Sending back a JSON response when failing Passport.js authentication

Question

I'm using Node.js as a backend API server for an iPhone client. I'm using Passport.js to authenticate with a local strategy. The relevant code is below:

// This is in user.js, my user model
UserSchema.static('authenticate', function(username, password, callback) {
    this.findOne({ username: username }, function(err, user) {
        if (err){
            console.log('findOne error occurred');
            return callback(err);
        }
        if (!user){
            return callback(null, false);
        }
        user.verifyPassword(password, function(err, passwordCorrect){
            if (err){
                console.log('verifyPassword error occurred');
                return callback(err);
            }
            if (!passwordCorrect){
                console.log('Wrong password');
                return callback(err, false);
            }
            console.log('User Found, returning user');
            return callback(null, user);
        });
    });
});

and

// This is in app.js
app.get('/loginfail', function(req, res){
    res.json(403, {message: 'Invalid username/password'});
});

app.post('/login',
    passport.authenticate('local', { failureRedirect: '/loginfail', failureFlash: false }),
    function(req, res) {
       res.redirect('/');
});

Right now, I have managed to redirect a failed login to /loginfail, where I send back some JSON to the iPhone client. However, this doesnt have enough granularity. I want to be able to send back the appropriate errors to the iPhone client, such as: "No user found" or "Password is wrong". With my existing code, I don't see how this can be accomplished.

I tried to follow the examples for a custom callback on the passport.js site, but I just cant get it to work due to lack of node understanding. How could I modify my code so that I'd be able to send back a res.json with an appropriate error code/message?

EDIT: I am trying something like this now:

// In app.js
app.post('/login', function(req, res, next) {
    passport.authenticate('local', function(err, user, info) {
        if (err) { return next(err) }
        if (!user) {
            console.log(info);
            // *** Display message without using flash option
            // re-render the login form with a message
            return res.redirect('/login');
        }
        console.log('got user');
        return res.json(200, {user_id: user._id});
    })(req, res, next);
});

// In user.js
UserSchema.static('authenticate', function(username, password, callback) {
    this.findOne({ username: username }, function(err, user) {
        if (err){
            console.log('findOne error occurred');
            return callback(err);
        }
        if (!user){
            return callback(null, false);
        }
        user.verifyPassword(password, function(err, passwordCorrect){
            if (err){
                return callback(err);
            }
            if (!passwordCorrect){
                return callback(err, false, {message: 'bad password'});
            }
            console.log('User Found, returning user');
            return callback(null, user);
        });
    });
});

But back when I try to console.log(info), it just says undefined. I don't know how to get this custom callback working...Any help would be appreciated!

Answer 1

I had a similar issue with Passport and failed login responses. I was building an API, and wanted all responses to be returned as JSON. Passport responds to an invalid password with status: 401 and body: Unauthorized. That's just a text string in the body, not JSON, so it broke my client which expected all JSON.

As it turns out, there is a way to make Passport just return the error to the framework instead of trying to send a response itself.

The answer is to set failWithError in the options passed to authenticate: https://github.com/jaredhanson/passport/issues/126#issuecomment-32333163

From jaredhanson's comment in the issue:

app.post('/login',
  passport.authenticate('local', { failWithError: true }),
  function(req, res, next) {
    // handle success
    if (req.xhr) { return res.json({ id: req.user.id }); }
    return res.redirect('/');
  },
  function(err, req, res, next) {
    // handle error
    if (req.xhr) { return res.json(err); }
    return res.redirect('/login');
  }
);

This will invoke the error handler after Passport calls next(err). For my app, I wrote a generic error handler specific to my use case of just providing a JSON error:

// Middleware error handler for json response
function handleError(err,req,res,next){
    var output = {
        error: {
            name: err.name,
            message: err.message,
            text: err.toString()
        }
    };
    var statusCode = err.status || 500;
    res.status(statusCode).json(output);
}

Then I used it for all api routes:

var api = express.Router();
...
//set up some routes here, attached to api
...
// error handling middleware last
api.use( [
        handleError
        ] );

I didn't find the failWithError option in the documentation. I stumbled upon it while tracing through the code in the debugger.

Also, before I figured this out, I tried the "custom callback" mentioned in the @Kevin_Dente answer, but it didn't work for me. I'm not sure if that was for an older version of Passport or if I was just doing it wrong.