I've been trying to create a Google Spreadsheet plugin from some existing Google App Scripts that I have, and one important part of this app script is Custom Functions.

Though the documentation for the add-ons doesn't indicate that this is supported, the documentation for Custom Functions does indicate that you can.

https://developers.google.com/apps-script/guides/sheets/functions

Through testing, I have not once been able to get Custom Functions exposed through a add-on. Does anyone know the secret sauce to get this to work?

Answer

According to Eric Koleda in [Code.gs - date_add_and_subtract]( https://github.com/google/google-apps-script-samples/blob/master/date_add_and_subtract/Code.gs) to make available the custom fuctions in an add-on it's required to include at least one add-on menu, but this is not working at this time on testing mode.

NOTE: The originally referred page was removed but the Date add and substract add-on sample code is available at https://github.com/googlesamples/apps-script/tree/master/sheets/dateAddAndSubtract.

The "solution" to test a custom function add-on is to publish the add-on privately so you could avoid the Google review of an untested add-on. Related Q&A: Publish an add-on privately