A Duck, an Alligator, and a Goat

animals.js

function newAlligator () {
  var alligator = {
    name: 'alligator',
    word: 'grrr'
  };
  extend(alligator, withAnimal);
  extend(alligator, withWalking);
  extend(alligator, withSwimming);
  return alligator;
}

function newDuck () {
  var duck = {
    name: 'duck',
    word: 'quack'
  };
  extend(duck, withAnimal);
  extend(duck, withWalking);
  extend(duck, withFlying);
  extend(duck, withSwimming);
  return duck;
}

function newGoat () {
  var goat = {
    name: 'goat',
    word: 'baa'
  };
  extend(goat, withAnimal);
  extend(goat, withWalking);
  return goat;
}

var alligator = newAlligator();
alligator.talk();
alligator.swim();
alligator.walk();

var duck = newDuck();
duck.talk();
duck.fly();
duck.walk();

var goat = newGoat();
goat.talk();
goat.walk();

extend.js

function extend (target, source) {
  Object.keys(source).forEach(function (key) {
    if (typeof target[key] !== 'undefined') {
      return;
    }
    target[key] = source[key];
  });
}

types.js

var withAnimal = {
  name: 'name',
  word: 'word',
  talk: function () {
    console.log(this.name + ' says ' + this.word);
  }
};

var withFlying = {
  fly: function () {
    console.log('flap flap');
  }
};

var withSwimming = {
  swim: function () {
    console.log('splish splash');
  }
};

var withWalking = {
  walk: function () {
    console.log('stomp stomp');
  }
};