Patient #9

CORS error when calling API from frontend

fetch('https://api.example.com/data')

The patient is experiencing a Cross-Origin Resource Sharing (CORS) error. This occurs because the browser's security policy prevents a web page from making requests to a different domain than the one that served the web page, unless the server explicitly allows it. In this case, the frontend running on 'http://localhost:3000' is trying to access 'https://api.example.com', which is a different origin. The API server at 'https://api.example.com' is not sending the necessary 'Access-Control-Allow-Origin' HTTP header in its response, or it's sending one that does not include 'http://localhost:3000', thus blocking the request.

The primary solution is to configure the API server ('https://api.example.com') to include the appropriate 'Access-Control-Allow-Origin' header in its responses. This header should specify which origins are allowed to access the resource. For development, 'http://localhost:3000' should be added. For production, the specific domain(s) of the frontend application should be listed. If multiple origins need to be supported, a comma-separated list or a wildcard '*' can be used (though '*' is generally not recommended for production due to security implications, especially with credentials). Additionally, other CORS headers like 'Access-Control-Allow-Methods' and 'Access-Control-Allow-Headers' might be needed depending on the request type and custom headers used. If server-side modification is not immediately possible, a proxy server can be set up on the same origin as the frontend to forward requests to the API, effectively bypassing the browser's CORS restriction.