broadcastChannel.unref()

如果这是事件系统中唯一的活动句柄，则在广播通道上调用 unref() 允许线程退出。 如果广播通道已经 unref()，则再次调用 unref() 无效。

Calling unref() on a BroadcastChannel allows the thread to exit if this is the only active handle in the event system. If the BroadcastChannel is already unref()ed calling unref() again has no effect.