var myObject = {};
	
Object.defineProperty( myObject, '_myProp',
{
	value:			'myDefault',
	writable:		false,
	enumberable:	false,
	configurable:	true
});
	
Object.defineProperty( myObject, 'myProp',
{
	enumberable: true,
	configurable: true,
	set: function( v )
	{
		Object.defineProperty( this, '_myProp', { writable:true } );
		this._myProp = v;
		Object.defineProperty( this, '_myProp', { writable:false } );
	},
	get: function()
	{
		return this._myProp;
	}

});

alert( myObject.myProp );
myObject.myProp = 'myValue';
alert( myObject.myProp );
myObject._myProp = 'teehee, not writable';
alert( myObject._myProp );

ECMAScript 5 Getter/Setter Alt

JavaScript