Table of Contents script

<script>
   // Get ToC div
    toc = document.getElementById("ToC");

    //Add a header
    tocHeader = document.createElement("h2");
    tocHeader.innerText="Table of contents";
    toc.appendChild(tocHeader);
   
    // Create a list for the ToC entries
    tocList = document.createElement("ul");    

    // Get the h3 tags - ToC entries
    headers = document.getElementsByTagName("h3");
   
    // For each h3
    for (i = 0; i < headers.length; i++){
     
      // Create an id
      name = "h"+i;
      headers[i].id=name;
     
      // a list item for the entry
      tocListItem = document.createElement("li");

      // a link for the h3
      tocEntry = document.createElement("a");
      tocEntry.setAttribute("href","#"+name);
      tocEntry.innerText=headers[i].innerText;
     
      tocListItem.appendChild(tocEntry);
      tocList.appendChild(tocListItem);
    }
    toc.appendChild(tocList);
</script>

Source: https://projectcodeed.blogspot.com/2020/04/an-automatic-table-of-contents.html