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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.

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