Prevent redirection of Xmlhttprequest
在发送XMLHttpRequest-s（即重新获取重定向状态代码并自己处理它）时，是否有可能阻止浏览器遵循重定向？
Not according to the W3C standard for the XMLHttpRequest object (emphasis added):
If the response is an HTTP redirect:
If the origin of the URL conveyed by the Location header is same origin with the XMLHttpRequest origin and the redirect does not violate infinite loop precautions, transparently follow the redirect while observing the same-origin request event rules.
They were considering it for a future release:
This specification does not include the following features which are being considered for a future version of this specification:
but the latest specification no longer mentions this.
The new Fetch API supports different modes of redirect handling: follow , error , and manual , but I can't find a way to view the new URL or the status code when the redirection has been canceled. You just can stop the redirection itself, and then it looks like an error (empty response). If that's all you need, you are good to go. Also you should be aware that the requests made via this API are not cancelable yet. They are now.
As for XMLHttpRequest, you can HEAD the server and inspect whether the URL has changed:
var http = new XMLHttpRequest();
http.open('HEAD', '/the/url');
http.onreadystatechange = function() {
if (this.readyState === this.DONE) {
console.log(this.responseURL);
}
};
http.send();
You won't get the status code, but will find the new URL without downloading the whole page from it.
You can use responseURL property to get the redirect destination or check whether the response was ultimately fetched from a location you accept.
This of course means the result is fetched anyway, but at least you can get the necessary info about the redirect destination and for example detect conditions when you would like to discard the response.
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