How to define a class in JavaScript_Basic knowledge

This is how I originally wrote it:

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

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

Later I saw another way of writing that was a little more complicated and seemed unnecessary: ​​

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

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

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

  return fn;
}

The Dog function here is actually a class-making function, which returns the real Dog class.
I feel that the advantage of doing this is better encapsulation.
For example, privateVariable here is a private variable:

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

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

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

This way Dog is no longer Dog~

Later understanding:
The above method of creating a new class directly overrides the prototype object. In this way, the original built-in attributes of the prototype are gone (arguments, call, apply, etc.).
The following method of creating a new class seems better:

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