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OK this is an open issue with Passport:
https://github.com/jaredhanson/passport/issues/582#issuecomment-506283986
I should use the href and not fetch.
Possible duplicate of
Cors error when sending request from React frontend to Google Oauth PassportJS on backend
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solved Using fetch with Passport gets “No ‘Access-Control-Allow-Origin’ header is present on the requested resource.”