function Dog(){
  this.name = 'hachi';
}

Dog.prototype = {
  makeNoise:function(){
    alert('wangwangwang');
  }
};

function Dog(){
  var privateVariable = 'secret';

  var fn = function(){
    //...
  }

  fn.prototype = {
    makeNoise:function(){
      alert('wangwangwang');
    }
  }

  return fn;
}

var d = new Dog;
d.privateVariable //undefined

Dog.prototype = {
  //e...WTF??
}

var Dog = function(name){
  this.name = name;
  var privateVariable = 'you cannot see me.';
  this.getPrivate = function(){return privateVariable;};
}