title: what is the solution? I am using node express for backend. I my localhost this is ok but after deployin this error occured
body: <blockquote>
  <p>Access to fetch at 'https://still-ravine-45870.herokuapp.com/user/undefined' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.</p>
  </blockquote>