lib/jsdom/living/helpers/mutation-observers.js
function notifyMutationObservers() {
  mutationObserverMicrotaskQueueFlag = false;

  const notifyList = [...activeMutationObservers].sort((a, b) => a._id - b._id);
  activeMutationObservers.clear();

  const signalList = [...signalSlotList];
  signalSlotList.splice(0, signalSlotList.length);

  for (const mo of notifyList) {
    const records = [...mo._recordQueue];
    mo._recordQueue = [];

    for (const node of mo._nodeList) {
      node._registeredObserverList = node._registeredObserverList.filter(registeredObserver => {
        return registeredObserver.source !== mo;
      });

      if (records.length) {
        try {
          mo._callback(
            records.map(idlUtils.wrapperForImpl),
            idlUtils.wrapperForImpl(mo)
          );
        } catch (e) {
          const { target } = records[0];
          const window = target._ownerDocument._defaultView;

          reportException(window, e);
        }
      }
    }
  }

  for (const slot of signalList) {
    const slotChangeEvent = Event.createImpl(
      [
        "slotchange",
        { bubbles: true }
      ],
      { isTrusted: true }
    );

    slot._dispatch(slotChangeEvent);
  }
}
function notifyMutationObservers() {
  mutationObserverMicrotaskQueueFlag = false;

  const notifyList = [...activeMutationObservers].sort((a, b) => a._id - b._id);
  activeMutationObservers.clear();

  const signalList = [...signalSlotList];
  signalSlotList.splice(0, signalSlotList.length);

  for (const mo of notifyList) {
    const records = [...mo._recordQueue];
    mo._recordQueue = [];

    for (const node of mo._nodeList) {
      node._registeredObserverList = node._registeredObserverList.filter(registeredObserver => {
        return registeredObserver.source !== mo;
      });

      if (records.length) {
        try {
          mo._callback(
            records.map(idlUtils.wrapperForImpl),
            idlUtils.wrapperForImpl(mo)
          );
        } catch (e) {
          const { target } = records[0];
          const window = target._ownerDocument._defaultView;

          reportException(window, e);
        }
      }
    }
  }

  for (const slot of signalList) {
    const slotChangeEvent = Event.createImpl(
      [
        "slotchange",
        { bubbles: true }
      ],
      { isTrusted: true }
    );

    slot._dispatch(slotChangeEvent);
  }
}
https://dom.spec.whatwg.org/#notify-mutation-observers