Typescript 1

type Dog = {
  numberOfLegs: number
  collar: {
    name: string
    material: "leather" | "plastic"
  }
}

type Cat = {
  numberOfLegs: number
  microchip: {
    id: string
    name: string
  }
}

type Pet = Dog | Cat

function numberOfLegs(pet: Pet): number {
  return pet.numberOfLegs // 
  this is fine as all pets share a common property
}

function getName(pet: Pet): string {
  // If it's a dog, read the collar
  // If it's a cat, read the microchip
  // ^-- Figuring this out is type narrowing
}