code: function encodeNavigation(output) {
      var book = output.getBook();
      var pages = output.getPages();
      var summary = book.getSummary();
      var articles = summary.getArticlesAsList();
  
  
      var navigation = articles
          .map(function(article, i) {
              var ref = article.getRef();
              if (!ref) {
                  return undefined;
              }
  
              var prev = articles.get(i - 1);
              var next = articles.get(i + 1);
  
              return [
                  ref,
                  {
                      index: i,
                      title: article.getTitle(),
                      introduction: (i === 0),
                      prev: prev? encodeArticle(pages, prev) : undefined,
                      next: next? encodeArticle(pages, next) : undefined,
                      level: article.getLevel()
                  }
              ];
          })
          .filter(function(e) {
              return Boolean(e);
          });
  
      return Immutable.Map(navigation).toJS();
  }
docs: this.navigation is a deprecated property from GitBook v2
  
  @param {Output}
  @return {Object}