Base64 (data url) to File functions

dataUrlToFIle.js

function dataUrlToFileSync(dataUrl, fileName) {
  var mimeType = dataUrl.substring(
    dataUrl.indexOf(":") + 1,
    dataUrl.indexOf(";")
  );
  var arr = dataUrl.split(",");
  var b64str = atob(arr[1]);
  var n = b64str.length;
  var u8arr = new Uint8Array(n);
  while (n--) {
    u8arr[n] = b64str.charCodeAt(n);
  }
  var newFile = new File([u8arr], fileName + "." + mimeType.substring(6), {
    type: mimeType,
  });
  return newFile;
}

function dataUrlToFileAsync(dataUrl, fileName) {
  var mimeType = dataUrl.substring(
    dataUrl.indexOf(":") + 1,
    dataUrl.indexOf(";")
  );
  return fetch(dataUrl)
    .then(function (res) {
      return res.arrayBuffer();
    })
    .then(function (buf) {
      return new File([buf], fileName, { type: mimeType });
    });
}

// Testing
var testData =
  "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAADElEQVQImWNgoBMAAABpAAFEI8ARAAAAAElFTkSuQmCC";

console.log(dataUrlToFileSync(testData, "sync-generated-image"));

dataUrlToFileAsync(testData, "async-generated-image").then(function (file) {
  console.log(file);
});