Custom collections in Eleventy based on your posts’ front matter

eleventy.config.js

module.exports = function(eleventyConfig) {

	// Make a custom collection based on a key in your posts' front matter
	function addCollectionByKey(collectionName, postKey, paginate=true, postsPerPage=100) {

		eleventyConfig.addCollection(collectionName, function(collectionApi) {

			const collectionObjects = {};
			let collectionFinal = [];

			// Filter the posts that have the chosen key in their metadata
			let collectionPosts = collectionApi.getAll().filter(function(post) {
				return postKey in post.data;
			});

			// Sort the posts in descending order by date
			// (Comment this part out if date is unavailable or irrelevant)
			collectionPosts = collectionPosts.sort(function(a, b) {
				return b.date - a.date;
			});

			// Loop through the posts
			collectionPosts.forEach(function(post) {

				let postValue = post.data[postKey]; // The chosen key's value in the current post

				// If the chosen key is `date`, the value needs some manipulation
				if (postKey === "date") {
					const postDate = new Date(post.data.date);
					// postValue = "YYYY/MM";
					postValue = postDate.getFullYear().toString() + "/" + (postDate.getMonth() + 1).toString().padStart(2, "0");
				}

				// This function finds every unique value (tag) associated with the chosen key
				function collectionizeTag(tag) {

					// If the tag isn't represented in collectionObjects,
					// make it a key in collectionObjects with an empty array value
					if (!collectionObjects.hasOwnProperty(tag)) {
						collectionObjects[tag] = [];
					}

					// Add the current post to the array associated with this tag
					collectionObjects[tag].push(post);
				}

				// If the chosen key has multiple tags
				if (typeof postValue === "object") {

					// Loop through the tags and run collectionizeTag on each tag
					for (let i = 0; i < postValue.length; i++) {
						collectionizeTag(post.data[postKey][i]);
					}

				// Otherwise run collectionizeTag on the key's single tag
				} else {
					collectionizeTag(postValue);
				}

			});
			
			/*
			collectionObjects now looks something like this:
			{
				"tag1": [ {post2}, {post1} ],
				"tag2": [ {post2}, {post1} ]
			}
			*/

			// If you want each tag archive to be paginated, the contents of
			// collectionObjects need to be reformatted
			if (paginate) {

				// Loop through the tags
				for (const tag in collectionObjects) {
					let page = 0;

					// Loop through the posts associated with the tag
					for (let i = 0; i < collectionObjects[tag].length; i++) {

						// Determine how many total pages there will be based on
						// the number of posts and postsPerPage
						let pageTotal = Math.ceil(collectionObjects[tag].length / postsPerPage);

						// If this is the first post, or the current page is at capacity, add a new page
						if (page === 0 || collectionFinal[collectionFinal.length - 1]["pageData"].length === postsPerPage) {
							page++;
							collectionFinal.push({"name": tag, "pageNumber": page, "pageTotal": pageTotal, "pageData": []});
						}

						// Add this post to the latest page
						collectionFinal[collectionFinal.length - 1]["pageData"].push(collectionObjects[tag][i]);
					}

					// Reset the page count for each tag
					page = 0;
				}

				/*
				collectionFinal now looks something like this:
				[
					{ "name": "tag1", "pageNumber": 1, "pageTotal": 2, "pageData": [ {post4}, {post3} ] },
					{ "name": "tag1", "pageNumber": 2, "pageTotal": 2, "pageData": [ {post2}, {post1} ] },
					{ "name": "tag2", "pageNumber": 1, "pageTotal": 2, "pageData": [ {post4}, {post3} ] },
					{ "name": "tag2", "pageNumber": 2, "pageTotal": 2, "pageData": [ {post2}, {post1} ] }
				]
				*/

			// If each tag archive doesn't need to be paginated,
			// no reformatting is necessary
			} else {
				collectionFinal = collectionObjects;
			}
			return collectionFinal;
		});
	};

	addCollectionByKey("postsByMonth", "date");
	addCollectionByKey("postsByTopic", "topic", false);

paginatedArchives.njk

---
pagination:
  data: collections.postsByMonth
  size: 1
  alias: month
permalink: "/blog/{{ month.name }}/{% if month.pageNumber %}{{ month.pageNumber }}/{% endif %}"
layout: layouts/base.njk
---
<h1>Month: “{{ month.name }}” {% if month.pageNumber %}({{ month.pageNumber }} of {{ month.pageTotal }}){% endif %}</h1>

{% set monthArchives = month.pageData %}
<ul>
{% for post in monthArchives %}
  <li>{{ post.fileSlug }}</li>
{% endfor %}
</ul>

unpaginatedArchives.njk

---
pagination:
  data: collections.postsByTopic
  size: 1
  alias: topic
permalink: "/blog/topic/{{ topic | slugify }}/ %}"
layout: layouts/base.njk
---
<h1>Topic: “{{ topic }}”</h1>

{% set topicArchives = collections[postsByTopic][topic] %}
<ul>
{% for post in topicArchives %}
  <li>{{ post.data.title }}</li>
{% endfor %}
</ul>