js原型链继承的优点

function Cat(){};

var tom=new Cat();

function Animal(){}

Animal.prototype.eat= function (food){

  console.log("I eat "+food);

}

var cat=new Animal();

Cat.prototype=cat;

var kitty=new Cat();

kitty.eat("fish");

console.log(tom instanceof Cat); // false

console.log(tom instanceof Animal); // false

console.log(kitty instanceof Cat); // true

console.log(kitty instanceof Animal); // true