medium.js

const getAverageRGB = (frame) => {
  const length = frame.data.length / 4;

  let r = 0;
  let g = 0;
  let b = 0;

  for (let i = 0; i < length; i++) {
    r += frame.data[i * 4 + 0];
    g += frame.data[i * 4 + 1];
    b += frame.data[i * 4 + 2];
  }

  return {
    r: r / length,
    g: g / length,
    b: b / length,
  };
};

// ...

for (let y = 0; y < height; y += fontHeight) {
  for (let x = 0; x < width; x += fontWidth) {
    const frameSection = hiddenContext.getImageData(x, y, fontWidth, fontHeight);
    const { r, g, b } = getAverageRGB(frameSection);

    outputContext.fillStyle = `rgb(${r},${g},${b})`;
    outputContext.fillRect(x, y, fontWidth, fontHeight);
  }
}