Express Passport (node.js) error handling

Express Passport (node.js) error handling

Problem

I’ve looked at how error handling should work in node via this stack exchange, but I’m not sure what passport’s doing when it fails authentication. I have the following LocalStrategy:

passport.use(new LocalStrategy({ usernameField: 'email', passwordField: 'password' },
  function(email, password, next) {

    User.find({email: UemOrUnm}, function(err, user){
      if (err) { console.log('Error > some err'); return next(err); }
      if (!user) { console.log('Error > no user'); return next('Incorrect login or password'); } 

      if (password != user.password) {
        return next(Incorrect login or password);
      }
      return next(null, user);
    });
  }
));

After I see ‘Error > some err’ console printout, nothing else happens. I would think it should continue on the the next path with an error parameter, but it doesn’t seem to do that. What’s going on?

Problem courtesy of: Omid Ahourai

Solution

The strategy-implementation works in conjunction with passport.authenticate to both authenticate a request, and handle success/failure.

Say you’re using this route (which is passed an e-mail address and a password):

app.post('/login', passport.authenticate('local', {
  successRedirect: '/loggedin',
  failureRedirect: '/login', // see text
  failureFlash: true // optional, see text as well
});

This will call the code in the strategy, where one of three conditions can happen:

  1. An internal error occurred trying to fetch the users’ information (say the database connection is gone); this error would be passed on: next(err); this will be handled by Express and generate an HTTP 500 response;
  2. The provided credentials are invalid (there is no user with the supplied e-mail address, or the password is a mismatch); in that case, you don’t generate an error, but you pass a false as the user object: next(null, false); this will trigger the failureRedirect (if you don’t define one, a HTTP 401 Unauthorized response will be generated);
  3. Everything checks out, you have a valid user object, so you pass it along: next(null, user); this will trigger the successRedirect;

In case of an invalid authentication (but not an internal error), you can pass an extra message along with the callback:

next(null, false, { message : 'invalid e-mail address or password' });

If you have used failureFlash and installed the connect-flash middleware, the supplied message is stored in the session and can be accessed easily to, for example, be used in a template.

EDIT: it’s also possible to completely handle the result of the authentication process yourself (instead of Passport sending a redirect or 401):

app.post('/login', function(req, res, next) {
  passport.authenticate('local', function(err, user, info) {
    if (err) {
      return next(err); // will generate a 500 error
    }
    // Generate a JSON response reflecting authentication status
    if (! user) {
      return res.send({ success : false, message : 'authentication failed' });
    }
    // ***********************************************************************
    // "Note that when using a custom callback, it becomes the application's
    // responsibility to establish a session (by calling req.login()) and send
    // a response."
    // Source: http://passportjs.org/docs
    // ***********************************************************************
    req.login(user, loginErr => {
      if (loginErr) {
        return next(loginErr);
      }
      return res.send({ success : true, message : 'authentication succeeded' });
    });      
  })(req, res, next);
});
Solution courtesy of: robertklep

Discussion

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