Fun to see other people looking into my code. Great job :)
Concertmaster is built over Electron and, as such, it is 100% JavaScript. In fact, it's not a more convenient website only because of Spotify's limitations.
It would be possible to make Concertmaster open source, but it's far from professional - it's more like a hobby I'm keeping for 3, 4 years.
(Concertmaster is more than the client. It has several server-side functions as well.)