wonderfulfrog.com/config/collections/index.js

import dayjs from "dayjs";

export const postsByTag = (collection) => {
  const posts = collection.getFilteredByTag("post");

  const postsByTag = {};

  for (const post of posts) {
    for (const tag of post.data.tags) {
      postsByTag[tag] ??= [];
      postsByTag[tag].push(post);
    }
  }

  return postsByTag;
};

export const collectionByTag = (collection, collectionName) => {
  const items = collection.getFilteredByTag(collectionName);

  const itemsByTag = {};

  for (const item of items) {
    for (const tag of item.data.tags) {
      itemsByTag[tag] ??= [];
      itemsByTag[tag].push(item);
    }
  }

  return itemsByTag;
};

export const catalogueByType = (collection) => {
  const allItems = collection.getFilteredByTag("catalogue");

  const catalogueByType = {};

  for (const item of allItems) {
    const type = item.data.tags[1];
    if (!type) continue;

    catalogueByType[type] ??= [];
    catalogueByType[type].push(item);
  }

  return catalogueByType;
};