async styles - callbacks.js

function GetUser(userId, callback) {
  redisClient.get(userId, function(redisError, redisResult){
    if (redisError !== null) callback(redisError, null);
    else if (redisResult !== null) callback(null, redisResult);
    else {
      mysqlClient.getUser(userId, function(mysqlError, mysqlResult){
        if (mysqlError !== null) callback(mysqlError, null);
        else callback(null, mysqlResult);
      });
    }
  });
}

// usage :
GetUser('dnitems', function(error, user){
  if (error !== null) {
    console.error('error', error);
  }
  else {
    console.log('user', user);
  }
});