null and undefined

type Car = { 
  brand: string | null; 
  year?: number 
};

Valid cars

const car1: Car = { brand: "bmw", year: 2020 };
const car2: Car = { brand: "toyota" };
const car3: Car = { brand: null, year: 2018 };

Invalid cars

const car4: Car = { year: 2015 };
const car5: Car = { brand: "mazda", year: null };