Handling Facebook App Center auto-login request with express and passport-facebook (in CoffeeScript)

app.coffee

# ... Do your usual Facebook passport strategy setup here ...

# Handle GET on /
app.get "/", (req, res, next) ->
  if req.isAuthenticated()
    # ... Render logged in page ...
  else
    # Check for Facebook app center login
    if req.query.code? and req.query.fb_source?
      # The callback URL *must* be the same as the request URL minus the "code" variable
      callbackQuery = {}
      for key, value of req.query
        if key != 'code'
          callbackQuery[key] = value
      
      doAuth = passport.authenticate('facebook', { callbackURL: 'http://example.com/?' + querystring.stringify(callbackQuery) } )
      doAuth req, res, -> res.redirect '/'
      return
    
    # Not a Facebook login request
    # ... Render logged out page ...