Why Am I Getting a 'No 'Access-Control-Allow-Origin' Header' Error When Accessing a REST API?

No 'Access-Control-Allow-Origin' Header in Response

Understanding CORS and Preflighted Requests

Cross-Origin Resource Sharing (CORS) is a mechanism that allows browser requests to access resources from origins different from the requesting origin. When a request is made from a different origin, the browser sends a preflight OPTIONS request to the server to check if the server allows the request.

In the case described in the question, the browser is sending a preflight OPTIONS request to the HP ALM REST API. However, the server is not responding with the necessary CORS headers, causing the error "No 'Access-Control-Allow-Origin' header is present on the requested resource."

Solving the Issue

There are several ways to solve this issue:

Using a CORS Proxy:

A CORS proxy forwards the request to the destination server and adds the necessary CORS headers to the response. This can be convenient if you don't have control over the destination server.

Avoiding Preflighted Requests:

To avoid the preflight request, you can ensure that the request doesn't require Authorization or Content-Type headers. This can be achieved by using alternative authentication methods or encoding the JSON data as a query parameter.

Solving the Wildcard Issue:

The error "Access-Control-Allow-Origin header must not be the wildcard" occurs when the server specifies a wildcard ('*') for the Access-Control-Allow-Origin header instead of the specific origin of the requesting client. To solve this, configure the server to set the Access-Control-Allow-Origin header to the requesting origin.

Fixing the JavaScript Code

The JavaScript code in the question has some incorrect headers that are triggering the preflight request:

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

Remove these headers as they should not be sent in the request.

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