Nil

The nil type represents the absence of a value, and is considered undefined:

nil
defined(nil)  # -> false
nil.class     # -> error, methods cannot be called on undefined values
nil.ref       # -> error, methods cannot be called on undefined values

The nil type has only one possible representation, nil.

Instead of an error or exception, an undefined value is represented by nil:

Hash(){:key}    # -> nil