构造函数本身也是函数只不过通过new声明的话有类的特点，内部会创建一个对象实例并返回

   // 3. 构造函数模式

        function Animal() {
            this.name = 'Dog';
            this.age = 1;
            this.bark = function () {
                console.log('wangwang');
            }
        }

        var dog = new Animal();

        dog.bark();
        dog.height = 1.0;
        Animal.prototype.height = 20;
        var dog2 = new Animal();
        console.log(dog2.height);