In the Basic property configuration form, for the Return value data type field you specify the data type of the object by selecting it from the drop-down menu. However, if the type is not in the list, you must edit the Property:Javascript_data_type and add the data type to the list.

The problems with this are, 1 - keeping the list in some order that makes sense (alphabetical), and 2 - dealing with a potentially endless list of types. 

We need to come up with a better way of specifying the type if it is not one of the primitive types. Perhaps an additional text input field under the drop-down where you can specify the type if you choose "Other" from the drop-down. The "Other" type would not be persistent, but defined for the page.