A basic Person class in JavaScript

PersonClass.js

/***********************************
 * First method to create a classe *
 * *********************************/
 
var Person = function (firstname, lastname, age) {
  this.firstName = firstname;
  this.lastName = lastname;
  this.age = age;
  
  this.setAge = function (age) {
    this.age = age;
  };
  
  this.toString = function () {
    return ["Hi ! I'm ", this.firstName, " ", this.lastName, " and I'm ", this.age, " year old."].join("");
  };
};

var yann = new Person("Yannick", "Comte", 28);
console.log(yann.toString());

/************************************
 * Second method to create a classe *
 * **********************************/

var Person = function (firstname, lastname, age) {
  this.firstName = firstname;
  this.lastName = lastname;
  this.age = age;
};

// Prototypes are better for performances if you have many call of this method
Person.prototype.setAge = function (age) {
  this.age = age;
};

Person.prototype.toString = function () {
  return ["Hi ! I'm ", this.firstName, " ", this.lastName, " and I'm ", this.age, " year old."].join("");
};

var yann = new Person("Yannick", "Comte", 28);
console.log(yann.toString());

/***********************************
 *          Inheritance            *
 * *********************************/
var Developer = function (firstname, lastname, age, skills) {
  Person.call(this, firstname, lastname, age);
  this.skills = skills;
};

Developer.prototype.setAge = function (age) {
  Person.prototype.setAge.call(this, age); // Super(age)
}

Developer.prototype.toString = function () {
  return ["Hi ! I'm ", this.firstName, " ", this.lastName, " and I'm ", this.age, " year old and I'm a developer with a skill level of ", this.skills].join("");
};


/*******************************************
 *     Private attributes and functions    *
 * *****************************************/
 
var Person = function (firstname, lastname, age) {
  // Private because of var keyword
  var _firstName = firstname;
  var _lastName = lastname;
  var _age = age;
  
  // Public because of this keyword
  this.setAge = function (age) {
    _age = age;
  };
  
  this.getAge = function () {
   return _age;
  };
  
  var getToStringValue = function () {
   return ["Hi ! I'm ", _firstName, " ", _lastName, " and I'm ", _age, " year old."].join("");
  };
  
  this.toString = function () {
    return getToStringValue();
  };
};

var yann = new Person("Yannick", "Comte", 0);
yann.setAge(28);
yann._age = 45; // Add a property "_age" to the object, not changing the private "_age" variable !
yann.getToStringValue(); // Doesn't work because it's private !
yann.toString(); // It's good because it's public ;)