7.6.1.2 Future Reserved Words
The following words are used as keywords in proposed extensions and are therefore reserved to allow for the
possibility of future adoption of those extensions.
Syntax
FutureReservedWord :: one of
class enum extends super
const export import
The following tokens are also considered to be FutureReservedWords when they occur within strict mode code
(see 10.1.1). The occurrence of  any of these tokens within strict mode code in any context where the
occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:
implements let private public yield
interface package protected static