I recently got a question about this add-in created as a sample by the Microsoft Office Developer Team. It is provided as a sample and is a challenging bit of code to follow. It all started because I was trying to provide some advice to someone who is new to the OfficeJS world (but not Office development in general). They wanted to build a cross-platform add-in for a cause. After tinkering with it for a few hours, I managed to get it to work. But then I looked it over and it was quite a bit of tinkering on my part.
Anyway, as I said, I spent a few hours today and worked on getting it into ES6 format by moving most of the code into classes, removing redundant code, removing redundant HTML pages, consolidating CSS and adding a lot of additional JSDoc comments throughout the code.
Hopefully, this version is a bit easier to follow for those that develop in a more modern ES6 style.
NOTE: This looks and behaves exactly the same, but I have not thoroughly tested it. So, it might be a bit rough here and there. If you encounter issues please let me know. 😁