code: function verifyEndpoint(request, respond)
  {
    var challenge;
  
    if (typeof request.query == 'string')
    {
      request.query = qs.parse(request.query);
    }
  
    if (typeof request.query == 'object'
      && ((request.query['hub.verify_token'] === this.credentials.secret)
          // for hapi@10 and restify/express with queryParser check for `hub` object
      || (typeof request.query.hub == 'object' && request.query.hub['verify_token'] === this.credentials.secret)
      )
    )
    {
      challenge = request.query['hub.challenge'] || (request.query.hub ? request.query.hub.challenge : null);
  
      this.logger.debug({message: 'Successfully verified', challenge: challenge});
      respond(challenge);
      return;
    }
  
    this.logger.error({message: 'Unable to verify endpoint', query: request.query});
    respond(400, 'Error, wrong validation token');
  }
docs: Verifies endpoint by replying with the provided challenge
  
  @this  Fbbot#
  @param {EventEmitter} request - incoming http request object
  @param {function} respond - http response function