How Can I Resolve 'No 'Access-Control-Allow-Origin' Header' CORS Errors?

"No 'Access-Control-Allow-Origin' Header Present in API Response"

CORS Proxy for Header Issue Resolution

If you lack control over the server, you can bypass the header deficiency by employing a CORS proxy. One easily deployable option is cors-anywhere (https://github.com/Rob--W/cors-anywhere), which can be set up with a few commands. This proxy adds the necessary Access-Control-Allow-Origin header to responses.

Avoiding CORS Preflight

The code in question triggers a CORS preflight due to the Authorization header. Additionally, Content-Type: application/json can also provoke a preflight. To avoid this, one must modify the server to respond appropriately to the preflight OPTIONS request with the necessary headers. Alternatively, designing the request to avoid these triggers (e.g., using different headers or embedded JSON) can be considered.

"Wildcard" Access-Control-Allow-Origin Issue

For responses with credentials, the value of the Access-Control-Allow-Origin header cannot be '*'. It must precisely match the origin of the frontend code, such as 'http://127.0.0.1:3000'. Server configuration can be adjusted to automatically reflect the origin value in the header.

Removing Unnecessary Request Headers

Remove these lines from the JavaScript code as they represent response headers and should not be included in requests:

headers.append('Access-Control-Allow-Origin', 'http://localhost:3000');
headers.append('Access-Control-Allow-Credentials', 'true');

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