The transpilation that typescript does doesn’t really have anything to do with typescript, it’s just there because typescript wants to support the latest ecmascript features, so transpilation is necessary for that, but technically you could simply strip out the type info and have another transpiler like babel handle the backwards compatibility. I think there are a few minor exceptions to that, like enums. There was even a proposal to add some typescript types to native JavaScript that would be ignored by the interpreter and just act as comments.