So I wrote some code that goes to a book's ID based on a "dictionary" and takes the data from all the sheets in that book after the second sheet. That data is then aggregated into a single array. My question centers around the idea of improving the for loop in the if statement. That is adding the "supplier name" to the front of the subarray that represents a row in the sheet/data. I was told that this is inefficient as the code has to go through each subarray and add a value.

Is there a more efficient way of doing this? I did it this way because I am copying the values of a range which are stored as an array of arrays. So to add data, I have to reaccess the subarrays. Is it possible to add the new data (supplier name) at the same time that the values are being copied? would this be more efficient? It was recommended to use an arrow function, however, I am not familiar with their usage.

function aggregate() {
   var combinedData = []
   var idArray = {
   "suppliername":"id",
   };
for (var supplierName in idArray){
   var sBook = SpreadsheetApp.openById(idArray[supplierName]);
   var sheets = sBook.getSheets();

   for (var index = 2; index <sheets.length; index++){
      var sheet = sheets[index];
      var dataLength = sheet.getRange("E5:E").getValues().filter(String).length;

      if(dataLength != 0){
         var dataRange = sheet.getRange(5,2,dataLength,14);
         var dataValues = dataRange.getValues();
      
         for (row in dataValues) {
           dataValues[row].unshift(supplierName);
         };
         combinedData = combinedData.concat(dataValues);
      };
   };
};

var dataLength = combinedData.length;
const dSheet = SpreadsheetApp.openById("id").getSheets()[0];
dSheet.getRange(2,1,dSheet.getMaxRows(),dSheet.getMaxColumns()).clearContent();
var dRange = dSheet.getRange(2,1,dataLength,15);
dRange.setValues(combinedData);
};