property position

This property contains the current position of the stream relative to the start of the attached stream.

var fileStreamObj = shell.serviceManager.basics.fileStream;
var streamReaderObj = shell.serviceManager.basics.rawStreamReader;

fileStreamObj.openForRead("c:\\temp\\test.txt");
streamReaderObj.stream = fileStreamObj;
streamReaderObj.seekRelative(4);

var position =  streamReaderObj.position;

fileStreamObj.close();