Private variable in javascript

private.js

  var SlideshowController = {};

  SlideshowController.getPropertyA = function() {
    var i = this.instance();
    return i.getPropertyA();
  }
  SlideshowController.getPropertyB = function() {
    var i = this.instance();
    return i.getPropertyB();
  }

  SlideshowController.instance = function() {
    if (!this._instance) {
      this._instance = (function() {
        function RealInstance() {
          this.propertyA = 'FOO'; // Can only be accessed by getPropertyA()
          this.propertyB = 'BAR'; // Can only be accessed by getPropertyB()
        }
        RealInstance.prototype.getPropertyA = function() {
          return this.propertyA;
        }
        RealInstance.prototype.getPropertyB = function() {
          return this.propertyB;
        }
        var instance = new RealInstance();
        return instance;
      })()
    }
    return this._instance;
  }