Fun with TS Extract/Exclude

optional-partial.ts

type PartialOptional<T extends U, U> =
  { [P in Extract<keyof T, keyof U>]: T[P] } &
  { [P in Exclude<keyof T, keyof U>]?: T[P] };

interface DetailedPerson {
  name: string
  age: number
  isCool: boolean
}

interface Person {
  name: string
}

type OptionallyDetailedPerson = PartialOptional<DetailedPerson, Person>;

const person1: Person = {
  name: 'Alice'
};
  
const person2: DetailedPerson = {
  name: 'Bob',
  age: 35,
  isCool: false
};
  
const optional1: OptionallyDetailedPerson = person1; // works
const optional2: OptionallyDetailedPerson = person2; // works
const person3: Person = optional2; // works
const person4: DetailedPerson = optional1; // doesn't work