webmidi-reader.js

// Generic MIDI message reader

function initWebMidiReader() {

  // Turns a generic MIDI message into { timeStamp, channel, command, note, velocity }
  function parseMidiMessage(midiMessage) {
    var data = midiMessage.data
    var channel = (data[0] & 0xf) + 1;
    var command = data[0] >> 4;
    // cf _parseChannelEvent() from https://github.com/djipco/webmidi/blob/master/src/webmidi.js
    return {
      timeStamp: midiMessage.timeStamp,
      channel,
      command,
      note: data[1],
      velocity: data[2]
    };
  }

  function listenToMidiMessages(handler) {

    const emit = (message) => setTimeout(() => handler(message), 0);

    const onMIDIFailure = () => emit({
      error: 'Could not access your MIDI devices.'
    });

    function onMIDISuccess(midiAccess) {
      // attach message handler to all MIDI inputs
      for (var input of midiAccess.inputs.values()) {
        console.log('Attaching MIDI device:', input.name);
        input.onmidimessage = emit;
      }
    }

    if (!navigator.requestMIDIAccess) {
      onMIDIFailure();
    } else {
      navigator.requestMIDIAccess().then(onMIDISuccess, onMIDIFailure);
    }

  }

  return {
    parseMidiMessage,
    onMidiMessage: listenToMidiMessages,
  };

}