Another way to achieve private class variables using closure.

class-variables-through-closure.js

var Cat = (function () {
  var numberOfCats = 0;

  var Cat = function (name) {
    this.name = name;
    numberOfCats += 1;
  }

  Cat.prototype.numberOfCats = function () { 
    return numberOfCats;
  };

  return Cat;
}())

cat = new Cat('fluffy');
cat.numberOfCats();        // 1
cat2 = new Cat('garfield')
cat.numberOfCats();        // 2