readyState的五种状态

There are five possible values for readyState: 0 (Uninitialized): The object has been created but the open() method hasn't been called. 1 (Loading): The open() method has been called but the request hasn't been sent. 2 (Loaded): The request has been sent. 3 (Interactive). A partial response has been received. 4 (Complete): All data has been received and the connection has been closed. readyState有五种可能的值： 0 (未初始化)： (XMLHttpRequest)对象已经创建，但还没有调用open()方法。 1 (载入)：已经调用open() 方法，但尚未发送请求