The undefined type has exactly one value, called undefined. It is returned by variables that have not yet been assigned with a value.
This value is also returned in some browsers when referring to a part of the document object model that is non-existent.
Sending a subsequent message to values that are currently undefined results in a run-time error.
ECMA 262 edition 2 - section - 4.3.10
ECMA 262 edition 2 - section - 8.1
ECMA 262 edition 3 - section - 4.3.10
ECMA 262 edition 3 - section - 8.1