According to the API docs, the Office.Settings objects are saved per add-in and per document. That is, they are available only to the add-in that created them, and only from the document in which they are saved.
I'm a bit confused regarding the Office.Settings interface. I've created a gist in the ScriptLab Add-in where I set 2 setting objects (queries and queryIDs) and then retrieve them (log them to console).
Gist: https://gist.github.com/VivianVenter/e5489628384f96f2d3bf9a50aace8728
If I run this gist on Excel Desktop, then I can create the settings and retrieve them successfully. If I save this workbook and open it in Excel Online and run the gist again then I cannot retrieve the settings, it returns null for both settings object.
According to my knowledge the ScriptLab Add-in is the same for the Desktop and Online version, am I missing something, or is it the intention of OfficeJS to not make the settings available/visible in the Online version if it was created and set in the Desktop version?
Settings desktop found: The settings objects was found in Excel Desktop
Settings online null: The settings objects was NOT found in Excel Online
The issue was caused by a bug in office-js which has now been fixed upstream.