javascript - CHRis "readyState==3"是如何工作的？

Question

这是代码片段

xhr.onreadystatechange = function(){    if(xhr.readyState == 3){      console.log("readyState response length " + xhr.response.length);   }} 

控制台是

readyState response length: 3854 readyState response length: 33214 readyState response length: 35296 

我不知道onreadystatechange触发的时机是什么，它似乎与响应的大小有点关系，它是如何工作的？

Answer 1

What is XHR readyState=3?

拥有readyState值为 3 表示当前状态为 LOADING。

So when is a readyStateChange event for that state fired?

每当异步请求确实切换到 LOADING 状态时。特别是triggered by :

Once the first byte (or more) of the response entity body has been received […] or If there is no response entity body […] Then switch to the LOADING state.

因此，当浏览器收到响应字节时，它们可以自由地使用 readyState 3 触发此事件。有些人更频繁地触发它(在每次缓冲区刷新时)，其他人则不会。 The spec even says对于 readystatechange 事件:

The readyState attribute changes at some seemingly arbitrary times for historical reasons.