eleventy.config.js

const fs = require('node:fs');
const path = require('node:path');
const { parseHTML } = require('linkedom');

const packagePath = path.join(__dirname, 'package.json');
const package = JSON.parse(fs.readFileSync(packagePath, 'utf-8'));

const TRANSFORM_NAME = package.name;

module.exports = function(eleventyConfig, options = {}) {
  eleventyConfig.addTransform(TRANSFORM_NAME, async function(content) {
    const { page } = this;

    if (!page?.outputPath?.endsWith?.('.html')) {
      return content;
    }

    const transform = await options.handler?.({ content, page });

    if (!transform) {
      return content;
    }

    const DOMEnv = parseHTML(content);

    await transform(DOMEnv);

    return DOMEnv.document.toString();
  })
}