undefined being treated as a missing optional argument

[Brandon Benvie]

This is also a shortcut to making a dense array of a given size.

Array.apply(null, Array(5)) // [undefined, undefined, undefined, undefined,
undefined]

Array(5).map(...) // won't do anything with a sparse array
Array.apply(null, Array(5)).map(Function.prototype.call.bind(Number)) //
[0,1,2,3,4,5]
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