Why 'Unexpected Token' JSON Errors Are Usually Not a Parser Problem

When developers encounter a SyntaxError from JSON.parse() or res.json(), the root cause is often not a malformed JSON file but a flawed HTTP request returning HTML instead of JSON. Common triggers include wrong URLs, missing authentication, or expired tokens, which cause the response to begin with HTML markup rather than valid JSON. In cases where the JSON itself is genuinely malformed, typical culprits are trailing commas, single quotes, unquoted keys, comments, or Python-style values like True and None. Double-parsing a fetch response is another frequent mistake, since res.json() already handles parsing and calling JSON.parse() on top of it breaks the flow. Developers can diagnose and fix these issues using online validators to locate the exact error line and repair tools that convert lenient syntax to strict JSON.