How to transpose MIDI?

[skepsys]

I'm looking for an ability to adjust the tonic of a melody being loaded into a player (+/- halftone). Any ideas and workarounds on how can I transpose a MIDI file?

[cifkao]

All the notes in the file are accessible through player.noteSequence.notes (player.noteSequence is a NoteSequence object from the Magenta.js library). So you can loop through them and change their pitches:

for (const note of player.noteSequence.notes) {
  note.pitch += 1;
}

Then you will want to assign the note sequence back to the player (and visualizer, if you have one) to make sure it gets updated immediately:

player.noteSequence = player.noteSequence;
visualizer.noteSequence = player.noteSequence;