test.js

function loadDocument(document) {
  TXTextControl.addEventListener("textControlLoaded", () => {
    blobToBase64(document, (encoded) => {
      TXTextControl.loadDocument(TXTextControl.StreamType.InternalUnicodeFormat, encoded);
    });
  });
}

function blobToBase64(blob, callback) {
  var reader = new FileReader();
  reader.onload = function() {
    callback(reader.result.split(',')[1]);
  };
  reader.readAsDataURL(blob);
}