zoo.js

function Animal(name, numLegs){
  this.name = name;
  this.numLegs = numLegs;
}

Animal.prototype.identify = function(){
  return "I am a " + this.name +  " with " + this.numLegs + " legs.";
};

var Zoo = {
  init: function(animals){
    this.animals = animals;
  },

  animalByLegs: function(numLegs){
    animalArray = [];
    this.animals.forEach(function(animal){
      if(animal.numLegs == numLegs)
        animalArray.push(animal);
      });
      return animalArray;
    },

  bipeds: function(){
    return this.animalByLegs(2);
  },

  quadrupeds: function(){
    return this.animalByLegs(4);
  }
}

var animals = [
  new Animal("Human", 2),
  new Animal("Monkey", 2),
  new Animal("Kangaroo", 2),
  new Animal("Horse", 4),
  new Animal("Cow", 4),
  new Animal("Centipede", 100)
];

Zoo.init(animals);

assert(
  Zoo.bipeds().length === 3, "the Zoo should have 3 bipeds"
);
assert(
  Zoo.quadrupeds().length === 2, "the Zoo should have 2 bipeds"
);
assert(
  animals[0].identify() === "I am a Human with 2 legs.", "humans have 2 legs"
);
assert(
  animals[2].name === "Kangaroo", "expected 'Kangaroo'"
);
assert(
  animals[0].identify === animals[5].identify, "only one implementation of the identify() function should exist"
);