code_snippet_8 for "Playing Mortal Kombat with TensorFlow.js. Transfer learning and data augmentation" translation

code_snippet_8.js

const grayscale = (canvas: HTMLCanvasElement) => {
  const imageData = canvas.getContext('2d').getImageData(0, 0, canvas.width, canvas.height);
  const data = imageData.data;
  for (let i = 0; i < data.length; i += 4) {
    const avg = (data[i] + data[i + 1] + data[i + 2]) / 3;
    data[i] = avg;
    data[i + 1] = avg;
    data[i + 2] = avg;
  }
  canvas.getContext('2d').putImageData(imageData, 0, 0);
};