JScript has a number of reserved words that you cannot use as identifiers. Reserved words have a specific meaning to the JScript language, as they are part of the language syntax. Using a reserved word causes a compilation error when loading your script.
JScript also has a list of future reserved words. These words are not currently part of the JScript language, although they are reserved for future use.
When choosing identifiers it is also important to avoid any words that are already the names of intrinsic JScript objects or functions, such as String or parseInt.