Boolean type behavior in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are the two possible values of a Boolean type in TypeScript?

The Boolean type in TypeScript can only have two values: true or false.

intermediate

How does TypeScript treat non-Boolean values in a Boolean context?

TypeScript uses JavaScript's truthy and falsy rules. Values like 0, '' (empty string), null, undefined, and NaN are treated as false. Others like non-empty strings or numbers (except 0) are true.

beginner

What is the result of Boolean('hello') in TypeScript?

The result is true because non-empty strings are considered truthy.

intermediate

Explain the difference between == and === when comparing Boolean values.

== compares values after type coercion, so 1 == true is true. === compares both value and type, so 1 === true is false. It's best to use === to avoid unexpected results.

beginner

How can you explicitly convert a value to a Boolean in TypeScript?

You can use the Boolean() function or double negation !!value to convert any value to a Boolean.

Which of the following values is considered falsy in TypeScript?

A {}

B 'false'

C []

D 0

0 is falsy. Non-empty strings ('false'), empty arrays ([]), and empty objects ({}) are truthy.

What does the expression !!'0' evaluate to?

A false

B true

C 0

D undefined

The string '0' is non-empty, so it is truthy. Double negation converts it to true.

Which operator checks both value and type equality for Booleans?

A ===

B !=

C ==

D ||

=== checks both value and type equality.

What is the Boolean value of null in TypeScript?

A true

B null

C false

D undefined

null is falsy, so its Boolean value is false.

How do you convert a number to a Boolean explicitly?

A Using <code>Boolean()</code>

B Using <code>Number()</code>

C Using <code>String()</code>

D Using <code>parseInt()</code>

Use Boolean() to convert any value to a Boolean.

Describe how TypeScript treats different values when used in a Boolean context.

Think about which values become true or false when converted.

Explain the difference between == and === when comparing Boolean values in TypeScript.

Focus on how type affects equality.

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