I was thrilled when I heard (in 2014 ish?) Excel was supporting Excel add-ins written in JavaScript. Until I found out what was involved. View source (and look askance) at http://kalx.net/ExcelUDF.html. Hmmm. Looks like that has succumbed to bit rot. Good riddance.

JavaScript add-ins part deux are quite simple now. Clone https://github.com/OfficeDev/Excel-Custom-Functions and follow the directions. You may need to start IIS if you don’t have that running.

Next step: put some tooling around TypeScript to automate everything.