mycode:

function sendTxn(rawTx, callback) {
  var privateKeyBuffer = new Buffer(privateKey, 'hex');
  var transaction = new tx(rawTx);
  transaction.sign(privateKeyBuffer);
  var serializedTx = transaction.serialize().toString('hex');
  web3.eth.sendSignedTransaction(
  '0x' + serializedTx, function(err, txnHash) {
    if(err) {
      console.log("txn err", err);
      callback(err, null);
    } else {
      console.log("txn result", txnHash);
      callback(null, txnHash);
    }
  }).catch((err) => {
    callback(err, null);
  });
}