Eleventy config to rewrite links

.eleventy.js

const markdownIt = require("markdown-it");
const markdownItReplaceLink = require('markdown-it-replace-link');

module.exports = function(eleventyConfig) {
  let markdownItOptions = {
    html: true,
    breaks: true,
    linkify: true,
    replaceLink: function (link, env) {
      link = link.toLowerCase()
      const doNothing = ['http://', 'https://', '#', 'mailto:'];
      if (doNothing.some((protocol) => link.startsWith(protocol))) { return link; }

      // Convert links such as `/page.md` to `/page/`
      if (link.startsWith("/")) {
        const extensions = ['md', 'mmd', 'mdl', 'markdown']
        if (extensions.some((ext) => link.endsWith(ext))) { 
          const regex = /(?!(?:http:|https:))(.*?)(.md)/gm;
          const newLink = link.replace(regex, "$1")
          return newLink
        }
      }

      // Convert links such as `./page.md` to `.././page/`
      const extensions = ['md', 'mmd', 'mdl', 'markdown']
      if (extensions.some((ext) => link.endsWith(ext))) { 
        const regex = /(?!(?:http:|https:))(.*?)(.md)/gm;
        const lastSegment = env.page.filePathStem.split('/').slice(-1)[0].toLowerCase()

        if (link.endsWith("index.md")) {
          link = link.substring(0, link.indexOf("index.md"))
        }

        if (lastSegment === "index") {
          return link.replace(regex, "$1")
        }

        return link.replace(regex, "../$1")
      }
      return link
    }
  };
  
  let md = markdownIt(markdownItOptions)
  md.use(markdownItReplaceLink)
 
  return {};
};