understanding an es6 function [on hold]

I have to unit test this function, but I need to understand what it does and how it works first. I mean it could be great if someone could describe me its workflow and different steps it goes through.

 saveDataToGoogleSpreadSheet(conversationData: ConversationData): Promise<void> {
        return new Promise((resolve, reject) => {
          Spreadsheet.load(this.getGoogleAPISettings(), (err, spreadsheet) => {
            if (err) return reject(err);

            return spreadsheet.receive((receivedError, rows, info) => {
              if (receivedError) return reject(receivedError);

              const rowData = this.getSpreadsheetRowData(conversationData, info.totalRows);
              spreadsheet.add(rowData);
              return spreadsheet.send(sendError => (sendError ? reject(sendError) : resolve()));
            });
          });
        });
      }

via laurent miller