How To Implement Private & Privileged Functions in javaScript 2020

Private members in javaScript:

In an object-oriented language like C#, Java they are having private and public members like variables and functions. If I declared a member of the class is private then this is available inside that class only outside of class it is not accessible.
Javascript is object-oriented so it's having private and public members.
Below is code for the private variable.
Private variables are declared using var keyword inside the object and this can access only private functions.

Code:

function DisplayFullname(firstname,lastname)
{

    // this are public fields
    this.FirstName=firstname;
    this.LastName=lastname;

    // this is Private variable which is accessible only inside of this constructor function.
    var fullname=this.FirstName+" "+this.Lastname; 

}

In the above code fullname is private variable and it is not accessible outside of that function and accessible only inside of that function.
Below is code for that private function which is accessible inside of that constructor function

Code:

function DisplayFullname(firstname,lastname)
{

    // this are public fields
    this.FirstName=firstname;
    this.LastName=lastname;

    // this is Private variable which is accessible only inside of this constructor function.
    var fullname="";

    var getFullName=function()
    {
        fullname=this.FirstName+" "+this.LastName;
        return fullname;
    }

}

Privileged Function:

The function which is declared using this keyword is called a privileged function. In below code previlageFunction this is a privileged function

Code:

function DisplayFullname(firstname,lastname)
{

    // this are public fields
    this.FirstName=firstname;
    this.LastName=lastname;

    // this is Private variable which is accessible only inside of this constructor function.
    var fullname="";

    var getFullName=function()
    {
        fullname=this.FirstName+" "+this.LastName;
        return fullname;
    }

    this.previlageFunction=function()
    {
        return getFullName();
    }
}

So what is a Privileged Function?
1) The Privileged function is created using this keyword and in the constructor function, public methods are created using a prototype property of the constructor function.
2) The Privileged function can access the private variables and private method of the constructor function.
3) Public methods have to access privileged methods but not private methods.
4) Privileged methods are also accessible outside the constructor function.

Code:

function DisplayFullname(firstname,lastname)
{

    // this are public fields
    this.FirstName=firstname;
    this.LastName=lastname;

    // this is Private variable which is accessible only inside of this constructor function.
    var fullname="";

    //private function
    var getFullName=function()
    {
        fullname=this.FirstName+" "+this.LastName;
        return fullname;
    }

    //privileged function
    this.previlageFunction=function()
    {
        return getFullName();
    }

    //public fucntion
    DisplayFullname.prototype.publicgetFullName=function(){
            return this.previlageFunction();
    }
}

var dis=new DisplayFullname("Sagar", "Jaybhay");
document.writeln(dis.FirstName); // Sagar
document.writeln(dis.LastName);// Jaybhay
document.writeln(dis.previlageFunction());// undefined
document.writeln("<br/>");
document.writeln(dis.fullname);//undefined
document.writeln("<br/>");
document.writeln(dis.publicgetFullName());//undefined

Private Functions:
This function can only be called by the privileged function. Below is the syntax of creating private function

Code:

var getFullName=function()
    {
        fullname=this.FirstName+" "+this.LastName;
        return fullname;
    }