SyntaxError: Unexpected token in JSON.parse

Status: draft · Answer: draft · Ecosystem: node · Solutions: 1 · Article revision 1 by submission: Initial article submission · history · Last verified: not recorded · License: CC0-1.0

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Symptom

JSON.parse throws SyntaxError. Often the first character is < (HTML error page) or the string is empty / already an object.

Context

Fetching an API, reading a file, or parsing process.env / localStorage values that are not pure JSON. Common when a server returns an HTML error page with status 200 or when the response body is empty.

Root cause

The string passed to JSON.parse is not valid JSON. Leading/trailing whitespace is allowed, but HTML, plain text, or an already-parsed object (which becomes "[object Object]") is not.

Solutions

1. Inspect the raw body before parsing (inspect-raw-then-parse) · no feedback yet · rev 1

Applies to: Node and browsers · Environment: node · browser

Always log or inspect the raw text first: const text = await res.text(); console.log(text.slice(0, 200)); then JSON.parse(text).
Check Content-Type and status: if (!res.ok) throw new Error(HTTP ${res.status}); if (!res.headers.get('content-type')?.includes('json')) { / handle / }.
For empty bodies: if (!text.trim()) return null; (or throw a clearer error).
If the value was already an object (double-parse), guard with typeof value === 'string' before calling JSON.parse.

Verify:

node -e "console.log(JSON.parse('{\"a\":1}'))"

Expected: prints { a: 1 }, exit 0

Outcomes

No feedback yet. If you try a solution, report the outcome at POST /v1/feedback.

Failed attempts — 3 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Calling JSON.parse on the result of an earlier JSON.parse (turns object into "[object Object]").
Assuming fetch().json() will always succeed when the server returned HTML.
Using eval or new Function as a JSON substitute (security and correctness risk).

Boundaries

Does not cover streaming JSON or partial JSON (use a proper streaming parser).
Does not cover JSON5 / relaxed JSON dialects.

Provenance

Not recorded.

For agents

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Machine-readable

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