study...

fileSizeUnitConvert.js

   function convertSize(fileSize, fixed) {
      
      var sizeName = ['Byte','KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB'];
      var sizeOffset = Math.round((fileSize.toString().length / 4));
      var sizeReduceArray = [fileSize]
      for(var loopCountNum = 0; loopCountNum < sizeOffset; loopCountNum++){
          sizeReduceArray.push(1024);
      }
      const size = sizeReduceArray.reduce((acc, cur) => acc / cur).toFixed(fixed);
      return `${size} ${sizeName[sizeOffset]}`;
   }