Provide an event or callback that notifies when the imported Excel file is complete and available in the Spreadsheet. A similar event from the Kendo jQuery widget is render:

https://docs.telerik.com/kendo-ui/api/javascript/ui/spreadsheet/events/render

The main advantage of this event/callback will provide the ability to see the data in advance and customize the cells if needed or adjust any other parts of the workbook. 

Currently, to achieve the same, a setTimeout needs to be used:

 public onExcelImport(ev: SpreadsheetExcelImportEvent) {
    setTimeout(() => {
      const desc = ev.sender.toJSON();
    }, 1000);
  }