Collection Data Structures

 •   •  eleventy

Eleventy uses tags to group pages into collections. Pages that share a tag are in the same collection. For example, a template with the following front matter would generate a page the belongs to the transportation and fantasy collections.

title: Flying Machines
- transportation
- fantasy
. . .

Collections are accessed by name as properties of the global collections object:

{{ collections.posts }}

Eleventy treats the tags page property special. For every tag name, there’s a corresponding array of all the templates with that tag.[1]

When you want to create a new collection that’s not a tag, you can use addCollection() in your .eleventy.js configuration file.

addCollection() takes two arguments:

  eleventyConfig.addCollection("articles", function(collection) {
return collection.getAllSorted().filter(function(item) {
return item.url && item.inputPath.startsWith('./src/articles/');

So, what’s in this collection parameter?

Property Type Description
items [item] All of the templates that Eleventy processed. Is this always the same as the number of pages?
sortAscending Boolean ? Whether the items are sorted in ascending order?
sortNumeric Boolean ? Whether the items are sorted in numeric order?
sortFunctionStringMap object Something to do with the sorting stuff. We’re not going to talk about it here.
Collection properties

We’re interested in the items array. This table shows what an item looks like.[2]

Property Type Description
template object Content and metadata about this item’s template. In other words, its page.
inputPath string A path relative to the source directory. Though really it’s more like ./src/ where src is the source directory.
fileSlug string The base name of the input file. The final element in the path. Never index. If the file is, the slug is an empty string.
data object Whoa
date string An the file’s date in ISO 8601 format. There’s some trickery about dates.
_pages [stuff] Something to do with Elventy’s caching that I’m not even going to think about.
url string The URL of this page. Actually just the path without the scheme, host, port info etc.
outputPath string Where the processed file ended up. Relative to where .eleventy.js is
item properties

Let’s put links in these tables.

Eff that. Use the VS Code debugger

Because what we really want to do is: to create a page-level property categories that works in a way similar to tags.

How do the tagged items end up in the collections? Haven’t we done this already? Yes we have.

The place where that happens looks like this:[3]

async getUserConfigCollectionsData() {
let collections = {};
let configCollections =
this.configCollections || eleventyConfig.getCollections();
for (let name in configCollections) {
collections[name] = this.createTemplateMapCopy(
`Collection: collections.${name} size: ${collections[name].length}`
return collections;

This is a typical way of creating a collection.

eleventyConfig.addCollection("articles", collection =>
.filter(item => item.url &&

We’re creating a collection made up of pages that were rendered from templates in the ./src/articles/ directory.[4]

And this collection is made up of pages that were rendered from templates that have a fun-examples tag.

  eleventyConfig.addCollection("fun-examples", collection =>

Let’s look at how getFilteredByTag() is implemented:[5]

getFilteredByTag(tagName) {
return this.getAllSorted()
.filter(item => => tag === tagName))

This is how the the individual collections get into to the collections property:

async getTaggedCollectionsData() {
let collections = {};
collections.all = this.createTemplateMapCopy(
debug(`Collection: collections.all size: ${collections.all.length}`);

let tags = this.getAllTags();
for (let tag of tags) {
collections[tag] = this.createTemplateMapCopy(
debug(`Collection: collections.${tag} size: ${collections[tag].length}`);
return collections;

Where does getTaggedCollectionsData get called? In TemplateMap.cache(). Hmmm. That’s not terribly useful. Is that where the magic happens?

I’m getting ahead of myself.

What is a category?

It’s like a newspaper section. The idea is to build silos for things that shouldn’t go together. Or that don’t necessarily go together.

What are the categories then?

    • movies
    • books
    • writing
    • poetry
  • TECH
    • code
    • gadgets
    • tools
    • work
  • LIFE
    • parenting
    • relationships
    • divorce
    • religion
date: 10/30/2018
title: Loomings
category: Tech
- tools
- git
- eleventy

What’s the default category?

Maybe a special category name that never gets rendered. It doesn’t really matter.

OK, so what we’re going to do is to make our getCatList function do several things:

Have we talked about how we’re actually going going to use these categories? Something like this:

{{ collections.categories.Culture }}

Now what?

Let’s see if this works. Let’s try to make category pages analogous to the tag pages.

Where to start? The tag list in tags-list.njk looks like this:

permalink: /tags/
layout: page.njk

{% for tag in collections.tagList | sort %}
{% set tagUrl %}/tags/{{ tag }}/{% endset %}
<a href="{{ tagUrl | url }}" class="tag">{{ tag }}</a>
{% endfor %}

Let’s make a copy of it called category-list.njk. And copy tags.njk to categories.njk.

OK, this is weird.

This is how I create the categories. There’s a new categories property in data.collections. Then for each category, the idea is to create an array element / object property. I want to be able to write collections.categories.Culture.

  sortedCollection.forEach(function(item) {
if (! ("categories" in {
// no categories collection? make one = {}

if (typeof === "string") {
if (!Array.isArray([])) {[] = []

So I’ll get something like this:

collections: {
all: [ items ],
categories: {
Culture: [ items ],
Life: [ items ],
Thinking: [ items ]

I was getting an error:

TypeError: collections[collectionName] is not iterable

This turned out to be a bug, where Eleventy was expecting that every property was some sort of array. This gets fixed in version 0.6.0..

  1. I should probably write this in English at some point. ↩︎

  2. I’m using the notation [item] to indicate that the property called items is an array of item objects. There really isn’t an item object. In other words, you’ll never see JSON like this:

    "item" : { ... }
  3. getUserConfigCollectionsData is actually implemented like this:

    async getUserConfigCollectionsData() {
    let collections = {};
    let configCollections =
    this.configCollections || eleventyConfig.getCollections();
    for (let name in configCollections) {
    let ret = configCollections[name](this.collection);

    // work with arrays and strings returned from UserConfig.addCollection
    if (
    Array.isArray(ret) &&
    ret.length &&
    ret[0].inputPath &&
    ) {
    collections[name] = this.createTemplateMapCopy(ret);
    } else {
    collections[name] = ret;

    `Collection: collections.${name} size: ${collections[name].length}`
    return collections;
  4. The current directory . is the directory that you're executing eleventy in. It contains .eleventy.js. ↩︎

  5. getFilteredByTag is actually implemented like this:

    getFilteredByTag(tagName) {
    return this.getAllSorted().filter(function(item) {
    let match = false;
    if (!tagName) {
    return true;
    } else if (Array.isArray( { => {
    if (tag === tagName) {
    match = true;
    // This branch should no longer be necessary per TemplateContent.cleanupFrontMatterData
    } else if (typeof === "string") {
    match = === tagName;
    return match;
  6. I am embarrassed about this side-effect thing. ↩︎

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