Strong typing in JS

var Finitio       = require('finitio.js');
var TypeFactory   = require('finitio.js/lib/support/factory');
var _             = require('underscore');

var $ = function(name, obj){
  var source = obj.toString().match(/[^]*\/\*([^]*)\*\/\}$/)[1];
  var result = Finitio.parse(source);
  return result;
};

var User = $('User', function(){/*
  UUID        = .String( s | s.match(new RegExp("^[a-zA-Z0-9]{17}$")) )

  {
    id        :   UUID,
    name      :   .String,
    lastname  :?  .String,
    birthdate :   .Date
  }
*/});

function toObject(system){
  var fields = system.main.heading.attributes;
  var obj = {};

  Object.defineProperty(obj, '__system', {
    value: system,
    enumerable: false
  });

  // Create the properties on the object
  _.each(fields, function(attr){
    var value = null;
    var name = attr.name;
    var type = attr.type;

    Object.defineProperty(obj, name, {
      enumerable: true,
      get: function(){ return value; },
      set: function(newval){
        console.log(value, '->', newval);
        value = type.dress(newval);
      }
    });
  });
  return obj;
}

var user = toObject(User);

user.name = 'Louis';
user.birthdate = new Date().getTime();
user.id = '3' // will throw