XMLHttpRequest

PropertyDescriptiononreadystatechangeStores a function (or the name of a function) to be called automatically each time the readyState property changesreadyStateHolds the status of the XMLHttpRequest. Changes from 0 to 4: 
0: request not initialized 
1: server connection established
2: request received 
3: processing request 
4: request finished and response is readystatus200: "OK"
404: Page not found