Boolean type behavior in Typescript - Time & Space Complexity

Time Complexity: Boolean type behavior

O(n)

Understanding Time Complexity

We want to understand how the time it takes to work with Boolean values changes as we use more of them.

How does the program's work grow when handling many Boolean values?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function countTrue(values: boolean[]): number {
  let count = 0;
  for (const val of values) {
    if (val) {
      count++;
    }
  }
  return count;
}
    

This code counts how many true values are in a list of Booleans.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each Boolean in the array once.
• How many times: Exactly once for each item in the input list.

How Execution Grows With Input

As the list gets longer, the program checks each Boolean one by one.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The work grows directly with the number of Booleans.

Final Time Complexity

Time Complexity: O(n)

This means the time to count true values grows in a straight line as the list gets longer.

Common Mistake

[X] Wrong: "Checking Booleans is instant and does not depend on list size."

[OK] Correct: Even though each check is quick, you must check every item, so more items mean more total work.

Interview Connect

Understanding how simple loops over Booleans scale helps you explain basic efficiency clearly and confidently.

Self-Check

"What if we changed the array to a nested array of Booleans? How would the time complexity change?"

Practice

1. Which of the following is the correct way to declare a boolean variable in TypeScript?

easy

A. let isDone: Boolean = 'true';

B. let isDone: bool = true;

C. let isDone = Boolean;

D. let isDone: boolean = true;

Solution

1. Step 1: Understand TypeScript boolean type

   TypeScript uses the lowercase boolean type to declare true/false values.

2. Step 2: Check each option's syntax

   let isDone: boolean = true; uses boolean with a true value correctly. let isDone: bool = true; uses bool which is invalid. let isDone = Boolean; assigns the Boolean constructor, not a value. let isDone: Boolean = 'true'; uses uppercase Boolean and a string, which is incorrect.

3. Final Answer:

   let isDone: boolean = true; -> Option D

4. Quick Check:

   TypeScript boolean type = let var: boolean = true [OK]

Hint: Use lowercase 'boolean' type for true/false in TypeScript [OK]

Common Mistakes:

• Using 'bool' instead of 'boolean'
• Assigning string 'true' instead of boolean true
• Using uppercase 'Boolean' type incorrectly

2. Which of the following is the correct syntax to assign a boolean value in TypeScript?

easy

A. let flag: boolean = false;

B. let flag: boolean = 'false';

C. let flag: boolean = 0;

D. let flag: boolean = True;

Solution

1. Step 1: Identify valid boolean values

   Boolean values in TypeScript are only true or false without quotes and lowercase.

2. Step 2: Evaluate each option

   let flag: boolean = false; correctly assigns false. let flag: boolean = 'false'; uses a string 'false' which is invalid. let flag: boolean = 0; uses number 0 which is not boolean. let flag: boolean = True; uses capitalized True which is invalid in TypeScript.

3. Final Answer:

   let flag: boolean = false; -> Option A

4. Quick Check:

   Boolean values are true or false without quotes [OK]

Hint: Assign boolean values as true or false without quotes [OK]

Common Mistakes:

• Using strings 'true' or 'false' instead of boolean literals
• Using numbers like 0 or 1 as booleans
• Capitalizing true or false

3. What will be the output of the following TypeScript code?

let a: boolean = true;
let b: boolean = false;
console.log(a && b);

medium

A. true

B. false

C. undefined

D. TypeError

Solution

1. Step 1: Understand boolean AND operation

   The && operator returns true only if both operands are true.

2. Step 2: Evaluate the operands

   Variable a is true, b is false, so true && false is false.

3. Final Answer:

   false -> Option B

4. Quick Check:

   true AND false = false [OK]

Hint: AND returns true only if both are true [OK]

Common Mistakes:

• Assuming true && false is true
• Confusing && with || operator
• Expecting runtime error for boolean operations

4. Identify the error in the following TypeScript code:

let isActive: boolean = True;
console.log(isActive);

medium

A. No error, prints true

B. TypeError: Cannot assign True to boolean

C. SyntaxError: True is not defined

D. Runtime error: invalid boolean

Solution

1. Step 1: Check boolean literal case sensitivity

   In TypeScript, boolean literals are lowercase true and false. True with uppercase T is undefined.

2. Step 2: Understand error type

   Using True causes a ReferenceError because it is not declared or recognized as a boolean.

3. Final Answer:

   SyntaxError: True is not defined -> Option C

4. Quick Check:

   Boolean literals must be lowercase true or false [OK]

Hint: Boolean literals are lowercase true/false only [OK]

Common Mistakes:

• Using uppercase True or False
• Expecting TypeScript to auto-correct case
• Confusing boolean literals with strings

5. Consider the following TypeScript code snippet:

function checkValue(val: any): boolean {
  return !!val;
}
console.log(checkValue(0));
console.log(checkValue('hello'));
console.log(checkValue(null));

What will be the output of this code?

hard

A. false true false

B. true true false

C. false false true

D. true false true

Solution

1. Step 1: Understand double negation (!!) in TypeScript

   The !! converts any value to its boolean equivalent: falsy values become false, truthy become true.

2. Step 2: Evaluate each input

   0 is falsy, so !!0 is false. 'hello' is a non-empty string (truthy), so !!'hello' is true. null is falsy, so !!null is false.

3. Final Answer:

   false true false -> Option A

4. Quick Check:

   !!value converts to boolean true/false correctly [OK]

Hint: Double negation (!!) converts any value to boolean [OK]

Common Mistakes:

• Assuming 0 is true
• Confusing !! with single !
• Expecting null to be true