Answer: The onreadystatechange event is part of the XMLHttpRequest object and is triggered every time the readyState property changes. Developers can set up event listeners for this event to define actions or handle data based on the current state of the AJAX request. Common states include 0 (request not initialized), 1 (server connection established), 2 (request received), 3 (processing request), and 4 (request finished and response is ready).
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