How to decode JSON parse errors and fix fetch failures in JavaScript

When a JavaScript app throws a SyntaxError while parsing a server response, the error message itself reveals what the server actually sent — such as plain text, an HTML error page, or the string 'undefined'. V8's error output truncates the unexpected content at 20 characters, giving developers a clue about the response type before they dig into network logs. A common but overlooked cause is fetch silently following redirects, delivering a 200-status login page instead of the expected JSON payload. The recommended fix is to read the response body as text first, then check the Content-Type header before attempting JSON.parse, so failures surface meaningful context rather than a cryptic parser error. Python's json.loads, by contrast, always reports 'char 0' for these same bad inputs, offering no equivalent diagnostic detail.