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

?

• 100:Continue
• 101:Switching protocols
• 200:OK
• 201:Created
• 202:Accepted
• 203:Non-Authoritative Information
• 204:No Content
• 205:Reset Content
• 206:Partial Content
• 300:Multiple Choices
• 301:Moved Permanently
• 302:Found
• 303:See Other
• 304:Not Modified
• 305:Use Proxy
• 307:Temporary Redirect
• 400:Bad Request
• 401:Unauthorized
• 402:Payment Required
• 403:Forbidden
• 404:Not Found
• 405:Method Not Allowed
• ?406:Not Acceptable
• 407:Proxy Authentication Required
• 408:Request Timeout
• 409:Conflict
• 410:Gone
• 411:Length Required
• 412:Precondition Failed
• 413:Request Entity Too Large
• 414:Request-URI Too Long
• 415:Unsupported Media Type?
• 416:Requested Range Not Suitable?
• 417:Expectation Failed?
• 500:Internal Server Error?
• 501:Not Implemented
• 502:Bad Gateway
• 503:Service Unavailable
• 504:Gateway Timeout?
• 505:HTTP Version Not Supported