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Last active Nov 8, 2021
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Memoize an eleventy collection.
const {memoize} = require('find-by-slug');
// Turn collection.all into a lookup table so we can use findBySlug
// to quickly find collection items without looping.
config.addCollection('memoized', function(collection) {
return memoize(collection.getAll());
const chalk = require('chalk');
const warn =;
let memo;
* Memoize an eleventy collection into a hash for faster lookups.
* Important: Memoization assumes that all post slugs are unique.
* @param {Array<Object>} collection An eleventy collection.
* Typically collections.all
* @return {Array<Object>} The original collection. We return this to make
* eleventy.addCollection happy since it expects a collection of some kind.
const memoize = (collection) => {
if (memo && Object.keys(memo).length) {
/* eslint-disable-next-line */
console.warn(warn(`Overwriting existing memoized collection!`));
memo = {};
collection.forEach((item) => {
if (memo[item.fileSlug]) {
throw new Error(`Found duplicate post slug: '${item.fileSlug}'`);
memo[item.fileSlug] = item;
// Just return the collection back to eleventy.
return collection;
* Look up a post by its slug.
* Requires that the collection the post lives in has already been memoized.
* @param {string} slug The post slug to look up.
* @return {Object} An eleventy collection item.
const findBySlug = (slug) => {
if (!slug) {
throw new Error(`slug is either null or undefined`);
if (!memo) {
throw new Error(`No collection has been memoized yet.`);
const found = memo[slug];
if (!found) {
throw new Error(`Could not find post with slug: ${slug}`);
return found;
module.exports = {memoize, findBySlug};
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