Primitive data types in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of typeof with different primitives

What is the output of the following code snippet?

Javascript

console.log(typeof null);
console.log(typeof undefined);
console.log(typeof 42);
console.log(typeof 'hello');

A

"object"
"undefined"
"number"
"string"

B

"null"
"undefined"
"number"
"string"

C

"object"
"null"
"number"
"string"

D

"undefined"
"undefined"
"number"
"string"

💡 Hint

Remember that typeof null is a known JavaScript quirk.

In JavaScript, typeof null returns "object" due to legacy reasons. typeof undefined returns "undefined". Numbers and strings return "number" and "string" respectively.

❓ Predict Output

intermediate

Boolean conversion of different values

What is the output of this code?

Javascript

console.log(Boolean(0));
console.log(Boolean(''));
console.log(Boolean('false'));
console.log(Boolean(null));

A

false
false
false
false

B

true
false
true
false

C

false
true
true
false

D

false
false
true
false

💡 Hint

Check which values are considered falsy in JavaScript.

In JavaScript, 0, empty string '', and null are falsy values, so Boolean conversion returns false. The string 'false' is a non-empty string, so it is truthy and converts to true.

🧠 Conceptual

advanced

Understanding Symbol uniqueness

Which statement about JavaScript Symbols is true?

A Symbols can be used as unique object property keys.

B Symbols are primitive but can be converted to strings automatically.

C Two Symbols created with Symbol() are always equal.

D Symbols are mutable primitive values.

💡 Hint

Think about what makes Symbols special compared to strings.

Symbols are unique and immutable primitive values. Each call to Symbol() creates a new unique symbol, so two symbols are never equal. Symbols can be used as unique keys for object properties. They cannot be automatically converted to strings; explicit conversion is needed.

❓ Predict Output

advanced

Output of typeof with BigInt and other primitives

What is the output of this code?

Javascript

console.log(typeof 9007199254740991n);
console.log(typeof true);
console.log(typeof '123');
console.log(typeof undefined);

A

"number"
"boolean"
"string"
"undefined"

B

"bigint"
"boolean"
"string"
"undefined"

C

"bigint"
"boolean"
"number"
"undefined"

D

"bigint"
"boolean"
"string"
"null"

💡 Hint

BigInt is a newer primitive type in JavaScript.

The typeof operator returns "bigint" for BigInt values, "boolean" for booleans, "string" for strings, and "undefined" for undefined. null is not returned by typeof undefined.

🔧 Debug

expert

Identify the error in primitive assignment

What error will this code produce?

Javascript

const str = 'hello';
str[0] = 'H';
console.log(str);

A TypeError: Cannot assign to read only property '0' of string

B SyntaxError: Invalid assignment

C hello

D undefined

💡 Hint

Strings are immutable in JavaScript. What happens when you try to change a character?

Strings are immutable primitives in JavaScript. Trying to assign to an index like str[0] = 'H' does not change the string and does not throw an error. The original string remains unchanged, so console.log(str) outputs hello.