Extract temporary variable to method. Also is the explicit check for `undefined` required, would falseyness do?

```
_emptyString: function(metadataExtent) {
    return (!metadataExtent.getBounds()) ? OpenLayers.i18n('unavailableExtent') : '';
} 

...
var map = new OpenLayers.Map({
    controls: [
        new OpenLayers.Control.MousePosition({
            emptyString: _emptyString(metadataExtent)
        })
    ]
});
```