Person5.ts

type Person = {
  birthday: Date;
};
type PersonJson = {
  birthday: string;
};
function isPerson(value: Person | PersonJson): value is Person {
  return value.birthday instanceof Date;
}
function calcAge(
  input: Date | null | undefined | string | Person | PersonJson
): number {
  if (!input) {
    return 0;
  } else if (typeof input === "string") {
    return diffInYears(parse(input));
  } else if (input instanceof Date) {
    return diffInYears(input);
  } else if (isPerson(input)) {
    return diffInYears(input.birthday);
  } else {
    return diffInYears(parse(input.birthday));
  }
}