xhr.onreadystatechange = function() { if (xhr.readyState === 4 && xhr.status === 200)什么意思

This code snippet sets a callback function for the `readystatechange` event of an `XMLHttpRequest` object (`xhr`). The `readystatechange` event is fired several times as the request progresses through different states, and the callback function is called each time to handle the response data.

The code checks if the `readyState` property of the `xhr` object is equal to 4, which means that the request has completed and the response data is available. If the condition is true, the callback function can access the response data using the `xhr.responseText` property and perform any necessary processing.