Why Am I Getting an 'Unexpected token < in JSON at position 0' Error in My React App?

Unexpected Token in JSON Parse

You encountered an "Unexpected token < in JSON at position 0" error while running a React app. This error typically arises when JSON.parse() is attempting to parse invalid JSON data.

In your case, the error is traced to the loadThreadsFromServer function, which makes an AJAX call to retrieve JSON data from the server. While the server is expected to return valid JSON, React is seemingly polling a different endpoint that results in receiving HTML content instead.

Possible Causes

The following are possible reasons for the invalid JSON response:

• The server may not be setting the correct Content-Type header to "application/json."
• Misconfigured proxy or CDN may be altering the response body.
• Network issues or race conditions could cause incorrect data to be received.

Troubleshooting Steps

To resolve this issue, consider these steps:

• Verify Server Response Content-Type: Use browser tools or a REST client to confirm that the server is sending the correct Content-Type header.
• Check Endpoint URL: Ensure that React is polling the intended endpoint (http://localhost:3001/api/threads) and not a spurious URL.
• Examine XHR Response Body: Add console.warn(xhr.responseText) to the loadThreadsFromServer function to inspect the raw response body. This will help you identify if HTML is being received instead of JSON.
• Inspect Network Activity: Use browser developer tools to monitor network traffic and ensure that the correct endpoint is being accessed and that the response is as expected.
• Restart Application: Sometimes a fresh app restart can resolve the issue, especially if it's related to race conditions or browser caching.

By following these steps, you should be able to pinpoint the cause of the invalid JSON parse and resolve the error.

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