[TS: Satisfies Operator] Example of the satisfies operator in TypesScript #typescript

satisfies.ts

type City = CityName | CityCoordinates;

type CityName = 'New York' | 'Mumbai' | 'Lagos';

type CityCoordinates = {
  x: number;
  y: number;
};

type User = {
  birthCity: City;
  currentCity: City;
};

const unsatisfied: User = {
  birthCity: 'Mumbai',
  currentCity: { x: 6, y: 3 },
};

// No bueno! TS knows birthCity can be type CityCoordinates,
// which does not have the toUpperCase method
unsatisfied.birthCity.toUpperCase();

const satisfied = {
  birthCity: 'Mumbai',
  currentCity: { x: 6, y: 3 },
} satisfies User;

// Hurray! TS has narrowed this property to type 'Mumbai'
satisfied.birthCity.toUpperCase();