code: function(syncOptions, serverResponses) {
      if (syncOptions.pixelEnabled) {
        const syncs = [];
        serverResponses.forEach(response => {
          response.body.bid.forEach(bidObject => {
            if (utils.isArray(bidObject.sync)) {
              bidObject.sync.forEach(syncElement => {
                if (syncs.indexOf(syncElement) === -1) {
                  syncs.push(syncElement);
                }
              });
            }
          });
        });
        return syncs.map(sync => ({ type: 'image', url: sync }));
      }
      return [];
    }
docs: Register the user sync pixels which should be dropped after the auction.
  
  @param {SyncOptions} syncOptions Which user syncs are allowed?
  @param {ServerResponse[]} serverResponses List of server's responses.
  @return {UserSync[]} The user syncs which should be dropped.