response indicating that there was no value assigned in the first place.

You can explicitly assign undefined to a variable though.

Another fun fact about JavaScript is that undefined never used to be a keyword. If you did var foo = undefined, foo would indeed have a value of undefined, but it was only because there was no variable called undefined in scope!

You could do var undefined = 42 then var foo = undefined would actually set foo to 42! window.undefined = 42 would break all sorts of things.

Thankfully this was fixed with ES5 in 2009, although it took a few years for browsers to make the change.