The interesting thing is, you can run the js you build in the spreadsheet, independent of the spreadsheet environment.

Unfortunately the project looks pretty dead. And further one of the bullet points is "is lightweight: ~2000 lines of source code (excluding libraries)" but looking into app.js it's something like 23k loc. It uses react but it looks to be pre-JSX. And because it's all in one giant file it feels too overwhelming for me to want to learn/extend it. He says it's compatible with IE 11. And maybe that's why it's structured that way, but I don't know. Despite it all the idea is really cool.