Support categories and tags migrated from WordPress in Eleventy. See https://micheleong.com/2023/09/23/migrate-wordpress-eleventy-colocated-images

categories.js

// File: lib/collections/categories.js

const slugify = require("slugify");

const getAllKeyValues = require("../utils/getAllKeyValues");

module.exports = (collection) => {
  let allCategories = getAllKeyValues(
    collection.getFilteredByGlob("src/posts/**/*.md"),
    "categories"
  );

  return allCategories.map((category) => ({
    title: category,
    slug: slugify(category),
  }));
};

categories.njk

---
layout: default
permalink: /category/index.html
---
<h1 class="text-3xl text-center font-bold leading-normal mt-0 mb-3">Categories</h1>

<div class="mt-3 text-center">
    {% for category in collections.categories %}
    <a href="{{ '/category/' | url }}{{ category.slug }}" class="inline-block bg-slate-200 rounded-full px-3 py-1 text-sm font-medium m-0.5">{{ category.title }}</a>
    {% endfor %}
</div>

category.njk

---
layout: default
eleventyExcludeFromCollections: true
pagination:
  data: collections.pagedPostsByCategory
  size: 1
  alias: paged
permalink: "/category/{{ paged.category.slug }}/{% if paged.number > 1 %}{{ paged.number }}/{% endif %}index.html"
---
<h1 class="text-3xl text-center font-bold leading-normal mt-0 mb-3">Category: {{ paged.category.title }}</h1>

<div class="flex flex-wrap -mx-2">
    {% include "partials/post-grid.njk" %}
</div>

{% if collections.pagedPostsByCategory[paged.category.slug].length > site.paginate %}
{% include "partials/paginator.njk" %}
{% endif %}

eleventy.config.images.js

module.exports = eleventyConfig => {
	...
	// Add custom categories and hashtag collections
	config.addCollection('categories', require('./lib/collections/categories'));
	config.addCollection('pagedPostsByCategory', require('./lib/collections/pagedPostsByCategory'));
	config.addCollection("hashtags", require('./lib/collections/hashtags'));
	config.addCollection('pagedPostsByHashtag', require('./lib/collections/pagedPostsByHashtag'));
	...
};

getAllKeyValues.js

// File: lib/utils/getAllKeyValues.js
// Source: https://www.webstoemp.com/blog/basic-custom-taxonomies-with-eleventy/

const lodash = require("lodash");

/**
 * Get all unique key values from a collection
 *
 * @param {Array} collectionArray - collection to loop through
 * @param {String} key - key to get values from
 */
module.exports = function getAllKeyValues(collectionArray, key) {
	// get all values from collection
	let allValues = collectionArray.map((item) => {
		return item.data[key] ? item.data[key] : [];
	});

	// flatten values array
	allValues = lodash.flattenDeep(allValues);
	// to lowercase
	allValues = allValues.map((item) => item.toLowerCase());
	// remove duplicates
	allValues = [...new Set(allValues)];
	// order alphabetically
	allValues = allValues.sort(function (a, b) {
		return a.localeCompare(b, "en", { sensitivity: "base" });
	});
	// return
	return allValues;
};

hashtags.js

// File: lib/collections/hashtags.js

const slugify = require("slugify");

const getAllKeyValues = require("../utils/getAllKeyValues");

module.exports = (collection) => {
  let allHashtags = getAllKeyValues(
    collection.getFilteredByGlob("src/posts/**/*.md"),
    "hashtags"
  );

  return allHashtags.map((hashtag) => ({
    title: hashtag,
    slug: slugify(hashtag),
  }));
};

include.js

// File: lib/filters/include.js

const lodash = require("lodash");

/**
 * Select all objects in an array
 * where the path includes the value to match
 * capitalisation and diacritics are removed from compared values
 *
 * @param {Array} arr - array of objects to inspect
 * @param {String} path - path to inspect for each object
 * @param {String} value - value to match
 * @return {Array} - new array
 */

module.exports = function (arr, path, value) {
  value = lodash.deburr(value).toLowerCase();
  return arr.filter((item) => {
    let pathValue = lodash.get(item, path);
    pathValue = Array.isArray(pathValue) ? pathValue : [pathValue];
    pathValue = pathValue.map((v) => lodash.deburr(v).toLowerCase());
    return pathValue.includes(value);
  });
};

pagedPostsByCategory.js

// File: lib/collections/pagedPostsByCategory.js

const siteData = require('../../src/_data/site');
const includes = require('../filters/include');

module.exports = (coll) => {
  const allPosts = require('./posts')(coll);
  const categoryList = require('./categories')(coll);

  const maxPostsPerPage = siteData.paginate;
  const pagedPosts = [];

  categoryList.forEach((category) => {
    const taggedPosts = includes(allPosts, "data.categories", category.title).reverse();
    const numberOfPages = Math.ceil(taggedPosts.length / maxPostsPerPage);

    for (let pageNum = 1; pageNum <= numberOfPages; pageNum++) {
      const sliceFrom = (pageNum - 1) * maxPostsPerPage;
      const sliceTo = sliceFrom + maxPostsPerPage;

      pagedPosts.push({
        category,
        number: pageNum,
        posts: taggedPosts.slice(sliceFrom, sliceTo),
        first: pageNum === 1,
        last: pageNum === numberOfPages
      });
    }
  });

  return pagedPosts;
};

pagedPostsByHashtag.js

// File: lib/collections/pagedPostsByHashtag.js

const siteData = require('../../src/_data/site');
const includes = require('../filters/include');

module.exports = (coll) => {
  const allPosts = require('./posts')(coll);
  const hashtagList = require('./hashtags')(coll);

  const maxPostsPerPage = siteData.paginate;
  const pagedPosts = [];

  hashtagList.forEach((hashtag) => {
    const taggedPosts = includes(allPosts, "data.hashtags", hashtag.title).reverse();
    const numberOfPages = Math.ceil(taggedPosts.length / maxPostsPerPage);

    for (let pageNum = 1; pageNum <= numberOfPages; pageNum++) {
      const sliceFrom = (pageNum - 1) * maxPostsPerPage;
      const sliceTo = sliceFrom + maxPostsPerPage;

      pagedPosts.push({
        hashtag,
        number: pageNum,
        posts: taggedPosts.slice(sliceFrom, sliceTo),
        first: pageNum === 1,
        last: pageNum === numberOfPages
      });
    }
  });

  return pagedPosts;
};

tag.njk

---
layout: default
eleventyExcludeFromCollections: true
pagination:
  data: collections.pagedPostsByHashtag
  size: 1
  alias: paged
permalink: "/tag/{{ paged.hashtag.slug }}/{% if paged.number > 1 %}{{ paged.number }}/{% endif %}index.html"
---
<h1 class="text-3xl text-center font-bold leading-normal mt-0 mb-3">Tagged with: {{ paged.hashtag.title }}</h1>

<div class="flex flex-wrap -mx-2">
    {% include "partials/post-grid.njk" %}
</div>

{% if collections.pagedPostsByHashtag[paged.hashtag.slug].length > site.paginate %}
{% include "partials/paginator.njk" %}
{% endif %}

tags.njk

---
layout: default
permalink: /tag/index.html
---
<h1 class="text-3xl text-center font-bold leading-normal mt-0 mb-3">Tags</h1>

<div class="mt-3 text-center">
    {% for tag in collections.hashtags %}
    <a href="{{ '/tag/' | url }}{{ tag.slug }}" class="inline-block bg-slate-200 rounded-full px-3 py-1 text-sm font-medium m-0.5">#{{ tag.title }}</a>
    {% endfor %}
</div>
<ul>