2.1.9 [ECMA-262-1999] Section 8, Types

V0020:

A value is an entity that takes on one of #nine# __eleven__ types. There are #nine# __eleven__ types (Undefined, Null, Boolean, String, Number, Object, __SafeArray, VarDate,__ Reference, List, and Completion). Values of type Reference, List, and Completion are used only as intermediate results of expression evaluation and cannot be stored as properties of objects.