Boolean type behavior in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a boolean variable with value true.

Typescript

let isActive: boolean = [1];

Drag options to blanks, or click blank then click option'

A"true"

Byes

C1

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around true, making it a string instead of a boolean.

Assigning numbers like 1 or 0 instead of true or false.

The boolean type in TypeScript accepts only true or false as values, not strings or numbers.

2fill in blank

medium

Complete the code to check if the variable is false.

Typescript

if ([1] === false) {
  console.log("It is false");
}

Drag options to blanks, or click blank then click option'

AisActive === false

B!isActive

CisActive == true

DisActive

Attempts:

3 left

💡 Hint

Common Mistakes

Writing the comparison twice like isActive === false === false.

Using negation !isActive when the code already compares to false.

To check if a variable equals false, use variable === false. Here, the if condition already compares to false, so just the variable name is needed.

3fill in blank

hard

Fix the error in the code to assign a boolean value correctly.

Typescript

let isDone: boolean = [1];

Drag options to blanks, or click blank then click option'

Afalse

B"false"

C0

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the string "false" instead of the boolean false.

Using numbers like 0 or null which are not boolean values.

Boolean variables must be assigned true or false without quotes. Using strings or other types causes errors.

4fill in blank

hard

Fill both blanks to create a boolean expression that checks if x is true and y is false.

Typescript

if (x [1] true && y [2] false) {
  console.log("Condition met");
}

Drag options to blanks, or click blank then click option'

A===

B==

C!=

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using loose equality == which can cause unexpected results.

Using inequality operators which check for difference, not equality.

Use the strict equality operator === to compare boolean values for exact match.

5fill in blank

hard

Fill all three blanks to create a boolean expression that checks if a is true and c is true.

Typescript

if ([1] [2] true && [3] === true) {
  console.log("All conditions met");
}

Drag options to blanks, or click blank then click option'

Aa

Bb

C===

Dc

Attempts:

3 left

💡 Hint

Common Mistakes

Using loose equality == instead of strict equality ===.

Mixing variable names or operators in wrong blanks.

Use the strict equality operator === to check if a and c are true.

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