zoo.js

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

Zoo.prototype = {
  bipeds: function() {
    return this.legFilter(2);
  },
  quadrupeds: function() {
    return this.legFilter(4);
  },
  legFilter: function(num) {
    return this.animals.filter(function(animal) {
      return animal.numLegs === num;
    });
  }
};

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

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

//------------------------------------------------------------------------------------------------------------------
// DRIVER CODE
//------------------------------------------------------------------------------------------------------------------

// setup
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)
];

var zoo = new Zoo(animals);

// assertion implementation
function assert(test, message) {
  if (!test) throw "ERROR: " + message;
  return true;
}

// assertions
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");