Node.js module error handling

Question

I've been struggling last week with error handling, especially error handling in Node.js modules. So first, here is my code:

user.js route

router.post('/register', function(req, res) {
    var newUser = new User({
        firstname: req.body.firstname,
        lastname: req.body.lastname,
        email: req.body.email,
        password: req.body.password,
    });

    User.addUser(newUser, function(err, user) {
        if(err) {
            return next(err)
        } else if(user) {
            res.status(403).send('User already exists');
        } else {
            res.sendStatus(200);
        }
    });
});

user.js module

module.exports.addUser = function(newUser, cb) {
  User.findOne({ email: newUser.email }, function(err, user) {
    if(err) {
      cb(err);
    } else if(user) {
      cb(null, user);
    } else {
      bcrypt.genSalt(10, function(err, salt) {
        if(err) {
          cb(err);
        } else {
          bcrypt.hash(newUser.password, salt, function(err, hash) {
            if(err) {
              cb(err)
            } else {
              newUser.password = hash;

              newUser.save(function(err, newUser) {
                if(err) {
                  cb(err);
                } else {
                  cb(null, false);
                }
              });
            }
          });
        }
      });
    }
  });
}

Everytime if there is error inside user.js module, call callback function and handle error inside user.js route. This works, but that mess inside my module doesn't look good, because there is so many if-else statements..

Is there better approach, or do I have to check everytime if there is error?

You can try using Promises to simplify the error handling.

Scimonster
– Scimonster

2018-01-25 09:35:27 +00:00
Commented Jan 25, 2018 at 9:35 — Scimonster
– Scimonster, Commented Jan 25, 2018 at 9:35

Hammerbot · Accepted Answer · 2018-01-25 09:38:14Z

You could simplify your code to something like:

module.exports.addUser = function(newUser, cb) {
  User.findOne({ email: newUser.email }, function(err, user) {
    if(err) {
      cb(err);
      return;
    } 

    if(user) {
      cb(null, user);
      return ;
    } 

    bcrypt.genSalt(10, function(err, salt) {
      if(err) {
        cb(err);
        return;
      } 

      bcrypt.hash(newUser.password, salt, function(err, hash) {
        if(err) {
          cb(err)
          return;
        } 

        newUser.password = hash;

        newUser.save(function(err, newUser) {
          if(err) {
            cb(err);
            return;
          } 

          cb(null, false);
        });
      });
    });
  });
}

However, if I were you and as @Scimonster stated in his comment, this is a typical use case for promises that would allow you to write more readable code and avoid the callback hell

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