qso

100% inspired by a Daniel's hack, this module brings a friendly MutationObserver like API to observe CSS selectors instead.

const so = new QuerySelectorObserver(records => {
  for (const record of records) {
    if (record.addedNodes.length) {
      record.target.textContent = 'Hello via QSO!';
      console.log(record.addedNodes);
    } else {
      console.log(record.removedNodes);
    }
  }
});

so.observe('.some-selector');
so.observe('#some-complex > .selector + p.cool');

button.onclick = () => so.disconnect();

Compatible with IE11 and other browsers, feel free to test it live.