This method changes the position of the stream to be at the end of the stream.
var memStreamObj = shell.serviceManager.basics.memoryStream;
var streamWriterObj = shell.serviceManager.basics.rawStreamWriter;
memStreamObj.size = 1024;
streamWriterObj.stream = memStreamObj;
streamWriterObj.seekToStart();
streamWriterObj.writeInt32(42);
streamWriterObj.writeInt32(41);
streamWriterObj.writeInt32(40);
//to make sure the position will be at the end for next write
//in case of seekRelative or seekToStart been called
streamWriterObj.seekToEnd();
memStreamObj.close();