I don't think we should silently return `undefined` when there no bod layer identified to by `id`. I think `undefined` should mean that `prop` is not defined for the layer, it should not mean that either the layer does not exist or `prop` is not defined for the layer.

So I would just implement getLayerProperty as follows:

```
this.getLayerProperty = function(id, prop) {
  return layers[id][prop];
};
```

And we might want to add another gaLayers unction to verify the existence of a layer by its id (idBod).