Downloading an Array Buffer via a "Save as" Dialog in the Browser

saveFile.ts

// This source code is taken from Firefox Send (https://github.com/mozilla/send) and slightly modified.

export default async function saveFile(plaintext: ArrayBuffer, fileName: string, fileType: string) {
  return new Promise((resolve, reject) => {
    const dataView = new DataView(plaintext);
    const blob = new Blob([dataView], { type: fileType });

    if (navigator.msSaveBlob) {
      navigator.msSaveBlob(blob, fileName);
      return resolve();
    } else if (/iPhone|fxios/i.test(navigator.userAgent)) {
      // This method is much slower but createObjectURL
      // is buggy on iOS
      const reader = new FileReader();
      reader.addEventListener('loadend', () => {
        if (reader.error) {
          return reject(reader.error);
        }
        if (reader.result) {
          const a = document.createElement('a');
          // @ts-ignore
          a.href = reader.result;
          a.download = fileName;
          document.body.appendChild(a);
          a.click();
        }
        resolve();
      });
      reader.readAsDataURL(blob);
    } else {
      const downloadUrl = URL.createObjectURL(blob);
      const a = document.createElement('a');
      a.href = downloadUrl;
      a.download = fileName;
      document.body.appendChild(a);
      a.click();
      URL.revokeObjectURL(downloadUrl);
      setTimeout(resolve, 100);
    }
  });
}