Type Guard

Type-Guard.ts

type Dog = {
  age: number;
  name: string;
  bark: () => void;
  type: 'dog'
}

type Person = {
  age: number;
  name: string;
  shout: () => void;
  type: 'person'
}

function makeNoise(dogOrPerson: Dog | Person): void {
  switch (dogOrPerson.type) {
    case 'dog':
      // Type is narrowed to "Dog", so we can "bark":
      dogOrPerson.bark();
      break;
    case 'person':
      // Type is narrowed to "Person", so we can "shout":
      dogOrPerson.shout();
      break;
  }
}