Pin the tail on the donkey

Image source: https://openclipart.org/detail/217463/jackass

// Variables
var donkey;
var tail = "Tail";

// Farm Animal Constructor
function FarmAnimal(numberOfLegs) {
  this.getNumberOfLegs = function() {
    return numberOfLegs;
  }
}
FarmAnimal.prototype = {
  isA: function() {
    return "Farm Animal";
  },
  hasA: function(bodyPart) {
    return this[bodyPart] != undefined;
  },
}

// Donkey Constructor
function Donkey() {
  FarmAnimal.call(this, 4);
}
Donkey.prototype = Object.create(FarmAnimal.prototype);
Donkey.prototype.isA = function() {
  return "Donkey";
}
Donkey.prototype.pinTheTail = function(theTail) {
  this.tail = theTail;
}

// Functions
function pinTailOnDonkey(d, t) {
  if (d != undefined) {
    d.pinTheTail(t)
    printFarmAnimal(d);
  }
  else {
    alert("Donkey is not defined!");
  }
}
function printFarmAnimal(a) {
  console.log(a);
  var withTail = document.getElementById("animalWithTail");
  var withoutTail = document.getElementById("animalWithoutTail");
  if (a.hasA("tail")) {
    withTail.setAttribute("style", "display:inline");
    withoutTail.setAttribute("style", "display:none");
  }
  else {
    withTail.setAttribute("style", "display:none");
    withoutTail.setAttribute("style", "display:inline");
  }
}
function newDonkey() {
  donkey = new Donkey();
  printFarmAnimal(donkey);
}

// Start
newDonkey();