index.d.ts: type FooArgs = {x: string} type FooResult = {y: string} type Foo = (_: FooArgs) => FooResult index.js: /// <reference path="index

index.d.ts: type FooArgs = {x: string} type FooResult = {y: string} type Foo = (_: FooArgs) => FooResult index.js: /// <reference path="index.d.ts"/> /** @type {Foo} */ export const foo = ({x}) => ({y: x}) Works as well