Logic Pro Scripter script for a latched MIDI keyboard

latch.js

// Plays note with infinite sustain when pressed.
// Unlatches note when it is pressed again.

// Initialize an empty set to store latched notes
var latchedNotes = new Set();

function HandleMIDI(event) {
    // Check if the event is a note on or note off
    if (event instanceof NoteOn) {
        if (latchedNotes.has(event.pitch)) {
            // If the note is already latched, release it
            var noteOff = new NoteOff(event);
            noteOff.send();
            latchedNotes.delete(event.pitch);  // Remove note from the set
        } else {
            // If the note is not latched, latch it
            event.send();  // Send the note-on message
            latchedNotes.add(event.pitch);  // Add note to the set
        }
    } else if (event instanceof NoteOff) {
        // Block the original note-off if the note is latched
        if (latchedNotes.has(event.pitch)) {
            // Do nothing; keep the note latched
        } else {
            // If not latched, pass the note-off through
            event.send();
        }
    }
}