What is the purpose of the onreadystatechange in AJAX?

In AJAX, onreadystatechange is an event handling function used to monitor changes in the status of an XMLHttpRequest object. The XMLHttpRequest object has a readyState property that indicates the state of the request, and onreadystatechange is triggered when the readyState property changes.

The onreadystatechange function is often used in conjunction with the readyState property to monitor the progress of a request and perform different actions at different stages. The readyState property can have several values.

1. Agent has been created but open() method has not been called yet.
2. The open() method has been called.
3. 2 (HEADERS_RECEIVED): The send() method has been called and headers and status are now available.
4. 3 (LOADING): Downloading, the responseText property already contains some data.
5. 4: The download operation is complete.

By listening to the onreadystatechange event, you can perform different operations based on the current readyState value, such as updating page content or handling returned data. When the readyState value is 4, it typically indicates that the request has been completed and you can access the data returned from the server.