readyState 5种状态(Ajax)

　　0: (Uninitialized) the send( ) method has not yet been invoked.

　　1: (Loading) the send( ) method has been invoked, request in progress.

　　2: (Loaded) the send( ) method has completed, entire response received.

　　3: (Interactive) the response is being parsed.

　　4: (Completed) the response has been parsed, is ready for harvesting.

　　0 - (未初始化)还没有调用send()方法

　　1 - (载入)已调用send()方法，正在发送请求

　　2 - (载入完成)send()方法执行完成，已经接收到全部响应内容

　　3 - (交互)正在解析响应内容

　　4 - (完成)响应内容解析完成，可以在客户端调用了