Подсчет количества элементов во вложенном массиве ExpressJS

let person = {
  firstname: "Benjamin",
  dog: {
    named: "Louie",
    owner: function() {
      return this.dog.named + " is " + this.firstname + "'s dog'";
    }
  }
}

console.log(person.dog.owner.call(person));