drawImage = async img_path => {
  let canvas = document.createElement("canvas");
  canvas.src = img_path;
  const context = canvas.getContext("2d");

  const img = await loadImage(img_path);
  canvas.width = img.width;
  canvas.height = img.height;
  context.drawImage(img, 0, 0);

  return { canvas, context };
};

function loadImage(img_path) {
  return new Promise(r => {
    let i = new Image();

    i.onload = () => r(i);
    i.src = img_path;
  });
}

calculateResult = (canvas, context) => {
  let store = {};
  const imgData = context.getImageData(0, 0, canvas.width, canvas.height);
  const data = imgData.data;

  const total_pixels = canvas.width * canvas.height;
  const coverage = total_pixels / 4;

  const max_pixel_index = total_pixels - 1;

  for (let i = 0; i < coverage; ++i) {
    const x = getPixelIndex(Math.floor(Math.random() * max_pixel_index));
    const key = `${data[x]},${data[x + 1]},${data[x + 2]}`;
    const val = store[key];
    store[key] = val ? val + 1 : 1;
  }

  const rgb_code = Object.keys(store).reduce((a, b) =>
    store[a] > store[b] ? a : b
  );

  return `rgb(${rgb_code})`;
};

function getPixelIndex(numToRound) {
  //Each pixel is 4 units long: r,g,b,a
  const remainder = numToRound % 4;
  if (remainder == 0) return numToRound;
  return numToRound + 4 - remainder;
}

main = async () => {
  const { canvas, context } = await drawImage("./img/wallpaper.jpg");
  const result = await calculateResult(canvas, context);
  console.log(result);
};

main();