code: function getTopLevelCallbackBookKeeping(topLevelType, nativeEvent, targetInst) {
    if (callbackBookkeepingPool.length) {
      var instance = callbackBookkeepingPool.pop();
      instance.topLevelType = topLevelType;
      instance.nativeEvent = nativeEvent;
      instance.targetInst = targetInst;
      return instance;
    }
    return {
      topLevelType: topLevelType,
      nativeEvent: nativeEvent,
      targetInst: targetInst,
      ancestors: []
    };
  }
docs: Used to store ancestor hierarchy in top level callback