How do I define a class in JavaScript

The way I used to write it was this:

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

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

Then I saw another slightly more complicated and seemingly unnecessary way to write it:

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

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

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

  return fn;
}

The Dog function here is actually a manufacturing class function that returns the real Dog class.
It feels like the benefit of doing this is better encapsulation.
For example, privateVariable is a privateVariable:

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

In addition, if I add a sentence at the end of the first example:

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

So a Dog is not a Dog

Later understanding:
This new class method overrides the prototype object directly. Then prototype's built-in properties (arguments, call, apply, etc.) are gone.
The following method of creating new classes seems better:

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