"Extends" does not mean it extends the type to contain more members, it means it extends instances to have more properties, thereby creating a subset or subtype

"Extends" does not mean it extends the type to contain more members, it means it extends instances to have more properties, thereby creating a subset or subtype. Perhaps it is confusing that TypeScript uses terms from both set-theory (union, intersection) and OO (extends, implements). But there is no contradiction.