JavaScript Interview Question #40: What is the type of `undefined` in JS?

#webdev #beginners #codenewbie #javascript

What's the output?

In JavaScript, the typeof operator always returns a string.

So, even though typeof undefined evaluates to undefined, it’s a string and not the primitive undefined value.

The string ’undefined’ is not equal to the value undefined.

ANSWER: the message false will appear in the console.

Top comments (5)

So, typeof always returns a string and if we do a check like

typeof undefined === undefined

it is actually checking whether the string undefined equal to the type undefined which is a false.
Instead if we do

typeof undefined === "undefined"

we will get true as we are comparing with the string "undefined" and with the result which is a string we get from typeof undefined.

Yes, you're absolutely right

If typeof(undefined) == undefined, that'd mean undefined has no type, so it makes sense for the string "undefined" to be returned instead :D

Hold on, typeof null is coming right after you :)

Hey, I never claimed JavaScript is a sane language overall :D