insertSheets.js

$("#file").change(() => tryCatch(insertSheets));

async function insertSheets() {
  const myFile = <HTMLInputElement>document.getElementById("file");
  const reader = new FileReader();

  reader.onload = (event) => {
    Excel.run((context) => {
      // strip off the metadata before the base64-encoded string
      const startIndex = (<string>(<FileReader>event.target).result).indexOf("base64,");
      const workbookContents = (<string>(<FileReader>event.target).result).substr(startIndex + 7);

      const sheets = context.workbook.worksheets;
      sheets.addFromBase64(
        workbookContents,
        null, // get all the worksheets
        Excel.WorksheetPositionType.end // insert them after the current workbook's worksheets
      );
      return context.sync();
    });
  };

  // read in the file as a data URL so we can parse the base64-encoded string
  reader.readAsDataURL(myFile.files[0]);
}

/** Default helper for invoking an action and handling errors. */
async function tryCatch(callback) {
  try {
    await callback();
  } catch (error) {
    // Note: In a production add-in, you'd want to notify the user through your add-in's UI.
    console.error(error);
  }
}