const createPayload = (leds) => {
  const buffer = new ArrayBuffer(1 + (leds.length * 3));
  const dv = new DataView(buffer);
  let n = 0;
  dv.setUint8(n++, leds.length);
  for (let i = 0; i < leds.length; i++) {
    dv.setUint8(n++, leds[i][0]);
    dv.setUint8(n++, leds[i][1]);
    dv.setUint8(n++, leds[i][2]);
  }
  return dv.buffer;
}

const readPayload = buffer => {
  let n = 0;
  const dv = new DataView(buffer);
  const count = dv.getUint8(n++);
  const leds = new Array(count).fill(0).map(() =>
    new Array(3).fill(0).map(() => dv.getUint8(n++)));
  return { count, leds };
}

const conn = new WebSocket('ws://0.0.0.0:81');
conn.onopen = () => console.log('Connection open.');
conn.onerror = (err) => console.log('Error: %o', err);
conn.binaryType = 'arraybuffer';
conn.onmessage = (e) => {
  const payload = readPayload(e.data);
  console.log(payload.count, payload.leds);
};

// To receive FastLED frame
conn.send('status');

// To send a FastLED frame
conn.send(createPayload([[0, 128, 255], [255, 255, 0], [255, 0, 0]]));