https://www.twilio.com/docs/studio/widget-library#run-function
When you invoke a Function, you have two possible options for using variables: (1) you can pass Flow variables as parameters into a Function (e.g. flow.data.foo), and (2) you may set Flow variables with data returned from the Function (TwiML or JSON can be returned).
Does anyone know how to set Flow variables with data returned from a Function?
You should be able to follow the Liquid Syntax documented here, to do so.
Liquid Template Language - Assigning Variables
https://www.twilio.com/docs/studio/user-guide/liquid-template-language#variable-assignment
Or just call the returned variable by addressing the Function widget as shown at the link below:
https://www.twilio.com/docs/studio/widget-library#run-function
widgets.MY_FUCTION_WIDGET_NAME.parsed.xxxxx (where xxxxx is the JSON path).
Alan