This can be written more simply with ``return !!skinClasses[skinClass]``.
(There is a small difference which has no impact here: using ``!== undefined`` returns true only if ``skinClasses[skinClass]`` is undefined, whereas my suggestion also returns ``true`` if ``skinClasses[skinClass]`` is ``false``, ``null``, ``0``, ``""``... but anyway, if a skin class exists, it must be an object, so those case will not happen).