lib/jsdom/living/helpers/shadow-dom.js
function assignSlotable(slot) {
  const slotables = findSlotable(slot);

  let shouldFireSlotChange = false;

  if (slotables.length !== slot._assignedNodes.length) {
    shouldFireSlotChange = true;
  } else {
    for (let i = 0; i < slotables.length; i++) {
      if (slotables[i] !== slot._assignedNodes[i]) {
        shouldFireSlotChange = true;
        break;
      }
    }
  }

  if (shouldFireSlotChange) {
    signalSlotChange(slot);
  }

  slot._assignedNodes = slotables;

  for (const slotable of slotables) {
    slotable._assignedSlot = slot;
  }
}
function assignSlotable(slot) {
  const slotables = findSlotable(slot);

  let shouldFireSlotChange = false;

  if (slotables.length !== slot._assignedNodes.length) {
    shouldFireSlotChange = true;
  } else {
    for (let i = 0; i < slotables.length; i++) {
      if (slotables[i] !== slot._assignedNodes[i]) {
        shouldFireSlotChange = true;
        break;
      }
    }
  }

  if (shouldFireSlotChange) {
    signalSlotChange(slot);
  }

  slot._assignedNodes = slotables;

  for (const slotable of slotables) {
    slotable._assignedSlot = slot;
  }
}
https://dom.spec.whatwg.org/#assign-slotables