Ajax Events

The onreadystatechange event

When a request to a server is sent, we want to perform some actions based on the response.
The onreadystatechange event is triggered every time the readyState changes.
The readyState property holds the status of the XMLHttpRequest.

Three important properties of the XMLHttpRequest object:
onReadyStateChangeIt is called whenever readystate attribute changes. It must not be used with synchronous requests.
readyStaterepresents the state of the request. It ranges from 0 to 4.
  • 0: request not initialized (open() is not called.)
  • 1: server connection established (open is called but send() is not called.)
  • 2: request received (send() is called, and headers and status are available.)
  • 3: processing request (Downloading data; responseText holds the data.)
  • 4: request finished and response is ready (The operation is completed fully.)
status200: "OK" 
404: Page not found
In the onreadystatechange event, we specify what will happen when the server response is ready to be processed. When readyState is 4 and status is 200, the response is ready:


  if (xmlhttp.readyState==4 && xmlhttp.status==200)