In Logic Pro, this script passes all events through and sends an “all notes off” message when receiving a MIDI continuous controller 20 event.
Text following /* shows comments that explain the JavaScript code.
function HandleMIDI(event) {
event.send(); /* pass through the event if the event is MIDI CC 20 */
if (event instanceof ControlChange && event.number == 20)
MIDI.allNotesOff(); /* send an all notes off message */
}