test.js

function Hamster() {}

Hamster.prototype.food = [];
Hamster.prototype.feed = function (item) {
  this.food.push(item);
};

const speedy = new Hamster();
const lazy = new Hamster();

speedy.feed("яблоко");
speedy.feed("лимон");

console.log(speedy.food.length); 
console.log(lazy.food.length);