Yeah. Although, in modern browsers, the overhead is really trivial.
I just tried timing it in modern Chrome. The overhead of calling a function like isDefined(), versus typeof x !== 'undefined', seems trivial. On my machine, after 100,000 calls there's a difference of 50ms.
Still, I concede that there are some situations where it would matter, if you were driving an animation. And in older browsers it might matter a lot even in other circumstances.
I just tried timing it in modern Chrome. The overhead of calling a function like isDefined(), versus typeof x !== 'undefined', seems trivial. On my machine, after 100,000 calls there's a difference of 50ms.
Still, I concede that there are some situations where it would matter, if you were driving an animation. And in older browsers it might matter a lot even in other circumstances.