JavaScript - create private property and function for Object

1.Create private property

1:  function Person(first,last,age) {  
2:    this.firstname = first;  
3:    this.lastname = last;  
4:    this.age = age;  
5:    var bankBalance = 7500; // private property which could not be called outside 
6:  }  
7:  // create your Person   
8:  var john = new Person("Guangyuan","Piao",29);  
9:  // try to print his bankBalance  
10:  console.log(john.bankBalance);  // will print undefined

2.Create and call private function

1:  function Person(first,last,age) {  
2:    this.firstname = first;  
3:    this.lastname = last;  
4:    this.age = age;  
5:    var bankBalance = 7500;  
6:    var returnBalance = function() {  
7:     return bankBalance;  
8:    };  
9:    // create the new function here  
10:    this.askTeller = function () {  
11:      return returnBalance;    
12:    }  
13:  }  
14:  var john = new Person('John','Smith',30);  
15:  console.log(john.returnBalance);  
16:  var myBalanceMethod = john.askTeller();  // return function
17:  var myBalance = myBalanceMethod(); // call function
18:  console.log(myBalance);