JavaScript Special Values & typeof

Question 1

What does undefined mean in JavaScript?

Answer 1

Absence of a defined value. A variable has been declared, but hasn’t been assigned a value. Or a property that doesn’t exist.

N/A

Question 2

Give an example of how undefined can occur.

Answer 2

let myVar;
console.log(myVar); (Uninitialized variable)
Function with missing arguments
Accessing a non-existent object property.

N/A

Question 3

What is the result of typeof undefined?

Answer 3

“undefined” (a string)

N/A

Question 4

What does null mean in JavaScript?

Answer 4

The intentional absence of a value. It signifies that a variable currently holds no object (or is intentionally empty). It’s a deliberate assignment.

N/A

Question 5

Give an example of how null is used.

Answer 5

let myObject = null; (Explicitly setting a variable to represent “no object here”). Clearing a variable’s value.

N/A

Question 6

What is the result of typeof null?

Answer 6

“object” (A historical quirk/bug! Be aware of this).

N/A

Question 7

Explain the key difference between undefined and null.

Answer 7

undefined: Hasn’t been assigned a value yet.
null: Intentionally holds no value (is explicitly empty).

N/A

Question 8

What does NaN mean in JavaScript?

Answer 8

“Not-a-Number”. Represents a value that is not a valid number. Result of an invalid numerical operation.

N/A

Question 9

Give an example of how NaN can occur.

Answer 9

0 / 0
Math.sqrt(-1)
Parsing a non-numeric string (parseInt(“hello”))

N/A

Question 10

What is the result of typeof NaN?

Answer 10

“number” (Even though it’s not a valid number).

N/A

Question 11

How do you properly check if a value is NaN? Why can’t you use === NaN?

Answer 11

Use Number.isNaN(value). NaN === NaN is always false. Number.isNaN() does NOT type coerce.

N/A

Question 12

What is the typeof operator?

Answer 12

A unary operator that returns a string indicating the data type of a value.

N/A

Question 13

List the main return values of the typeof operator.

Answer 13

“undefined”, “object”, “boolean”, “number”, “string”, “symbol”, “function”

N/A

Question 14

What is the typeof an array?

Answer 14

“object” (Arrays are objects in JavaScript).

N/A

Question 15

Explain the benefit of using the strict equality operator (===) when comparing to null or undefined.

Answer 15

It avoids type coercion, which can lead to unexpected results.

N/A

Question 16

When should you use optional chaining (?.) and nullish coalescing (??)

Answer 16

To handle situations where values might be null or undefined, providing safer property access (?.) and fallback defaults (??).

N/A