refactored-gpix.js

let $photoInput = document.getElementById("input");
let image = new Image();
let $editor = document.getElementById("editor");
let $editorCtx = $editor.getContext("2d");

function opacitor(op) {
  let imgData = $editorCtx.getImageData(0, 0, $editor.width, $editor.height);
  for (let x = 0; x < image.width; x++) {
    for (let y = 0; y < image.height; y++) {
      let index = (x + y * image.width) * 4;
      imgData.data[index + 3] = op;
    }
  }
  $editorCtx.putImageData(imgData, 0, 0);
}

$photoInput.addEventListener("change", e => {
  let file = e.target.files[0];
  createImageBitmap(file).then(bitmap => {
    $editor.width = bitmap.width;
    $editor.height = bitmap.height;
    $editorCtx.drawImage(bitmap, 0, 0);
    opacitor(257);
  });
});