reaper-jsfx-midi

name	reaper-jsfx-midi
description	MIDI processing patterns for JSFX plugins. Use when building MIDI utilities, arpeggiators, chord generators, note filters, or CC processors. Triggers on requests for MIDI effects, arpeggiator, chord, transpose, or MIDI routing in JSFX.
disable-model-invocation	true
license	MIT
metadata	{"author":"mthines","author_name":"Mads Thines Coello","author_email":"[e-mail]","version":"1.0.0","workflow_type":"implementation","tags":["jsfx","reaper","midi","arpeggiator","chord"]}

REAPER JSFX MIDI Processing

Patterns for MIDI utility plugins including note processing, arpeggiators, chord generators, and CC manipulation.

Requires: reaper-jsfx-core for language fundamentals.

Rules

Rule	Description
midi-basics	MIDI message structure, receive/send, SysEx handling
midi-transform	Note transposition, velocity scaling, CC mapping
midi-generate	Arpeggiators, sequencers, chord generators
midi-routing	Multi-bus routing, channel filtering, splitting

Official Documentation

• JSFX MIDI Functions

Key Principles

1. MIDI-Only Plugin Structure

desc:MIDI Utility
tags:midi

in_pin:none
out_pin:none

@block
while(midirecv(offset, msg1, msg2, msg3)) (
  // Process and forward
  midisend(offset, msg1, msg2, msg3);
);

2. Always Forward or Handle All Messages

@block
while(midirecv(offset, msg1, msg2, msg3)) (
  status = msg1 & $xF0;

  status == $x90 || status == $x80 ? (
    // Process notes
    process_note(offset, msg1, msg2, msg3);
  ) : (
    // Pass through everything else unchanged
    midisend(offset, msg1, msg2, msg3);
  );
);

3. Preserve Timing

@block
// Use original offset for timing-critical events
while(midirecv(offset, msg1, msg2, msg3)) (
  // Transform but keep original offset
  midisend(offset, new_msg1, new_msg2, new_msg3);
);

Quick Reference

Use Case	Rule
Understanding MIDI	midi-basics
Transpose / Velocity	midi-transform
Arpeggiator	midi-generate
Channel Filter	midi-routing