Something

function requestMe() {
  var req = opensocial.newDataRequest();
  req.add(req.newFetchPersonRequest(
            opensocial.DataRequest.PersonId.VIEWER),
          "viewer");
  req.send(handleRequestMe);
};

function handleRequestMe(data) {
  var viewer = data.get("viewer");
  if (viewer.hadError()) {
    //Handle error using viewer.getError()...
    return;
  }

  //No error. Do something with viewer.getData()...
}