Add GitLab's-like [History] button on GitHub pages via Tampermonkey

gitlab-history-button-on-github.js

// ==UserScript==
// @name         Add GitLab's-like [History] button on GitHub pages
// @namespace    http://tampermonkey.net/
// @version      1.1
// @author       foka
// @match        *://github.com/*
// @icon         https://www.google.com/s2/favicons?domain=github.com
// @grant        none
// @require      http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js
// @require      https://gist.github.com/raw/2625891/waitForKeyElements.js
// ==/UserScript==

(function() {
  'use strict';

  const goToFileSelector = 'a[data-hotkey="t"]:not(.dropdown-item)';
  const historyIconSelector = 'svg.octicon-history';

  waitForKeyElements(goToFileSelector, goToFile => addHistoryBeforeGoToFile(goToFile[0], document.querySelector(historyIconSelector)));
  waitForKeyElements(historyIconSelector, historyIcon => addHistoryBeforeGoToFile(document.querySelector(goToFileSelector), historyIcon[0]));
})();

function addHistoryBeforeGoToFile(goToFile, historyIcon) {
  if (!goToFile || !historyIcon || document.getElementById('gitlab-history-button')) {
    return;
  }
  const historyUrl = historyIcon.closest('a').getAttribute('href');
  const addRightMargin = ! hasLeftMargin(goToFile);
  goToFile.before(createHistoryElement(historyUrl, addRightMargin));
}

function createHistoryElement(historyUrl, addRightMargin) {
  const history = document.createElement('a');
  history.id = 'gitlab-history-button';
  history.setAttribute('href', historyUrl);
  history.innerText = 'History';
  history.classList.add('btn');
  if (addRightMargin) {
    history.classList.add('mr-2');
  }
  return history;
}

function hasLeftMargin(element) {
  return !!element.classList.contains('ml-2');
}