Ajax only gets response.readyStatus=4, but what's the reason for response.status=0?

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want to cross-domain request third-party API, front end uses ajax request, backend uses superAgent as relay, but ajax can not get a response, try to output response.status=0, but is response.readyStatus=4, because the back-end response is not written correctly?

Superagent javascript node.js ajax

Apr.07,2021

readystate
status

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