My music theory teacher has built from scratch, without much coding knowledge, an Electron app with games. Example of games are: guess the 2-3 notes randomly played by the piano, find the intervals etc. Those are multiple choice games mostly.

He is now on a journey to build games that interact with notes on scores, like colorize all the Cs, click on the notes that are wrong vs what you hear etc.

He is building everything from scratch, with the scores, the notes, and I'm wondering if there are some tools or libraries out there that provide much flexibility to build stuff for music theory.

https://github.com/tonaljs/tonal does a bunch of theory stuff. I think there’s another JS library I’ve used too but I can’t think of the name.

If he wants to generate audio, Tone.js can be a useful higher level abstraction for WebAudio.

I am not sure if it is only JS libraries you are looking for (since it is an Electron app), but I released a fairly comprehensive python music theory library that not only supports Western tunings, but all sorts of equal temperaments. It even has some support for post-tonal music theory.

https://xenharmlib.readthedocs.io/en/latest/

It's not really a framework, but I've been having a lot of fun live coding music in Sonic Pi: https://sonic-pi.net/

It has built-in functions for chords and scales and is pretty easy to make catchy loops.

https://github.com/JuliaMusic/MusicTheory.jl

https://strudel.cc