pure private class fields.js

/*
  Very simple. Uses [gs]etters and caller. Precache '_method in Class' relations using unique `Class.jbId_` and `_method.jbOwnerClassIdMap_`.
*/

(function()
{
  var nextClassId = 1;
  var _methodInClass = function(_fn, Class)
  {
    var classId = Class.jbId_;
    if(!classId)
    {  
      var classId = Class.jbId_ = nextClassId++;
      for(var i in Class.prototype)
      {
        if(!{}.hasOwnProperty.call(Class.prototype, i)) continue;
        var _method = Class.prototype[i];
        if(_method.constructor !== Function) continue;
        var ownerClassIdMap = _method.jbOwnerClassIdMap_ || (_method.jbOwnerClassIdMap_ = {})
        ownerClassIdMap[classId] = 1;
      }
    }
    return _fn.jbOwnerClassIdMap_ && _fn.jbOwnerClassIdMap_[classId];
  };
  var _defPrivateField = function(obj, name, v)
  {
    obj.__defineGetter__(name, function()
    {
      var _caller = arguments.callee.caller;
      if(_methodInClass(_caller, obj.constructor))
        return v;
      else
        throw ReferenceError(name);
    });
    obj.__defineSetter__(name, function(nv)
    {
      var _caller = arguments.callee.caller;
      if(_methodInClass(_caller, obj.constructor))
        v = nv;
      else
        throw ReferenceError(name);
    });
  };
  
  var Class = function()
  {
    _defPrivateField(this, 'foo', 1);
  };
  
  Class.prototype = {
    constructor: Class,
    _bar: function()
    {
      this.foo = 2;
      return this.foo;
    }
  };
  
  var c = new Class();
  console.log(c._bar()); // 2
  try
  {
    console.log(c.foo); // oops! `foo` is private
  }
  catch(err)
  {
    console.log(err);
  }

})();