code: function verifyClient (req, res, next) {
    var params = req.connectParams
  
    // missing redirect uri
    if (!params.redirect_uri) {
      return next(new AuthorizationError({
        error: 'invalid_request',
        error_description: 'Missing redirect uri',
        statusCode: 400
      }))
    }
  
    // missing client id
    if (!params.client_id) {
      return next(new AuthorizationError({
        error: 'unauthorized_client',
        error_description: 'Missing client id',
        statusCode: 403
      }))
    }
  
    Client.get(params.client_id, {
      private: true
    }, function (err, client) {
      if (err) { return next(err) }
  
      // The client must be registered.
      if (!client) {
        return next(new AuthorizationError({
          error: 'unauthorized_client',
          error_description: 'Unknown client',
          statusCode: 401
        }))
      }
  
      // Make client available to downstream middleware.
      req.client = client
  
      // Redirect URI must be configured for this client.
      if (client.redirect_uris.indexOf(params.redirect_uri) === -1) {
        return next(new AuthorizationError({
          error: 'invalid_request',
          error_description: 'Mismatching redirect uri',
          statusCode: 400
        }))
      }
  
      next()
    })
  }
docs: Verify Client
  
  This route-specific middleware retrieves a registered client and adds it
  to the request object for use downstream. It verifies that the client is
  registered and that the redirect_uri parameter matches the configuration
  of the registered client.