The onreadystatechange eventWhen 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:
Stores a function (or the name of a function) to be called automatically each time the readyState property changes
Holds 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 ready
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: