WeakMap and private

weakmap.js

var privateThings = new WeakMap();

function Private() {
  this.data = 'is private';
}

Private.prototype.somePrivateFunction = function() {
  this.foo = 'bar';
};

function Public() {
  privateThings.set(this, new Private());
}

Public.prototype.somePublicFunction = function() {
  var myPrivate = privateThings.get(this);
  // myPrivate.data === 'is private';

  myPrivate.somePrivateFunction.call(this);
};

Why couldn't I do:

var p = new Public();

privateThings.get(p)