As the other answers mentioned, at the moment there is no way to intercept undefined properties.
Would this be acceptable though?
var myObj = (function() {
var props = {
foo : 'foo'
}
return {
getProp : function(propName) { return (propName in props) ? props[propName] : 'Nuh-uh!' }
}
}());
console.log(myObj.getProp('foo')); // foo
console.log(myObj.getProp('bar')); // Nuh-uh