JSFiddle - React, Tailwind, and code Playground

JavaScript

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;
  } else {
    return false;
  }
};

Array.prototype.countForBreeding = function () {
  var numToBreed = 0;
  for (var i = 0; i < this.length; i++) {
    if (this[i].noCalvesYet) {
      numToBreed++;
    }
  }
  return numToBreed;
};

alert(forestCows.countForBreeding());