let Animal = function (type){
   this.type = type
}
//这是类的实例对象方法
Animal.prototype.eat = function (){
   Animal.walk()//引用类的静态方法

console.log('eat food') } 

//这是类的静态方法 
Animal.walk = function () { 
  console.log('walking') 
} 
let dog = new Animal('dog') 
dog.eat() 
dog.walk() //类的实例对象里没有walk这个方法

class Animal {
   constructor (type) {
      this.type = type
   }
   //类的实例对象方法
   eat (){
      Animal.walk()
      console.log('eat food')
   }
   //类的静态方法
   static walk (){
      console.log('walking...')
   }
}
let dog = new Animal('dog')
dog.eat()