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.