Person class wrote in JavaScript

person.js

class Person {
  
  constructor(name, age){
    this.name = name;
    this.age = age;
  }
  
  advanceAge(){
    this.age++;
  }
}

function doBirthday(citizen){
  citizen.advanceAge();
  console.log(`Happy ${citize.age} years!`);
}

const leonardo = new Person('Leonardo', 21);
doBirthday(leonardo);
leonardo.age = 21; // Note that this is possible, there is no way to make a attribute private on JS
leonardo.age = 'twenty'; // This is possible as well, there is no type check

This is how you set private variables.

function Foo(a) {
var bar = a;
this.getBar = function() {return(bar);}
this.setBar = function(a) {bar = a;}
}

var x = new Foo(3);
var y = x.getBar();
x.setBar(12);
var z = x.bar;