Type checking using typeof in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of typeof with different values

What is the output of the following code snippet?

Javascript

console.log(typeof null);

A "boolean"

B "object"

C "undefined"

D "null"

💡 Hint

Remember that typeof null is a known quirk in JavaScript.

In JavaScript, typeof null returns "object" due to legacy reasons.

❓ Predict Output

intermediate

Type of an array using typeof

What will be the output of this code?

Javascript

console.log(typeof [1, 2, 3]);

A "object"

B "array"

C "list"

D "undefined"

💡 Hint

Arrays are a special kind of object in JavaScript.

The typeof operator returns "object" for arrays because arrays are objects in JavaScript.

❓ Predict Output

advanced

Output of typeof with function and symbol

What is the output of this code snippet?

Javascript

console.log(typeof function() {});
console.log(typeof Symbol('id'));

A "function" and "symbol"

B "object" and "string"

C "function" and "object"

D "object" and "symbol"

💡 Hint

Functions have their own type, and symbols are a unique primitive type.

The typeof operator returns "function" for functions and "symbol" for symbols.

❓ Predict Output

advanced

Type of NaN using typeof

What will this code output?

Javascript

console.log(typeof NaN);

A "undefined"

B "NaN"

C "number"

D "object"

💡 Hint

NaN stands for Not-a-Number but is still a number type in JavaScript.

Despite its name, NaN is of type "number" in JavaScript.

❓ Predict Output

expert

Type checking with typeof and undefined variables

What is the output of this code?

Javascript

let a;
console.log(typeof a);
console.log(typeof b);

A ReferenceError and ReferenceError

B ReferenceError and "undefined"

C "undefined" and ReferenceError

D "undefined" and "undefined"

💡 Hint

typeof returns "undefined" for both declared but unassigned variables and undeclared variables without throwing errors.

Variable a is declared but not assigned, so typeof a returns "undefined". Variable b is undeclared, but typeof b also returns "undefined" safely without throwing a ReferenceError.