Save images scanned with Dynamic Web TWAIN

I am using Dynamic Web TWAIN with which I can manipulate the settings of a scanner with javascript, my problem is that I can not recover the images once scanned to save them in a database, I have this function that converts an image into base64 I put it inside the OnPostTransfer function where the scanned images are loaded, I try to save them in an array but although the images are stored sometimes the last image is superimposed, that is, I scan two images and only save one, I would appreciate your help.

 var imagenes = [];
 DWObject.RegisterEvent('OnPostTransfer', function(){
   DWObject.ConvertToBase64 ([0,3], EnumDWT_ImageType.IT_PDF, 
   asyncSuccessFunc, asyncFailureFunc);
     function asyncSuccessFunc (result) {
      var length=result.getLength();
       imagenes.push(result.getData(0,length));
       console.log(result.getData(0,length));
     }
     function asyncFailureFunc (errorCode, errorString) {
      alert("ErrorCode: "+errorCode +"\r"+"ErrorString:"+ errorString);
     }
   });