node.js Duplex流与Transform 流

Use-case	Class	Method(s) to implement
Reading only	Readable	_read
Writing only	Writable	_write
Reading and writing	Duplex	_read, _write
Operate on written data, then read the result	Transform	_transform, _flush

通过查node.js 官方API 了解到：

Duplex 流 只设计到读写动作，

而Transform 流，读入数据->操作数据_>写出结果。