Why Do I Get a SyntaxError in Fetch Requests with `mode: \'no-cors\'`?

Handling Response Errors: SyntaxError in 'no-cors' Mode

When using the fetch API in ReactJS to make requests to a REST-API, it's essential to handle the response appropriately. However, common errors can occur during this process. One such error is the "SyntaxError: Unexpected end of input."

This error is typically encountered when attempting to handle the response, specifically at the line where response.json() is called. The error indicates that the response received from the server is incomplete or in an unexpected format.

Understanding the 'no-cors' Mode

The crux of this issue lies in the use of the mode: 'no-cors' setting in the fetch request. This setting essentially prevents the browser from attaching CORS headers to the request, making the response opaque.

Opaque Responses: Opaque responses prohibit frontend JavaScript code from accessing the response body or headers. This restriction can cause problems when trying to process the response, as it prevents access to the necessary data.

Solution: Remove 'no-cors' Mode

To resolve the SyntaxError and successfully handle the response, it's crucial to remove the mode: 'no-cors' setting. By default, the fetch API uses the 'same-origin' mode, which allows the browser to handle CORS and provide access to the response.

Alternative CORS Solutions

If you're still encountering CORS issues after removing 'no-cors', consider the following alternative solutions:

• Configure the Server: Ensure that the server sending the response includes the Access-Control-Allow-Origin response header and handles OPTIONS preflight requests.
• CORS Proxy: Utilize a CORS proxy such as https://github.com/Rob--W/cors-anywhere to bypass CORS issues. See the linked resources in the provided answer for more information on CORS proxies.

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