See https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#including-declarations-in-your-npm-package ; all that is needed is a index.d.ts file that contains type definitions for all exported functions. This would be super helpful for Typescript (and plain javascript too) applications, so we don't need to reference the API documentation and get automatic type checking if we have an editor that supports type definitions!
