Insert ToC in TypeScript Handbook.

app.js

// ==UserScript==
// @name         PlusToC
// @namespace    https://marco.plus
// @version      0.1
// @description  Insert ToC in TypeScript Handbook.
// @author       Marco
// @match        https://www.typescriptlang.org/docs/handbook/*
// @grant        none
// ==/UserScript==

(function () {
    'use strict';
    let headingElements = document.querySelectorAll(".post-content h1, .post-content h2");

    let tocMother = document.createElement("div");
    let toc = document.createElement("ol");
    let h2After = false;

    for (let i = 0; i < headingElements.length; i++) {
        if (headingElements[i].nodeName === "H1") {
            let li = document.createElement("li");
            let a = document.createElement("a");

            a.setAttribute('href', `#${headingElements[i].id}`);
            a.insertAdjacentText('afterbegin', headingElements[i].textContent);
            li.appendChild(a);
            toc.appendChild(li);
            h2After = false;
        } else if (h2After) {
            let subList = toc.lastChild.lastChild;
            let li = document.createElement("li");
            let a = document.createElement("a");

            a.setAttribute('href', `#${headingElements[i].id}`);
            a.insertAdjacentText('afterbegin', headingElements[i].textContent);
            li.appendChild(a);
            subList.appendChild(li);
        } else {
            let h1 = toc.lastChild;
            let ol = document.createElement("ol");
            let childLi = document.createElement("li");
            let a = document.createElement("a");

            a.setAttribute('href', `#${headingElements[i].id}`);
            a.insertAdjacentText('afterbegin', headingElements[i].textContent);
            childLi.appendChild(a);
            ol.appendChild(childLi);
            h1.appendChild(ol);
            h2After = true;
        }
    }

    tocMother.appendChild(toc);
    tocMother.style.marginTop = "45px";
    tocMother.style.marginBottom = "-35px";

    let postHeader = document.querySelector(".post-header");
    postHeader.insertAdjacentElement('afterend', tocMother);
})();

Thanks!