Type algebra to detect empty objects

encodeQueryObject.ts

/**
 * This came out of some work where we were sending an encoded JSON object as a query param in a network call.
 * But, we can't send nothing, `param=`, or we crash the server, that's taken care of by restricting the type
 * to at least `Record<string, string>`, but it left open the possibility of empty objects. Encoded as `'%7B%7D'`.
 *
 * So I went looking and it's definitely possible to restrict empty objects, but the type algebra is wild. And
 * it makes use of something people are calling F-bounded quantification. And the error message isn't every helpful.
 *
 * See:
 *   - https://stackoverflow.com/a/72554458/769780
 *   - https://stackoverflow.com/a/72420133/769780
 */

// One of many "canonical" ways to describe an empty object:
type EmptyObject = Record<string, never>;

type NonEmptyObject<T extends Record<string, unknown>> = T extends EmptyObject
  ? never
  : T;

export default function encodeQueryObject<T extends Record<string, unknown>>(
  queryObject: NonEmptyObject<T>,
) {
  return encodeURIComponent(JSON.stringify(queryObject));
}

// This is I think why they say the generic is self-referential, when
// calling the function we don't provide a generic, but let the argument's
// own implicit type determine the type algebra:

const empty = {};
encodeQueryObject(empty) // TS2345: Argument of type '{}' is not assignable to parameter of type 'never'.


const notEmpty = {
  jquery: '123',
};
encodeQueryObject(notEmpty) // No error