var myObject = {};

//wrapping Object.defineProperty in a function like this feels very hacky
(function() {
	var myProp = 'myDefault';
	Object.defineProperty( myObject, 'myProp', 
	{
		enumerable:		false,
		configurable:	true,
		get: function()
		{
			return myProp;
		},
		set: function( v )
		{
			myProp = v+' is mo beta';
		}
	
	});
})();

alert( myObject.myProp );
myObject.myProp = 'myValue';
alert( myObject.myProp );


/*  COULDN'T IT HAVE JUST BEEN....

	Object.defineProperty( myObject, 'myProp', 
	{
		writable:		false,
		enumerable:		false,
		configurable:	true,
		value: 'myDefault',
		get: function()
		{
			return myProp;
		},
		set: function( v )
		{
			myProp = v+' is mo beta';
		}
	
	});
*/

ECMAScript 5 Getter/Setter

JavaScript