Number type behavior in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic type used for numbers in TypeScript?

TypeScript uses the number type to represent all numeric values, including integers and floating-point numbers.

beginner

What happens when you perform arithmetic operations with number types in TypeScript?

Arithmetic operations like addition, subtraction, multiplication, and division work as expected on number types, following JavaScript's number rules.

intermediate

How does TypeScript handle special numeric values like NaN and Infinity?

NaN (Not a Number) and Infinity are special number values in TypeScript, inherited from JavaScript, used to represent invalid or unbounded results.

intermediate

Can TypeScript distinguish between integers and floating-point numbers with the number type?

No, TypeScript's number type does not differentiate between integers and floating-point numbers; both are treated as the same type.

advanced

What is the range of safe integers in TypeScript's number type?

The safe integer range is from Number.MIN_SAFE_INTEGER (−(2⁵³ − 1)) to Number.MAX_SAFE_INTEGER (2⁵³ − 1). Outside this range, precision may be lost.

Which type does TypeScript use for all numeric values?

A int

B number

C float

D decimal

TypeScript uses the 'number' type for all numeric values, including integers and floats.

What is the result of dividing a positive number by zero in TypeScript?

A NaN

B Error

C 0

D Infinity

Dividing a positive number by zero results in 'Infinity' in TypeScript.

Does TypeScript's number type distinguish between integers and floating-point numbers?

A No, both are the same type

B Only for integers

C Yes, it has separate types

D Only for floating-point numbers

TypeScript treats both integers and floating-point numbers as the same 'number' type.

What does NaN represent in TypeScript?

A Zero

B A valid number

C Not a Number (invalid numeric result)

D Infinity

NaN means 'Not a Number', representing an invalid numeric result.

What is the significance of Number.MAX_SAFE_INTEGER?

A Maximum integer value that can be safely represented without precision loss

B Maximum number allowed in TypeScript

C Maximum floating-point value

D Maximum array length

Number.MAX_SAFE_INTEGER is the largest integer that can be safely represented without losing precision.

Explain how TypeScript handles the number type and its special values like NaN and Infinity.

Think about how JavaScript treats numbers and special cases.

Describe the concept of safe integers in TypeScript and why it matters.

Consider what happens when numbers get very big.

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