Sort post by categories with 11ty data files

cases.js

const cases = [
  {
    id: "123456",
    title: "This is a case red blue",
    categories: [
      { id: "1", slug: "red" },
      { id: "2", slug: "blue" }
    ]
  },
  {
    id: "23456",
    title: "This is another case red blue green",
    categories: [
      { id: "1", slug: "red" },
      { id: "2", slug: "blue" }
    ]
  },
  {
    id: "65478",
    title: "This is a case blue",
    categories: [{ id: "2", slug: "blue" }]
  },
  {
    id: "65478",
    title: "This is a case red green",
    categories: [
      { id: "1", slug: "red" },
      { id: "3", slug: "green" }
    ]
  }
];

function getUniqueCategories(cases) {
  let categories = [];

  cases.forEach(item => {
    categories.push(...item.categories);
  });

  let uniqueCategories = Array.from(
    new Set(categories.map(JSON.stringify))
  ).map(JSON.parse);

  return uniqueCategories;
}

function sortCases(cases, categories) {
  let sortedCases = [];

  categories.forEach(category => {
    let filteredCases = cases.filter(item => {
      let itemCats = JSON.stringify(item.categories);
      let currentCat = JSON.stringify(category);
      return itemCats.includes(currentCat);
    });

    let casesObject = {
      ...category,
      cases: [...filteredCases]
    };

    sortedCases.push(casesObject);
  });

  return sortedCases;
}

module.exports = sortCases(cases, getUniqueCategories(cases));

The in a cases.njk template

---
pagination:
    data: cases
    alias: category
    size: 1
permalink: /cases/category/{{ category.slug | slug }}.html
---
{% for post in category.cases %}
    <p>{{ post.title }}</p>
{% endfor %}