Stupid MIDI synthisizer

index.html

<script>
  // 5 octaves + 9 semitones under A440Hz is Midi Key Zero
  const MidiKeyZero = 440/(Math.pow(2, 5) * Math.pow(2, 9/12)) ;

  const ctx = new AudioContext();
  const oscis = new Map();
  const volumeCtrl = ctx.createGain();
  volumeCtrl.gain.value = 0;
  volumeCtrl.connect(ctx.destination);

  function processMidiEvent(event) {
    console.log(event.data);
    switch (event.data[0]) {
      case 0x90: // Note On
        if (oscis.has(event.data[1])) 
          return;
        const osci = ctx.createOscillator();
        osci.frequency.value = MidiKeyZero * Math.pow(2, event.data[1]/12);
        osci.connect(volumeCtrl);
        osci.start();
        oscis.set(event.data[1], osci);
      break;
      case 0x80: // Note Off
        if(oscis.has(event.data[1]))
          oscis.get(event.data[1]).disconnect();
        oscis.delete(event.data[1]);
      break;
      case 0xB0: // Sliders
        volumeCtrl.gain.value = event.data[2] / 128;
    }
  }

  navigator.requestMIDIAccess()
    .then(ma => ma.inputs.forEach(input => input.onmidimessage = processMidiEvent));
</script>