withTransaction.js

function rollback (conn, cb) {
  conn.query('ROLLBACK', function (err) {
    if (err) {
      console.log('Rollback failed');
      // need to remove conn from the pool 
      return cb(err);
    }
    return cb();
  })
}

function withTransaction (tasks, cb) {
  getConnection(function (err, conn) {
    if (err) return cb(err);
    conn.query('START TRANSACTION', function (err) {
      if (err) return cb(err);
      // here we give the connection to each task
      async.parallel(tasks.map(conn), function (err) {
        if (err) return rollback(conn, cb);
        conn.query('COMMIT', function (err) {
          if (err) return rollback(conn, cb);
          /* if we get here, the transaction succeeded */
          return cb();
        });
      });
    });
  })
}

// EXAMPLE

// We basically need to end up with a function that takes the DB conn 
// as an argument at a later time (i.e. from inside the withTransaction function)
function updateBalance (accountId, amount) {
  return function pretask (conn) {
    return function task (cb) {
      conn.query(`
        UPDATE accounts 
        SET balance = $2
        WHERE id = $1
      `, [accountId, amount], function (err) {
        if (err) return cb(err);
        return cb();
      });
    }
  }
}

app.post('/transfer/:fromAccount/:toAccount/:amount', function (req, res, next) {
  var tasks = [];
  tasks.push(updateBalance(req.params.fromAccount, -req.params.amount));
  tasks.push(updateBalance(req.params.toAccount,    req.params.amount));
  withTransaction(tasks, function (err) {
    if (err) return next(err);
    res.json({ transfer: 'success' });
  });
});

If you're on Node 6.x, you can clean up the db function:

function updateBalance (accountId, amount) {
  return (conn) => (cb) => {
    conn.query(`
      UPDATE accounts 
      SET balance = $2
      WHERE id = $1
    `, [accountId, amount], (err) => {
      if (err) return cb(err);
      return cb();
    });
  }
}