"TypeError: Failed to fetch" in a POST request

The "TypeError: Failed to fetch" error you're encountering in your React app using the fetch API can be caused by various factors. In your code, you are making a GET request to a server and receiving a valid response, but the fetch API still throws this error. Here are some possible reasons and solutions:

1. CORS Headers: This error could be due to an issue with Cross-Origin Resource Sharing (CORS). Check the value of the Access-Control-Allow-Origin header in the response from your server. If it doesn't match the origin of your request (e.g., localhost), it can trigger this error. You need to configure your server to allow requests from your app's origin.

   You can learn more about CORS headers in the nginx.org.

2. HTTP vs. HTTPS: If you are making requests to a server running on localhost, try using non-SSL (HTTP) instead of SSL (HTTPS). Fetch might fail on an invalid or self-signed certificate, especially on localhost.

3. Fetch Method Misuse: En that you are using the fetch API correctly, especially the response handling. You should return the result of res.json() in your .then() to properly handle errors during JSON parsing.

   fetch(url)
      .then(res => {
         if (!res.ok) {
            throw new Error('Network response was not ok');
         }
         return res.json();
      })
      .then(data => {
         // Handle your data here
      })
      .catch(error => {
         console.error('Fetch error:', error);
      });

4. Invalid URLs: Double-check your URL. In some cases, adding "http://" or "https://" before "localhost" can resolve the issue if it's missing.

5. Use of Mutations for GET: En you are using the correct method for your API calls. Use queries to fetch data from the server and mutations for create, update, or delete operations. If you are using mutations for GET requests, it may lead to this error.

   You can learn more about using queries and mutations in the tanstack.com and tanstack.com.

6. Payload Size: If you are uploading files or sending large payloads, the server might reject the request, causing this error. Make your payload size is within acceptable limits.

7. Sentry: If you are using Sentry for error tracking, en that the 'sentry-trace' value is added to your server's 'Access-Control-Allow-Headers' list.

8. Nginx Configuration: If you are using Nginx, en that you have correctly configured the 'add_header' directive with the 'always' parameter for the required headers to prevent CORS issues.

Remember to check your server logs for any additional information about the failed requests. These steps should help you diagnose and resolve the "TypeError: Failed to fetch" error in your React app.

The issue you're facing, "TypeError: Failed to fetch," in a POST request using JavaScript and Node.js, is related to CORS (Cross-Origin Resource Sharing) headers in the response message.

The problem arises because the response message contains two "access-control-allow-origin" headers. This issue can break CORS functionality. To reproduce this issue, follow these steps:

1. When you make an HTTP request in Tyk Portal, it first performs an "OPTIONS" HTTP preflight request to verify that the server will accept the request.

2. Next, Tyk performs the actual "GET" HTTP request to retrieve the data. Both requests contain the "origin" header with the IP address of the portal.

3. Both requests are processed from the Tyk Catalogue API to the Tyk API itself and forwarded to the backend microservice.

4. The backend microservice returns HTTP headers in the response message, including "Access-Control-Allow-Origin: *," indicating that any origin is allowed.

5. The Tyk API adds another "Access-Control-Allow-Origin" header to the HTTP response message, resulting in two of these headers in the response.

To resolve this issue, you should en that the backend application returns the "Access-Control-Allow-Origin" header with the value " * " and configure Tyk's options_passthrough to true.

By setting options_passthrough to "true" and disabling CORS in the Tyk configuration, you can avoid the duplication of "Access-Control-Allow-Origin" headers and resolve the "TypeError: Failed to fetch" issue in your POST request.

Here's a snippet of code that shows how to configure Tyk's options_passthrough:

"enable_options_passthrough": true,

Make to adjust your Tyk configuration accordingly, and the issue should be resolved.