Why is my React app receiving an 'Unexpected token < in JSON at position 0' error despite a seemingly correct JSON response?

Reacting to "_SyntaxError: Unexpected token < in JSON at position 0_"

In a React app featuring Facebook-esque feeds, you're encountering an enigmatic error: "Feed.js:94 undefined 'parsererror' 'SyntaxError: Unexpected token < in JSON at position 0'". This error typically arises when Google Chrome attempts to parse HTML as JSON, leading to suspicion that despite assigning a JSON content type, the server's response might actually contain HTML.

To delve deeper into the issue, you've examined the server's output using Chrome's developer tools and REST client, and it appears to adhere to the JSON format. However, a crucial detail suggests otherwise: React is inexplicably polling a different endpoint ("http://localhost:3000/?_=1463499798727") instead of the intended API ("http://localhost:3001/api/threads").

Initially, you hadn't suspected this disparity and had ruled out typos and recently modified code as potential culprits. However, it's likely that an unintended change has altered React's polling behavior, causing it to retrieve data from an erroneous location.

To resolve the issue, you could consider:

• Scrutinizing your webpack configuration and hot-reload server settings to ensure they're pointing to the correct backend address.
• Verifying that the express app is listening on port 3001 and handling API requests as expected.
• Including additional logging statements, such as console.warn(xhr.responseText), to gain further insight into the server's response. This might reveal the root cause of the HTML response instead of the intended JSON.

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