code: function encodeArticle(pages, article) {
      var articlePath = article.getPath();
  
      return {
          path: articlePath,
          title: article.getTitle(),
          level: article.getLevel(),
          exists: (articlePath && pages.has(articlePath)),
          external: article.isExternal()
      };
  }
docs: Encode an article for next/prev
  
  @param {Map<String:Page>}
  @param {Article}
  @return {Object}