requestMIDIAccess.js

// Focusrite-Novation Launchpad Mini
const midiDeviceMap = {};
const midi = await navigator.requestMIDIAccess();
midi.inputs.forEach(entry => {
  const key = entry.manufacturer+entry.name+entry.version;
  if (!midiDeviceMap[key]) midiDeviceMap[key] = {};
  midiDeviceMap[key].input = entry;
});
midi.outputs.forEach(entry => {
  const key = entry.manufacturer+entry.name+entry.version;
  if (!midiDeviceMap[key]) midiDeviceMap[key] = {};
  midiDeviceMap[key].output = entry;
});
for (const key in midiDeviceMap) {
  const {input, output} = midiDeviceMap[key];
  input.onmidimessage = ({data}) => {
    const [chn, pos, vel] = data;
    output.send([chn, pos, vel ? Math.floor(Math.random() * 127) + 1 : 0]);
  };
}