index.js

const lodash = require('lodash');
const { getAllUniqueKeyValues, slugifyString } = require('../utils');
const site = require('../../src/_data/site');
const { dir } = require('../constants');

/** Returns all blog posts as a collection. */
const getAllPosts = (collection) => {
  const posts = collection.getFilteredByGlob(`${dir.input}/_posts/**/*.md`);
  return posts.reverse();
};

/** Given a category name and an optional page number, returns the root-relative URL to that category's page.
 * @param {string} category
 * @param {number} [page] The one-indexed page number
 */
const getCategoryHref = (category, page) => {
  const slug = slugifyString(category);
  return page > 1 ? `/tags/${slug}/page/${page}/` : `/tags/${slug}/`;
};

/** Returns all unique categories as a collection.
 * NOTE: I'm calling these "categories" to distinguish them from 11ty's built-in "tags." However,
 * these are still referred to as tags in the UI since that's what's most common.
 * @returns {({ title: string; href: string; count: string })[]}
 */
const getAllUniqueCategories = (collection) => {
  const allPosts = getAllPosts(collection);
  const uniqueCategories = getAllUniqueKeyValues(allPosts, 'categories');

  // Do this separately for performance rather than inside the category map logic below, which would be several levels of nesting loops
  const categoryCounts = allPosts.reduce((categoryCounts, post) => {
    post.data.categories?.forEach((category) => {
      if (!categoryCounts[category]) {
        categoryCounts[category] = 0;
      }
      categoryCounts[category]++;
    });
    return categoryCounts;
  }, {});

  return (
    uniqueCategories
      .map((category) => ({
        title: category,
        href: getCategoryHref(category),
        count: categoryCounts[category],
      }))
      // Popular categories first
      .sort((a, b) => b.count - a.count)
  );
};

// Blog posts by category, for pagination
// Adapted for use from: https://www.webstoemp.com/blog/basic-custom-taxonomies-with-eleventy/
const getPostsByCategory = (collection) => {
  const postsPerPage = site.pagination.itemsPerPage;
  const blogPostsByCategory = [];

  const allPosts = getAllPosts(collection);
  const allUniqueCategories = getAllUniqueKeyValues(allPosts, 'categories');

  allUniqueCategories.forEach((category) => {
    // Get all posts belonging to this category
    const categoryPosts = allPosts.filter((post) => post.data.categories?.includes(category));

    // Create a 2D array of chunked posts, where each nested array represents a page.
    // e.g., if postsPerPage = 2 and we have three posts, then we'd get [[{post1}, {post2}], [{post3}]]
    const chunkedCategoryPosts = lodash.chunk(categoryPosts, postsPerPage);

    // Map each chunk to its page slug
    const pageHrefs = chunkedCategoryPosts.map((_, pageIndex) => getCategoryHref(category, pageIndex + 1));

    chunkedCategoryPosts.forEach((posts, index) => {
      // Massage the data into a format that 11ty's pagination will like
      // https://github.com/11ty/eleventy/issues/332#issuecomment-445236776
      blogPostsByCategory.push({
        title: category,
        href: pageHrefs[index],
        pageNumber: index,
        totalPages: pageHrefs.length,
        hrefs: {
          all: pageHrefs,
          next: pageHrefs[index + 1] || null,
          previous: pageHrefs[index - 1] || null,
          first: pageHrefs[0] || null,
          last: pageHrefs[pageHrefs.length - 1] || null,
        },
        posts,
      });
    });
  });

  return blogPostsByCategory.sort((category1, category2) => category2.posts.length - category1.posts.length);
};

module.exports = {
  getAllPosts,
  getAllUniqueCategories,
  getPostsByCategory,
};