Javascript: Pattern to create instance-specific private data

To make a property private, just make it local to the constructor. Any outside code trying to access age variable will result in an undefined. But the outside code can still call the property that accesses this private variable using the public function getAge.

function Person(name) {

   // Define a variable only accessible inside of the Person constructor
   var age = 25;

   this.name = name;

   this.getAge = function() {
      return age;
   };

   this.growOlder = function() {
      age++;
   };
}

var person = new Person("Nicholas");

console.log(person.name);     // "Nicholas"
console.log(person.getAge()); // 25

person.age = 100;
console.log(person.getAge()); // 25

person.growOlder();
console.log(person.getAge()); // 26