设备就绪或未就绪状态 device ready not ready
另外，未初始化的就绪状态在实际的应用程序中是没有真正的用处的。
Furthermore, the uninitialized ready state is pretty useless in practical applications.
第一种就绪状态的特点是 readyState 属性为 0（readyState == 0），表示未初始化状态。
The first ready state, signified by the readyState property 0 (readyState == 0), represents an uninitialized request.
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