If you look at their example, the "type information" is actually encoded in an ancillary JSON file: https://github.com/OfficeDev/Excel-Custom-Functions/blob/mas

If you look at their example, the "type information" is actually encoded in an ancillary JSON file: https://github.com/OfficeDev/Excel-Custom-Functions/blob/master/customfunctions.json https://github.com/OfficeDev/Excel-Custom-Functions/blob/mas... In theory, you could embed all of the metadata in JSDoc + TS annotations.