Monophonic filter that only keeps top note in a chord for Mainstage/Logic Pro

script.js

/*
Only play highest note in a chord
*/

var notes = [];

function highestPitch() {
	var highestPitch = -1;
	notes.forEach(function(note) {
		if (note.pitch > highestPitch)
		{
			highestPitch = note.pitch;
		}
	});
	return highestPitch;
}

function HandleMIDI(event)
{
	var previousPitch = highestPitch();
	if (event instanceof NoteOn)
	{
		notes.push(event);
		if (event.pitch > previousPitch && previousPitch > -1) {
			// New note is higher, so switch off previous note
			var off = new NoteOff();
			off.pitch = previousPitch;
			off.send();
			event.send();
		} else if (previousPitch == -1) {
			// No previous note
			event.send();
		}
	}
	else if (event instanceof NoteOff)
	{
		// Remove from notes
		var index = notes.findIndex(function(note) {
			return note.pitch === event.pitch;
		});
		notes.splice(index, 1);
		
		if (event.pitch === previousPitch && GetParameter('Restore Lower Keys') == 100)
		{
			// Highest note has been turned off, so turn on next highest
			previousPitch = highestPitch();
			var on = notes.find(function(note) {
				return note.pitch === previousPitch;
			});
			if (on) {
				on.send();
			}
		}
		event.send();
	}
}

var PluginParameters = [
	{
		name:'Restore Lower Keys',
		type:'menu',
		valueStrings:['off', 'on'],
		defaultValue:0,
		numberOfSteps: 100
	}
];