Inside a module, your constructors and prototype methods have access to any variables declared inside the IIFE. As an alternative to adding properties to the prototype, this is a way to share data between instances but make it hidden from any code outside the module. It looks like this:

(function (window) {
  'use strict';
  var App = window.App || {};
  var launchCount = 0;

  function Spaceship() {
    // Initialization code goes here
  }

  Spaceship.prototype.blastoff = function () {
    // Closure scope allows access to the launchCount variable
    launchCount++;
    console.log('Spaceship launched!')
  }

  Spaceship.prototype.reportLaunchCount = function () {
    console.log('Total number of launches: ' + launchCount);
  }

  App.Spaceship = Spaceship
  window.App = App;
})(window);

Other languages provide a way to declare a variable as private, but JavaScript does not. You can take advantage of closure scope (a function using variables declared in the outer scope) to simulate private variables.