decimals, and special values like Infinity and NaN.

🎯 Goal: You will create variables with different number types, use a helper variable, perform calculations, and finally print the results to see how TypeScript handles these numbers.

📋 What You'll Learn

Create variables with integer, decimal, Infinity, and NaN values

Add a helper variable to hold a multiplier

Use arithmetic operations to calculate new values

Print the final results to the console

💡 Why This Matters

🌍 Real World

Understanding number behavior is important when building calculators, games, or any app that does math calculations.

💼 Career

Many programming jobs require handling numbers correctly, especially when working with financial data, scientific calculations, or user input validation.

1

Create number variables

Create four variables called integerNum, decimalNum, infinityNum, and nanNum with these exact values: 42, 3.14, Infinity, and NaN respectively.

Typescript

// Create the four number variables with the exact values
// Your code here

Hint

Use let to declare each variable and assign the exact number values given.

2

Add a multiplier variable

Create a variable called multiplier and set it to 2.

Typescript

let integerNum = 42;
let decimalNum = 3.14;
let infinityNum = Infinity;
let nanNum = NaN;

// Create the multiplier variable
// Your code here

Hint

Use let multiplier = 2; to create the helper variable.

3

Calculate new values

Create four new variables called intResult, decResult, infResult, and nanResult. Multiply integerNum, decimalNum, infinityNum, and nanNum by multiplier respectively and assign the results to these new variables.

Typescript

let integerNum = 42;
let decimalNum = 3.14;
let infinityNum = Infinity;
let nanNum = NaN;

let multiplier = 2;

// Multiply each number by multiplier and store in new variables
// Your code here

Hint

Use the * operator to multiply each original number by multiplier.

4

Print the results

Use four console.log statements to print intResult, decResult, infResult, and nanResult in this exact order.

Typescript

let integerNum = 42;
let decimalNum = 3.14;
let infinityNum = Infinity;
let nanNum = NaN;

let multiplier = 2;

let intResult = integerNum * multiplier;
let decResult = decimalNum * multiplier;
let infResult = infinityNum * multiplier;
let nanResult = nanNum * multiplier;

// Print the results in order
// Your code here

Hint

Use console.log(variableName); to print each result on its own line.

Practice

1.

In TypeScript, which of the following is true about the number type?

easy

A. It represents both whole numbers and decimals using the same type.

B. It only represents whole numbers (integers).

C. It only represents decimal numbers (floats).

D. You must declare separate types for integers and decimals.

Solution

1. Step 1: Understand TypeScript number type

   TypeScript uses a single number type for all numeric values, whether whole or decimal.

2. Step 2: Compare options with this fact

   Only "It represents both whole numbers and decimals using the same type." correctly states that both whole and decimal numbers share the same number type.

3. Final Answer:

   It represents both whole numbers and decimals using the same type. -> Option A

4. Quick Check:

   TypeScript number = whole + decimal [OK]

Hint: Remember: one number type covers all numeric values [OK]

Common Mistakes:

• Thinking integers and decimals have separate types
• Assuming number type is only for floats
• Believing you must declare int or float explicitly

2.

Which of the following is the correct way to declare a variable x as a number with value 42 in TypeScript?

?

easy

A. let x: number = "42";

B. let x = "42";

C. let x: number = 42;

D. let x: string = 42;

Solution

1. Step 1: Check variable declaration syntax

   In TypeScript, to declare a number variable, use let x: number = value;.

2. Step 2: Validate each option

   let x: number = 42; correctly declares x as a number with value 42. Other options use wrong types or values, such as assigning a string to a number type, which is invalid.

3. Final Answer:

   let x: number = 42; -> Option C

4. Quick Check:

   Correct number declaration = let x: number = 42; [OK]

Hint: Use let x: number = value; for numbers [OK]

Common Mistakes:

• Assigning string values to number variables
• Missing the type annotation when needed
• Using quotes around numeric values

3.

What is the output of the following TypeScript code?

const a: number = 5;
const b: number = 2.5;
const c = a + b;
console.log(c);

medium

A. 7

B. Error

C. 52.5

D. 7.5

Solution

1. Step 1: Add the two numbers

   Variable a is 5 (integer), b is 2.5 (decimal). Adding gives 5 + 2.5 = 7.5.

2. Step 2: Check console output

   The console.log(c) prints the sum, which is 7.5.

3. Final Answer:

   7.5 -> Option D

4. Quick Check:

   5 + 2.5 = 7.5 [OK]

Hint: Add numbers normally; decimals stay decimals [OK]

Common Mistakes:

• Treating numbers as strings and concatenating
• Rounding automatically to integers
• Expecting a syntax or runtime error

4.

Identify the error in this TypeScript code snippet:

let num: number = 10;
num = "20";
console.log(num);

medium

A. Assigning a string to a variable declared as number causes a type error.

B. The variable num should be declared with const instead of let.

C. The console.log statement is missing parentheses.

D. There is no error; the code runs fine.

Solution

1. Step 1: Check variable type and assignment

   num is declared as number, but assigned a string "20" later, which is invalid in TypeScript.

2. Step 2: Identify the error type

   This causes a type error because TypeScript enforces type safety.

3. Final Answer:

   Assigning a string to a variable declared as number causes a type error. -> Option A

4. Quick Check:

   Type mismatch error = assigning string to number [OK]

Hint: Match variable type and assigned value exactly [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking TypeScript allows implicit type changes
• Confusing let and const as error causes

5.

Consider the following TypeScript code:

const values = [1, 2.5, 3, 4.75];
const integersOnly = values.filter(v => Number.isInteger(v));
console.log(integersOnly);

What will be the output?

hard

A. [1, 2.5, 3, 4.75]

B. [1, 3]

C. [2.5, 4.75]

D. []

Solution

1. Step 1: Understand Number.isInteger usage

   The filter keeps only values where Number.isInteger(v) returns true, i.e., whole numbers.

2. Step 2: Apply filter to array

   From [1, 2.5, 3, 4.75], only 1 and 3 are integers, so the filtered array is [1, 3].

3. Final Answer:

   [1, 3] -> Option B

4. Quick Check:

   Filter integers only = [1, 3] [OK]

Hint: Use Number.isInteger to filter whole numbers [OK]

Common Mistakes:

• Expecting decimals to pass Number.isInteger
• Confusing filter output with original array
• Using isInteger incorrectly as a method on numbers