Very clean design and great execution! I like the used of piano notes and audio in general, it really gives the game personality.
I saw below you are looking into Godot in the future. For my game this time, I tried out Phaser.js which takes care of some game engine tasks, but still let's you do a lot of things your own way (and code in JavaScript which it looks like you used for this game). I recommend checking that out, too ^-^
Nice job!