Why Am I Getting an 'Unexpected token

"Unexpected Token <" Error When Parsing JSON

In a React application that manages Facebook-like content feeds, you may encounter an "Unexpected token < in JSON at position 0" error. This error occurs when there is an issue parsing the JSON response received from the server.

Source of the Error

The root cause of this error is often mismatched content types. While the server claims to send Content-Type:application/json, the response body might actually be HTML. This can happen due to an inconsistency in the server-side configuration or a network issue.

Troubleshooting Steps

1. Check the Server Response Content-Type: Ensure that the server is correctly setting the Content-Type header to "application/json."
2. Inspect the Response Body: Print the xhr.responseText property in your JavaScript code to see the actual response body. If you see HTML instead of JSON, it confirms that the server is sending the wrong content type.
3. Check the URL Format: Verify that the URL used in the request is correct and points to the intended API endpoint.
4. Clear the Backend Cache: Restart the backend service or clear its cache to ensure it is generating the correct JSON data.
5. Examine Server-Side Configuration: If the content-type error persists, check the server-side configuration for potential misconfigurations that could result in HTML being served instead of JSON.

Network Issues

Occasionally, network issues can lead to 404 or 500 errors and redirect responses, resulting in the JSON body being malformed and triggering the "Unexpected token <" error.

Resolution

To resolve this error, ensure that the server is correctly generating and sending a valid JSON response with the "application/json" content type. Inspect the xhr.responseText property to verify the response body and make any necessary adjustments to your server or network configuration.

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