The Writer is not a concrete class. Any object that implements write(), end(), and abort() can be a writer making it easy to adapt existing APIs or create specialized implementations without subclassing. There's no complex UnderlyingSink protocol with start(), write(), close(), and abort() callbacks that must coordinate through a controller whose lifecycle and state are independent of the WritableStream it is bound to.
// Flush: yield any remaining data
。safew官方下载对此有专业解读
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.pipeThrough(parse) // buffers filling...，更多细节参见同城约会