Adding a Discriminant

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

type Cat = {
  kind: "cat" // 👀
  numberOfLegs: number
  microchip: {
    id: string
    name: string
  }
}

type Pet = Dog | Cat

function getName(pet: Pet): string {
  switch (pet.kind) {
    case "dog": return pet.collar.name
    case "cat": return pet.microchip.name
  }
}