Cant get recursive search to work

Hello,

I need to do a recursive search and get all the files in all the directories from a root folder.I grabbed this adobe script from the forums.Unfortunately it doesnt work on line it gives me error 21: undefined not a object

if (f.name.slice(-4).toLowerCase() == ".jpg")

 so i narrowed it down to f.name is undefined i tried logging what type of variable it is using instanceof keyword but f isnt a Folder,File,or String type.Iam at a lost on were to go from here any help is appreciated thanks

function find_files (dir)
    {
    return find_files_sub (dir, []);
    }

function find_files_sub (dir, array)
    {
    var f = (dir).getFiles ();
    for (var i = 0; i < f.length; i++)
        {
        if (f instanceof Folder)
            find_files_sub (f, array);
        else
        {
          if(f instanceof Folder)
             fileOut.writeln("Is Folder")
          if(f instanceof File)
             fileOut.writeln("Is File")
          if(f instanceof String)
             fileOut.writeln("Is String")

            if (f.name.slice(-4).toLowerCase() == ".jpg")
                array.push (f);
          }
        }
    return array;
    }