// 环状单向链表操作
  if (queue.pending === null) {
    update.next = update;
  } else {
    update.next = queue.pending.next;
    queue.pending.next = update;
  }
  queue.pending = update;

...
  } else {
    update.next = queue.pending;
    queue.pending.next = update;
  }
...