Boolean type behavior in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Boolean conversion of different values

What is the output of this TypeScript code?

Typescript

const values = [0, 1, "", "hello", null, undefined, [], {}];
const results = values.map(v => Boolean(v));
console.log(results);

A [false, true, false, true, false, false, true, true]

B [false, true, true, true, false, false, false, true]

C [false, true, false, true, true, false, true, false]

D [true, true, false, true, false, true, true, true]

💡 Hint

Remember that Boolean() converts values to true or false based on their truthiness.

In JavaScript/TypeScript, Boolean() converts values to true or false based on truthiness. 0, empty string "", null, and undefined are false. Non-empty strings, arrays, and objects are true.

❓ Predict Output

intermediate

Boolean logic with && and || operators

What is the output of this TypeScript code?

Typescript

const a = 0;
const b = "hello";
const c = null;
console.log(a && b);
console.log(b || c);

A

0
hello

B

hello
null

C

false
hello

D

0
null

💡 Hint

Remember how && returns the first falsy or last truthy value, and || returns the first truthy or last falsy value.

The && operator returns the first falsy value or the last value if all are truthy. Since a is 0 (falsy), a && b returns 0. The || operator returns the first truthy value or the last value if all are falsy. Since b is "hello" (truthy), b || c returns "hello".

🔧 Debug

advanced

Unexpected Boolean result in conditional

Why does this TypeScript code print "No" instead of "Yes"?

Typescript

const value: any = "false";
if (value) {
  console.log("Yes");
} else {
  console.log("No");
}

A Because the variable is typed as any, it causes a runtime error and prints "No".

B Because the string "false" is falsy, so the else block runs and prints "No".

C Because the string "false" is truthy, but the code prints "No" due to a syntax error.

D Because the string "false" is truthy, so the if block runs and prints "Yes".

💡 Hint

Check how non-empty strings behave in Boolean context.

In JavaScript/TypeScript, any non-empty string is truthy, even if it contains the text "false". So the if (value) condition is true and prints "Yes". If the code prints "No", it means the code was not run as shown or there is a misunderstanding.

📝 Syntax

advanced

Boolean type assignment error

Which option causes a TypeScript type error?

Typescript

let flag: boolean;
flag = ???;

A true

B "true"

C false

D Boolean(0)

💡 Hint

Check which values are allowed for a variable typed as boolean.

TypeScript's boolean type only accepts true or false (or expressions that evaluate to boolean). The string "true" is not a boolean, so assigning it causes a type error.

🚀 Application

expert

Count truthy values in an array

Given this TypeScript function, what is the output when called with the array [0, "", 1, "hello", null, undefined, [], {}]?

Typescript

function countTruthy(arr: any[]): number {
  return arr.reduce((count, item) => count + (item ? 1 : 0), 0);
}
console.log(countTruthy([0, "", 1, "hello", null, undefined, [], {}]));

A 5

B 6

C 4

D 7

💡 Hint

Count how many items are truthy in the array.

The truthy values are: 1, "hello", [], and {}. That's 4. But check carefully: [] and {} are truthy, so total is 4. Wait, let's count carefully: 1, "hello", [], {} → 4 truthy values. So answer is 4.

Correction: The options show 4,5,6,7. The correct count is 4.

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