Undefined Type - undefined
The undefined type represents a value that is nothing and may only be
undefined. It is used to indicate that a variable does not have a value or that an
object does not exist.
Note
Due to unions not being implemented yet, it is not possible to use the undefined type in combination with other types. Meaning that at the moment, its practical usage is fairly limited.
Examples
Simple undefined variable definitions
var var1: undefined = undefined;