download-from-base64.js

const download = (fileName, mimeType, fileData) => {
  if (window.navigator.msSaveOrOpenBlob === undefined) {
    const e = document.createElement('a');
    const href = `data:${mimeType};base64,${fileData}`;
    e.setAttribute('href', href);
    e.setAttribute('download', fileName);
    document.body.appendChild(e);
    e.click();
    document.body.removeChild(e);
  } else {
    const charCodeArr = new Array(fileData.length);
    for (let i = 0; i < fileData.length; ++i) {
      const charCode = fileData.charCodeAt(i);
      charCodeArr[i] = charCode;
    }
    const blob = new Blob([new Uint8Array(charCodeArr)], {
      type: mimeType
    });
    window.navigator.msSaveOrOpenBlob(blob, fileName);
  }
};