uechi.io/plugins/hexo-simple-toc/toc.js

const JSDOM = require("jsdom").JSDOM;

function generateToC(html) {
  const dom = JSDOM.fragment(html);

  const headings = Array.from(dom.querySelectorAll("h1, h2, h3, h4, h5, h6"))
    // Filter headings with ids
    .filter((el) => el && el.id)
    // {id: string, content: string|null, depth: number}
    .map((el) => ({
      id: el.id,
      content: el.textContent,
      depth: Number(el.nodeName.replace("H", "")), // H1 -> 1
    }));

  let pos = 0;
  function genToC(arr, lastDepth) {
    const stack = [];
    while (pos < arr.length) {
      const { depth, id, content } = arr[pos];

      if (depth < lastDepth) {
        // unwind
        pos -= 1;
        break;
      } else if (depth > lastDepth) {
        // nested ToC
        stack.push(stack.pop() + genToC(arr, depth));
      } else {
        // same depth
        stack.push(`<a href="#${id}">${content}</a>`);
      }

      pos += 1;
    }

    return "<ul>" + stack.map((s) => `<li>${s}</li>`).join("") + "</ul>";
  }

  return genToC(headings, 1);
}

exports.after_post_render = function (data) {
  data.content = data.content.replace("<!-- toc -->", () =>
    generateToC(data.content)
  );
  return data;
};