It's TypeScript bundled with esbuild. I needed to minify the output because the monaco editor resulted in file sizes over the 4MB limit imposed on FF addons. The background script is here: https://github.com/icholy/ContainerScript/blob/master/backgr...
Was all or part of the program written in another language and compiled to Javascript?
My Javascript is not the good but it looks that way to me, or perhaps was constructed from some table driven system.