属性描述onreadystatechange存储函数(或函数名),每当 readyState 属性改变时,就会调用该函数。readyState 存有 XMLHttpRequest 的状态。从 0 到 4 发生变化。 0: 请求未初始化1: 服务器连接已建立2: 请求已接收3: 请求处理中4: 请求已完成,且响应已就绪status 100:Continue101:Switching protocols200:OK201:Created202:Accepted203:Non-Authoritative Information204:No Content205:Reset Content206:Partial Content300:Multiple Choices301:Moved Permanently302:Found303:See Other304:Not Modified305:Use Proxy307:Temporary Redirect400:Bad Request401:Unauthorized402:Payment Required403:Forbidden404:Not Found405:Method Not Allowed 406:Not Acceptable407:Proxy Authentication Required408:Request Timeout409:Conflict410:Gone411:Length Required412:Precondition Failed413:Request Entity Too Large414:Request-URI Too Long415:Unsupported Media Type 416:Requested Range Not Suitable 417:Expectation Failed 500:Internal Server Error 501:Not Implemented502:Bad Gateway503:Service Unavailable504:Gateway Timeout 505:HTTP Version Not Supported