Router redirecting with error message

Question

I have a question on best practices for redirecting to a page and supplying an error message. For example, a person is registering as a new user and the username they have chosen is already in use. I want to redirect them back to the register page (basically a form), but display an error message. Should I set a cookie in the routing code and have javascript display the error or is there a best practice for this?

router.post('/register', function(req, res) {

  var username = req.body.username;
  db.fetchUser(username, function(err, user){
    if (!err && !user) {
      db.addUser(req.body.username, req.body.password, "vendor", function(err) {
        if (!err) {
          res.redirect('/portal');
        }
        else {
          // TODO: Set error.
          res.redirect('/register');
        }
      }
    }
    else {
      // return error already exists.
      res.redirect('/register');
    }
  });
});

Answer 1

I wound up using express-session, which seems a clean way to do it.

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var session = require('express-session');
var routes = require('./routes/index');
var portal = require('./routes/portal');

var FileStore = require('session-file-store')(session);
var app = express();

app.use(session({
    secret: "flibbernachosqueezance",
    store: new FileStore,
    saveUninitialized: true,
    resave: true
}));

And then using things like this for the session variables:

      req.session.userMessage = "This username already exists: "+username;