web-midi-api.js

let midiOutputs
let midiInputs

// check MIDI support/availability
if (navigator.requestMIDIAccess) {
  navigator.requestMIDIAccess().then(function (access) {
    // get output devices
    midiOutputs = Array.from(access.outputs.values())

    // get input devices
    midiInputs = Array.from(access.inputs.values())
    midiInputs.forEach(m => {
      // incoming MIDI message listener
      m.value.onmidimessage = messageRcvd
    })
  })
}

// handle received MIDI message
function messageRcvd (message) {
  let command = message.data[0]
  let note = message.data[1]
  let velocity = message.data[2]

  // MIDI message received, do something
}

// send a MIDI message
function messageSend (command, note, velocity) {
  midiOutputs.forEach(m => {
    m.send([command, note, velocity])
  })
}