Yup. It's an attempt to formalize the mess that already exists in many JS libraries. That's why, even if I don't plan on using Typescript itself for a project,

Yup. It's an attempt to formalize the mess that already exists in many JS libraries. That's why, even if I don't plan on using Typescript itself for a project, I prefer libraries that have type declarations or are natively Typescript. It shows the library authors at least attempted to understand the type system they were creating.