ssc-more-up.user.js

// ==UserScript==
// @name           SSC: More Up
// @namespace      https://github.com/bakkot/SlateStarComments/
// @description    Add links to OPs of threads on SSC and Unsong
// @match          http://slatestarcodex.com/20*
// @match          http://unsongbook.com/*
// @version        1.0.0
// ==/UserScript==

var comments = document.querySelectorAll('li.comment');

for (var i=0; i<comments.length; ++i) {
  if (comments[i].parentElement.tagName === 'UL') {
    var commentHolder = comments[i].querySelector('div.commentholder');
    var parent = comments[i];
    do {
      parent = parent.parentElement.parentElement;
    } while (parent.parentElement.tagName === 'UL');
    var parentID = parent.firstElementChild.id;

    var parentLink = document.createElement('a');
    parentLink.href = '#' + parentID;
    parentLink.className = 'comment-reply-link';
    parentLink.style.textDecoration = 'underline';
    parentLink.title = 'OP comment';
    parentLink.textContent = '⇑';

    var replyEle = commentHolder.querySelector('div.reply');
    replyEle.appendChild(document.createTextNode(' '));
    replyEle.appendChild(parentLink);
  }
}