xhr.readyState === 4 && xhr.status === 200

这段代码是用来检查 XMLHttpRequest 对象的 readyState 属性是否等于 4 的条件语句。readyState 属性表示请求的状态，4 表示请求已完成且响应已就绪。因此，这段代码可以用来检查 XMLHttpRequest 请求是否已经完成，如果已经完成，则可以通过 XMLHttpRequest 对象的 response 属性获取响应数据。

相关问题

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

This is an incomplete line of JavaScript code. It seems that the code block is checking for the readyState of an XMLHttpRequest object (represented by the variable 'xhr'). The number 4 represents the value of the readyState property when the request has been completed and the response is ready. However, the rest of the code is missing, so it's unclear what action is being taken based on the readyState value.

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.