File Reader to Base64

readFile.js

/*
 * @params - event: upload file event
 * @params - onComplete: callback provided by caller
*/
const uploadFile = ({ event, onComplete }) => {
  event.preventDefault();

  var base64File = null;
  const file = event.target.files[0];

  if (file) {
    const reader = new FileReader();

    reader.readAsBinaryString(file);

    reader.onload = (readerEvent) => {
      base64File = btoa(readerEvent.target.result);
      onComplete({ file, base64File });
    };
  }
};