Node.js/Async - How to avoid callback hell with async?

Question

I\'m new to Node.Js and JavaScript web development on the backend. I see that callbacks inside callbacks could be a pain and there are modules to avoid that. One of these mo

Answer 1

Here is the code modified to use async whenever needed.

function check_auth_user(username, password) {
    var client = new pg.Client("pg://user:pass@127.0.0.1/database");
    async.waterfall([
      client.connect,
      function(callback) {
        client.query('select * from "user" where username = $1 and password = $2', [username, password], callback);
      },
      function(result, callback) {
        if(result.rowCount > 0) {
                var res = result.rows[0];

                async.series([
                   function(callback) {
                     passport.serializeUser(res, function(res, done) {
                       callback(null, res);
                     }
                   },
                   function(res, callback){
                     if(res) {
                       passport.deserializeUser(res, function(res, done) {
                          callback(null, res);
                       });
                     } else {
                       callback(new Error('SerializeUser failed'));
                     }
                   }
                ], function(err, res) {
                    if(err) {
                       callback(err);
                    } else {
                       callback(null);
                    }
                });
        }
        else {
           callback(new Error("No result"));
        }
      }
    ], function(err, result) {
       if(err)
          console.err(err);
    });
}

Obviously, it does not make the code any more readable. I would suggest additional changes:

• Querying the database should be separated in its own method (it is indeed a model method), thus allowing error checking to happen in its own "dominion".
• Passport serialization/deserialization should be done in a separate method, for the same reasons.

These two methods would both take callbacks. Here is a re-write:

// Given a client, queries for user
function retrieveUser( client, username, password, cb) {
  client.query('select * from "user" where username = $1 and password = $2', [username, password], function(err, users){
    if(err) {
      cb(err);
    }
    else if(users.rowCount < 0) {
      cb(new Error("No user found for username ="+username));
    }
    else {
      cb(null, result.rows[0]);
  });
}

//Uses passport to serialize/deserializeUser
function passportSerialize(user, cb){
      async.series([
       function(callback) {
         passport.serializeUser(user, function(res, done) {
           callback(null, res);
         }
       },
       function(res, callback){
         if(res) {
           passport.deserializeUser(res, function(res, done) {
              if(res) {
                 callback(null, res);
              } else {
                 callback(new Error('deserializeUser failed'));
              } 
           });
         } else {
           callback(new Error('SerializeUser failed'));
         }
       }
    ], cb);
}

Thus, our main method now becomes:

function check_auth_user(username, password) {
    var client = new pg.Client("pg://user:pass@127.0.0.1/database");
    async.waterfall([
      client.connect,
      function(callback) {
        retrieveUser(client, username, password, callback);
      },
      function(user, callback) {
            passportSerialize(user, callback);
      }
    ], function(err, result) {
       if(err)
          console.err(err);
        else
          console.log("User authenticated, let her in");
    });
}

I hope you can see how this is much much better.