readyState Status Code
0: Request uninitialized
1: The server connection has been established
2: The request receiving
3: Request Processing
4: request has been completed, and the response is ready
HTTP status code
200: The server successfully returned the page
404: The requested page does not exist
503: The server is temporarily unavailable
XmlHttpRequest object
1: object type API
2: In the browser environment
3: transfer to the receiving client and server-side data
4: a request for the XML data (JSON, plain text text)