Private members hack...

private.js

function Private() {
	var val, NADA = {};

	return {
		open: function( safe, ret ) {
			val = NADA;
			safe();
			if ( val !== NADA ) {
				ret = val;
				val = NADA;
			}
			return ret;
		},
		safe: function( value ) {
			return function() {
				val = value;
			};
		}
	};
}

var WMap = this.WeakMap || (function(){
	var expando_postfix = ":" + ( "" + Math.random() ).slice(2),
		expando_counter = 0;

	return function() {
		var val,
			expando = ++expando_counter + expando_postfix;

		return {
			get: function( obj, ret ) {
				if ( expando in obj ) {
					val = void 0;
					obj[ expando ]();
					if ( val && val.obj === obj ) {
						ret = val.value;
					}
					val = void 0;
				}
				return ret;
			},
			set: function( obj, value ) {
				var a = { obj: obj, value: value };
				obj[ expando ] = function() {
					val = a;
				};
			}
		};
	};
})();


var PrivateData = function() {
	var map = WMap();

	return function( obj ) {
		var data = map.get( obj );

		if ( !data ) {
			data = {};
			map.set( obj, data );
		}

		return data;
	};
};


// EXAMPLE
var MyClass = (function(){
	var PRIVATE = PrivateData();

	function MyClass( pass, data ) {
		var p = PRIVATE(this);
		p.pass = pass;
		p.data = data;
	}

	MyClass.prototype.acces = function( pass ) {
		var priv = PRIVATE(this);
		if ( priv.pass !== pass ) {
			throw new Error("bad password");
		}
		return priv;
	};

	MyClass.prototype.changePass = function( oldPass, newPass ) {
		this.acces( oldPass ).pass = newPass;
	};

	MyClass.prototype.setData = function( pass, data ) {
		this.acces( pass ).data = data;
	};

	MyClass.prototype.getData = function( pass ) {
		return this.acces( pass ).data;
	};

	return MyClass;
})();