I think another reason they added refinement types in the form of literal string types and literal number types was because there are no data constructors in TS

I think another reason they added refinement types in the form of literal string types and literal number types was because there are no data constructors in TS & Flow either. This is a bit of speculation on my part. For example, in lots of strongly typed languages when you make a sum type there's a constructor. haskell, data Foo = Baz | Bar or rust, enum Foo { Baz, Bar } Baz and Bar are data constructors, they allow you to pattern match on them and make new values. In ts an equivalent sum type has to use a string/number refinement. type Foo = { kind: "baz" } | { kind: "bar" } This fits closer to js semantics and still allows sum types to be written. Unfortunately the pattern matching isn't anywhere near as good, but I applaud them for adding this to the language.