interface Bird { fly: () => void; }
interface Fish { swim: () => void; }
 
function isFish(pet: Bird | Fish): pet is Fish {
  return (pet as Fish).swim !== undefined;
}
 
function move(pet: Bird | Fish) {
  if (isFish(pet)) {
    pet.swim(); // TypeScript knows it's a Fish
  } else {
    pet.fly(); // TypeScript knows it's a Bird
  }
}