index.js

'use strict';

const btnContainer = document.getElementsByClassName(
  'd-flex my-2 mx-md-2 flex-md-justify-end'
)[0];

const copyCommitBtn = document.createElement('button');

copyCommitBtn.type = 'button';
copyCommitBtn.className = 'btn btn-secondary';
copyCommitBtn.style.margin = '0px 10px';
copyCommitBtn.innerHTML = 'Copy commits';

btnContainer.insertBefore(copyCommitBtn, btnContainer.firstChild);

const handleButtonClick = () => {
  const commitMessageContainers = document.getElementsByClassName(
    'TimelineItem-body'
  );

  let commitMessages = ``;

  commitMessageContainers &&
    Object.keys(commitMessageContainers).forEach((key) => {
      const commitElement = commitMessageContainers[key].querySelector(
        'div div div div code a'
      );

      if (commitElement) {
        const commitMessage = commitElement.title || '';

        commitMessages += `- ${commitMessage}\n`;
      }
    });

  copyCommitsToClipBoard(commitMessages);

  copyCommitBtn.innerHTML = 'Copied to clipboard!';

  setTimeout(() => {
    copyCommitBtn.innerHTML = 'Copy commits';
  }, 3000);
};

const copyCommitsToClipBoard = (commitMessages) => {
  const el = document.createElement('textarea');

  el.value = commitMessages;

  document.body.appendChild(el);

  el.select();

  document.execCommand('copy');
  document.body.removeChild(el);
};

btnContainer.addEventListener('click', handleButtonClick);