Why Am I Seeing the \"No Access-Control-Allow-Origin Header\" Error?

Why "No Access-Control-Allow-Origin header is present on the requested resource"

The issue is when a browser makes a cross-origin request (CORS), the browser first sends a preflight request to the server to check if the server supports CORS, and if so, which methods, headers, and origins are allowed.

The server should respond to the preflight request with the appropriate CORS headers. If the server does not respond with the required headers, the browser will block the actual request.

In this case, the server is not responding with the Access-Control-Allow-Origin header, which tells the browser that the origin is allowed to access the resource.

How to fix it

There are a few ways to fix this issue:

1. Add the Access-Control-Allow-Origin header to the response of the preflight request. This can be done by using the middleware in the code.
2. Set the Access-Control-Allow-Origin header to '*' to allow all origins. However, this is not recommended for security reasons.
3. Use a reverse proxy that can add the Access-Control-Allow-Origin header to the response.

Additional notes

• The Access-Control-Allow-Origin header can also be set for specific origins. For example, to allow requests from https://example.com, you can set the header to Access-Control-Allow-Origin: https://example.com.
• The Access-Control-Allow-Origin header is only required for cross-origin requests. If the request is from the same origin, the browser will not send a preflight request and the server does not need to respond with the Access-Control-Allow-Origin header.

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