如何验证上传的Excel文件为空并在JavaScript中显示错误消息

Question

在javascript中，我正在执行excel文件上传功能，因此，如果其中一种情况是excel文件为空，则在这里我使用js / xlsx lib如何验证excel文件为空，所以这是我的代码

function ExportToTable() {  
     var regex = /^([a-zA-Z0-9\s_\\.\-:])+(.xlsx|.xls)$/;  
     /*Checks whether the file is a valid excel file*/  
     if (regex.test($("#excelfile").val().toLowerCase())) {  
         var xlsxflag = false; /*Flag for checking whether excel is .xls format or .xlsx format*/  
         if ($("#excelfile").val().toLowerCase().indexOf(".xlsx") > 0) {  
             xlsxflag = true;  
         }  
         /*Checks whether the browser supports HTML5*/  
         if (typeof (FileReader) != "undefined") {  
             var reader = new FileReader();  
             reader.onload = function (e) {  
                 var data = e.target.result;  
                 /*Converts the excel data in to object*/  
                 if (xlsxflag) {  
                     var workbook = XLSX.read(data, { type: 'binary' });  
                 }  
                 else {  
                     var workbook = XLS.read(data, { type: 'binary' });  
                 }  
                 /*Gets all the sheetnames of excel in to a variable*/  
                 var sheet_name_list = workbook.SheetNames;  

                 var cnt = 0; /*This is used for restricting the script to consider only first sheet of excel*/  
                 sheet_name_list.forEach(function (y) { /*Iterate through all sheets*/  
                     /*Convert the cell value to Json*/  
                     if (xlsxflag) {  
                         var exceljson = XLSX.utils.sheet_to_json(workbook.Sheets[y]);  
                     }  
                     else {  
                         var exceljson = XLS.utils.sheet_to_row_object_array(workbook.Sheets[y]);  
                     }  
                     if (exceljson.length > 0 && cnt == 0) {  
                         BindTable(exceljson, '#exceltable');  
                         cnt++;  
                     }  
                 });  
                 $('#exceltable').show();  
             }  
             if (xlsxflag) {/*If excel file is .xlsx extension than creates a Array Buffer from excel*/  
                 reader.readAsArrayBuffer($("#excelfile")[0].files[0]);  
             }  
             else {  
                 reader.readAsBinaryString($("#excelfile")[0].files[0]);  
             }  
         }  
         else {  
             alert("Sorry! Your browser does not support HTML5!");  
         }  
     }  
     else {  
         alert("Please upload a valid Excel file!");  
     }  
 }  

请告诉我如何验证空的excel文件