var forestCows = [
  {name: "Legolas", type: "calf", hadCalf: null},
  {name: "Gimli", type: "bull", hadCalf: null},
  {name: "Arwen", type: "cow", hadCalf: null},
  {name: "Galadriel", type: "cow", hadCalf: null},
  {name: "Eowyn", type: "cow", hadCalf: "Legolas"}
];

Object.prototype.noCalvesYet = function () {
  if (this.type == 'cow' && this.hadCalf == null) {
    return true;
  }
};
Array.prototype.countForBreeding = function () {
  var numToBreed = 0;
  for ( var i = 0; i < this.length; i++) {
    if (this[i].noCalvesYet() == true) {
      numToBreed++;
    }
  }
  return numToBreed;
};
console.log('powinny byc 2 nieocielone krowy', forestCows.countForBreeding());