Callback code not executed - node.js

Question

Here is my code:
socket.on('add user', function (data) {
d('"add user" event');
d(data, true);
debugger;
UserEngine.login({
username: data.username,
password: data.password
}, function(err, user) {
if (err) {
d('Bad Login. Username: ' + data.username);
return;
}
/*
** Code after this comment is never executed
*/
debugger;
d('Login OK: ' + data.username);
socket.username = data.username;
usernames[data.username] = data.username;
++numUsers;
addedUser = true;
socket.emit('login', {
numUsers: numUsers
});
socket.broadcast.emit('user joined', {
username: socket.username,
numUsers: numUsers
});
return;
});
});
If there is an error in UserEngine.login(...), the If statement before the comment works correctly and the callback returns. But if the method works correctly, the code after the If statement isn't executed.
Why?
Edit:
Here is the code of the UserEngine module:
http://pastebin.com/u2DQJrV3

Answer 1

My guess is that your code is throwing an exception somewhere in your login() method and because much of that code is executed in async callbacks, you may not get any exception info in the console.
Here are several suggestions for debugging this problem:
You need to follow the exact flow through your login() method. If what you say in your question is true, then it isn't calling the callback sometimes and you need to find out why. The simplest way to follow the flow is to insert a uniquely tagged console.log() statement in every branch of the login() method so you can see exactly which way the control flow goes and what is executed and what is not. Once you find that, you can then output various values to see why it goes that way or set an appropriate breakpoint and trace through it.
There also could be an exception of some kind getting thrown in the login() method or in something that it calls. One you get into an async callback, exceptions may not get logged in the console so things could fail silently and the callback could just be skipped without anything showing in the console. If you suspect an exception in a particular place, you can put your own exception handler there and log what the exception is. Remember, each async scope needs its own exception handler - you can't just use one exception handler at the top level. This is a complication of async coding and one reason why using promises instead of plain callbacks is massively more useful because the promise system will catch all async exceptions for you and turn it into a rejected promise with the exception as the reason (so async exceptions don't fail silently).
There is a logic problem in your login() method if it goes does the user.save() branch of code. I've documented that issue below, though I don't think that is your main issue - but it is something to fix.
I'd also suggest you put logging where you call your UserEngine.login() so you are logging ALL possible callback values.
Suggested logging:
UserEngine.login({
username: data.username,
password: data.password
}, function(err, user) {
// =========== Add this ============
console.log("UserEngine.login() callback");
console.log(err, user);
if (err) {
d('Bad Login. Username: ' + data.username);
return;
}
Here's the issue with the user.save() path in your login() method:
In your login() code, as part of the self._verifyToken() function call, you need to change this:
login: function(args, callback) {
/**
* username [String]
* password [String]
*/
var self = this;
if (!args.username || !args.password) {
return callback(Error.genObj(Error.code.MISSING_PARAMS));
}
User.findOne({
username: args.username
}, function(err, user) {
console.log('[Debug] In User.findOne(...) Callback;');
if (err) {
return callback(Error.genObj(Error.code.INTERNAL));
}
if (!user) {
return callback(Error.genObj(Error.code.TOKEN_AUTH_FAILED));
}
if (self._generateHash({ password: args.password, salt: user.salt }) != user.password) {
return callback(Error.genObj(Error.code.TOKEN_AUTH_FAILED));
}
self._verifyToken({
token: user.token,
username: args.username,
key: Config.tokenKey
}, function(err) {
if (err) {
var token = self._generateToken({ username: args.username, key: Config.key });
user.token = token;
user.save(function(err) {
if (err) {
return callback(Error.genObj(Error.code.INTERNAL));
}
return callback(null, user);
});
}
return callback(null, user);
});
});
},
To this:
login: function(args, callback) {
/**
* username [String]
* password [String]
*/
var self = this;
if (!args.username || !args.password) {
return callback(Error.genObj(Error.code.MISSING_PARAMS));
}
User.findOne({
username: args.username
}, function(err, user) {
console.log('[Debug] In User.findOne(...) Callback;');
if (err) {
return callback(Error.genObj(Error.code.INTERNAL));
}
if (!user) {
return callback(Error.genObj(Error.code.TOKEN_AUTH_FAILED));
}
if (self._generateHash({ password: args.password, salt: user.salt }) != user.password) {
return callback(Error.genObj(Error.code.TOKEN_AUTH_FAILED));
}
self._verifyToken({
token: user.token,
username: args.username,
key: Config.tokenKey
}, function(err) {
if (err) {
var token = self._generateToken({ username: args.username, key: Config.key });
user.token = token;
user.save(function(err) {
if (err) {
return callback(Error.genObj(Error.code.INTERNAL));
}
return callback(null, user);
});
} else {
// ======== put this in an else clause ==========
return callback(null, user);
}
});
});
},
The issue is that if you get an error from verifyToken(), then it will start user.save(), but since that's async, it will continue on and execute return callback(null, user) before the user.save() operation is done and then it will call the callback again when user.save() is done.