Userscript that adds buttons for quick "support" and "no support" posts. (btw I hate these)

scratch-support-buttons.user.js

// ==UserScript==
// @author      Zro716
// @name        Scratch Support Buttons
// @namespace   ScratchSupportButtons
// @description Adds buttons to post "support" or "no support" (must include a reason)
// @include     https://scratch.mit.edu/discuss/topic/*
// @version     1.01
// @grant       none
// ==/UserScript==

var isOpen = !!document.getElementsByClassName("follow-button").length,
    isSuggestion = document.querySelectorAll("a")[21].innerHTML == "Suggestions",
    pageNum = window.location.href.match(/\?page=(\d+)/),
    isFirstPage = pageNum == null || pageNum[1] == 1;

console.log("ScratchSupportButtons Debug:\n  isOpen       -> " + isOpen
                                      + "\n  isSuggestion -> " + isSuggestion
                                      + "\n  pageNum      -> " + JSON.stringify(pageNum)
                                      + "\n  isFirstPage  -> " + isFirstPage);

if (isOpen && isSuggestion && isFirstPage) {
  var op = document.getElementsByClassName("post_body_html")[0];
  console.log(op);
  var btn_sup = document.createElement("button"),
      btn_nosup = document.createElement("button");
  btn_sup.setAttribute("class","follow-button button blue");
  btn_nosup.setAttribute("class","unfollow-button button grey");
  btn_sup.innerHTML = "I Support";
  btn_nosup.innerHTML = "I Don't Support";
  btn_sup.onclick = function() {
    var post = prompt("It is encouraged by the Scratch Team to give reason for supporting.\nPlease briefly write your thoughts below:");
    if (post) {
      document.getElementsByTagName("textarea")[0].value = "I support. " + post;
      document.getElementsByClassName("form-submit")[0].getElementsByTagName("button")[0].click();
    }
  };
  btn_nosup.onclick = function() {
    var post = prompt("It is encouraged by the Scratch Team to give reason for not supporting.\nPlease briefly write your thoughts below:");
    if (post) {
      document.getElementsByTagName("textarea")[0].value = "I don't support. " + post;
      document.getElementsByClassName("form-submit")[0].getElementsByTagName("button")[0].click();
    }
  };
  var div = document.createElement("div");
  div.setAttribute("class","ScratchSupportButtons");
  div.appendChild(document.createElement("br"));
  div.appendChild(btn_sup);
  div.appendChild(btn_nosup);
  op.appendChild(div);
}