nil? determines whether an expression's runtime type is
Nil. For example:
a = 1 a.nil? # => false b = nil b.nil? # => true
It is a pseudo-method because the compiler knows about it and it can affect type information, as explained in if var.nil?(...).
It has the same effect as writing
is_a?(Nil) but it's shorter and easier to read and write.
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