使构造函数返回特定的实例类型

// 定义基础构造函数
function Animal(name) {
  this.name = name;
}

Animal.prototype.speak = function() {
  console.log(`${this.name} makes a noise.`);
};

// 定义特定类型的构造函数
function Dog(name) {
  Animal.call(this, name);
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

Dog.prototype.speak = function() {
  console.log(`${this.name} barks.`);
};

// 工厂函数
function createAnimal(type, name) {
  switch (type) {
    case 'dog':
      return new Dog(name);
    default:
      return new Animal(name);
  }
}

// 使用工厂函数创建实例
const animal1 = createAnimal('dog', 'Rex');
animal1.speak(); // 输出: Rex barks.

const animal2 = createAnimal('unknown', 'Unknown');
animal2.speak(); // 输出: Unknown makes a noise.