writable._final(callback)

• callback <Function> 在写入剩余数据完成后调用此函数（可选择传入一个错误参数）。

_final() 方法 不能 被直接调用。它可以由子类实现，如果实现了，将只会被内部的 Writable 类方法调用。

🌐 The _final() method must not be called directly. It may be implemented by child classes, and if so, will be called by the internal Writable class methods only.

在流关闭之前，将调用此可选函数，并会延迟 'finish' 事件的触发，直到调用 callback。这对于在流结束前关闭资源或写入缓冲数据非常有用。

🌐 This optional function will be called before the stream closes, delaying the 'finish' event until callback is called. This is useful to close resources or write buffered data before a stream ends.