# Reserved Words
This article lists keywords that may not be used as variables, functions, methods, or object identifiers, because either Titanium or another source specifies its usage.
# ECMAScript Specification
The following keywords cannot be used as identifiers according to the ECMAScript Language Specification:
Do not use double underscore prefixes on variables, properties, or function names (e.g., __foo). They are reserved for use in Alloy.