Private variables

Closures are frequently used to encapsulate some information as private variables. JavaScript does not allow such encapsulation found in programming languages such as Java or C++, but by using closures, we can achieve similar encapsulation:

function privateTest(){
 var points=0;
  this.getPoints=function(){
    return points;
  };
  this.score=function(){
    points++;
  };
}

var private = new privateTest();
private.score();
console.log(private.points); // undefined
console.log(private.getPoints());

In the preceding example, we are creating a function that we intend to call as a constructor. In this privateTest() function, we are creating a var points=0 variable as a function-scoped variable. This variable is available only in privateTest(). Additionally, we create an accessor function (also called a getter)—getPoints()—this method allows us to read the value of only the points variable from outside privateTest(), making this variable private to the function. However, another method, score(), allows us to modify the value of the private point variable without directly accessing it from outside. This makes it possible for us to write code where a private variable is updated in a controlled fashion. This pattern can be very useful when you are writing libraries where you want to control how variables are accessed based on a contract and pre-established interface.