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HTML

              
                
              
            
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CSS

              
                
              
            
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JS

              
                console.clear();

class MIDIAccess {
  constructor(args = {}) {
    this.onDeviceInput = args.onDeviceInput || console.log;
  }

  start() {
    return new Promise((resolve, reject) => {
      this._requestAccess().then(access => {
        this.initialize(access);
        resolve();
      }).catch(() => reject('Something went wrong.'));
    });
  }

  initialize(access) {
    const devices = access.inputs.values();
    for (let device of devices) this.initializeDevice(device);
  }

  initializeDevice(device) {
    device.onmidimessage = this.onMessage.bind(this);
  }
  
  onMessage(message) {
    let [_, input, value] = message.data;
    this.onDeviceInput({ input, value });
  }

  _requestAccess() {
    return new Promise((resolve, reject) => {
      if (navigator.requestMIDIAccess)
        navigator.requestMIDIAccess()
          .then(resolve)
          .catch(reject);
      else reject();
    });
  }
}

class Instrument {
  constructor() {
    this.synth = new Tone.PolySynth(3, Tone.FMSynth);

    this.filter = new Tone.Filter();
    this.volume = new Tone.Gain();

    this.synth.connect(this.filter);
    this.filter.connect(this.volume);
    this.volume.toMaster();
    
    this.filter.frequency.value = 200; // 200 - 15000
    this.volume.gain.value = 0.8; // 0-0.8
  }

  toggleSound(value) {
    let method = value === 127 ? 'triggerAttack' : 'releaseAll';
    this.synth[method](['C4', 'E4', 'G4']);
  }

  handleVolume(value) { // 0-127
    let val = value / 127 * 0.8;
    this.volume.gain.value = val;
  }

  handleFilter(value) { // 0-127
    let val = value / 127 * 14800 + 200;
    this.filter.frequency.value = val;
  }
}

// UPDATE: there is a problem in chrome with starting audio context
//  before a user gesture. This fixes it.
var started = false;
document.documentElement.addEventListener('mousedown', () => {
  if (started) return;
  started = true;
  const inst = new Instrument();
  const midi = new MIDIAccess({ onDeviceInput });
  midi.start().then(() => {
    console.log('STARTED!');
  }).catch(console.error);

  function onDeviceInput({ input, value }) {
    if (input === 23) inst.toggleSound(value);
    else if (input === 2) inst.handleVolume(value);
    else if (input === 14) inst.handleFilter(value);
    else console.log('onDeviceInput!', input, value);
  }
});

              
            
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