Privates and WeakMaps

gistfile1.js

// Based on a gist by @rwaldron
// https://gist.github.com/2897761

function privatize() {
  var map = new WeakMap();
  return function private(obj) {
    var data = map.get(obj);
    if (!data) {
      map.set(obj, data = {});
    }
    return data;
  };
}

var Person = (function () {

  var private = privatize();

  function Person(name) {
    private(this).name = name;
  }
  Person.prototype.say = function (msg) {
    return private(this).name + ' says: ' + msg;
  };

  return Person;

}());