Source: modules/tracker/js/tracker-history.es6.js

/**
 * Attaches behaviors for the Tracker module's History module integration.
 *
 * May only be loaded for authenticated users, with the History module enabled.
 */
(function ($, Drupal, window) {
  function processNodeNewIndicators($placeholders) {
    const newNodeString = Drupal.t('new');
    const updatedNodeString = Drupal.t('updated');

    $placeholders.each((index, placeholder) => {
      const timestamp = parseInt(
        placeholder.getAttribute('data-history-node-timestamp'),
        10,
      );
      const nodeID = placeholder.getAttribute('data-history-node-id');
      const lastViewTimestamp = Drupal.history.getLastRead(nodeID);

      if (timestamp > lastViewTimestamp) {
        const message =
          lastViewTimestamp === 0 ? newNodeString : updatedNodeString;
        $(placeholder).append(`<span class="marker">${message}</span>`);
      }
    });
  }

  function processNewRepliesIndicators($placeholders) {
    // Figure out which placeholders need the "x new" replies links.
    const placeholdersToUpdate = {};
    $placeholders.each((index, placeholder) => {
      const timestamp = parseInt(
        placeholder.getAttribute('data-history-node-last-comment-timestamp'),
        10,
      );
      const nodeID = placeholder.previousSibling.previousSibling.getAttribute(
        'data-history-node-id',
      );
      const lastViewTimestamp = Drupal.history.getLastRead(nodeID);

      // Queue this placeholder's "X new" replies link to be downloaded from the
      // server.
      if (timestamp > lastViewTimestamp) {
        placeholdersToUpdate[nodeID] = placeholder;
      }
    });

    // Perform an AJAX request to retrieve node view timestamps.
    const nodeIDs = Object.keys(placeholdersToUpdate);
    if (nodeIDs.length === 0) {
      return;
    }
    $.ajax({
      url: Drupal.url('comments/render_new_comments_node_links'),
      type: 'POST',
      data: { 'node_ids[]': nodeIDs },
      dataType: 'json',
      success(results) {
        Object.keys(results || {}).forEach((nodeID) => {
          if (placeholdersToUpdate.hasOwnProperty(nodeID)) {
            const url = results[nodeID].first_new_comment_link;
            const text = Drupal.formatPlural(
              results[nodeID].new_comment_count,
              '1 new',
              '@count new',
            );
            $(placeholdersToUpdate[nodeID]).append(
              `<br /><a href="${url}">${text}</a>`,
            );
          }
        });
      },
    });
  }

  /**
   * Render "new" and "updated" node indicators, as well as "X new" replies links.
   */
  Drupal.behaviors.trackerHistory = {
    attach(context) {
      // Find all "new" comment indicator placeholders newer than 30 days ago that
      // have not already been read after their last comment timestamp.
      const nodeIDs = [];
      const $nodeNewPlaceholders = $(context)
        .find('[data-history-node-timestamp]')
        .once('history')
        .filter(function () {
          const nodeTimestamp = parseInt(
            this.getAttribute('data-history-node-timestamp'),
            10,
          );
          const nodeID = this.getAttribute('data-history-node-id');
          if (Drupal.history.needsServerCheck(nodeID, nodeTimestamp)) {
            nodeIDs.push(nodeID);
            return true;
          }

          return false;
        });

      // Find all "new" comment indicator placeholders newer than 30 days ago that
      // have not already been read after their last comment timestamp.
      const $newRepliesPlaceholders = $(context)
        .find('[data-history-node-last-comment-timestamp]')
        .once('history')
        .filter(function () {
          const lastCommentTimestamp = parseInt(
            this.getAttribute('data-history-node-last-comment-timestamp'),
            10,
          );
          const nodeTimestamp = parseInt(
            this.previousSibling.previousSibling.getAttribute(
              'data-history-node-timestamp',
            ),
            10,
          );
          // Discard placeholders that have zero comments.
          if (lastCommentTimestamp === nodeTimestamp) {
            return false;
          }
          const nodeID = this.previousSibling.previousSibling.getAttribute(
            'data-history-node-id',
          );
          if (Drupal.history.needsServerCheck(nodeID, lastCommentTimestamp)) {
            if (nodeIDs.indexOf(nodeID) === -1) {
              nodeIDs.push(nodeID);
            }
            return true;
          }

          return false;
        });

      if (
        $nodeNewPlaceholders.length === 0 &&
        $newRepliesPlaceholders.length === 0
      ) {
        return;
      }

      // Fetch the node read timestamps from the server.
      Drupal.history.fetchTimestamps(nodeIDs, () => {
        processNodeNewIndicators($nodeNewPlaceholders);
        processNewRepliesIndicators($newRepliesPlaceholders);
      });
    },
  };
})(jQuery, Drupal, window);