import { MergeDeep } from 'type-fest';

type Person = { name: string; age: number };

type PartialPerson = Partial<Person>;

// type of MergeDeepPersonPartialFirst makes sense, since all Person values will be used
type MergeDeepPersonPartialFirst = MergeDeep<PartialPerson, Person>;

// I assumed this would be inferred as the same type as MergeDeepPersonPartialLast
// but it gets infered as
// {
//   name?: string | undefined;
//   age?: number | undefined;
// }
type MergeDeepPersonPartialLast = MergeDeep<Person, PartialPerson>;

const person: Person = { name: 'a', age: 25 };
const partialPerson: PartialPerson = { name: 'b' };

// both values get inferred as
// {
//    name: string;
//    age: number;
// }
// equivalent to Person
const spreadPartialLast = { ...person, ...partialPerson };
const spreadPartiaFirst = { ...partialPerson, ...person };

// on a sideonde
// these get infered as string
type test1 = string & (string | undefined);
type test2 = (string | undefined) & string;

// these get infered as something equivalent to Person
type test3 = Partial<Person> & Person;
type test4 = Person & Partial<Person>;