JSON::ParserError: 859: unexpected token at 'undefined': what it means and how to fix it

What the error means

JSON::ParserError: 859: unexpected token at 'undefined' means the parser expected valid JSON but encountered a character/token that cannot appear there. In practice, it usually means the input is not JSON (or not strict JSON), or it is incomplete.

Most common real-world causes

• The input is not strict JSON (single quotes, comments, trailing commas).
• The response is truncated (missing closing } / ] / ").
• The response contains a prefix/suffix (logs, banners, debug output).
• The payload has invalid escapes or control characters in strings.
• You are parsing the wrong thing (already-parsed object, HTML, CSV, etc).

Fast debugging steps

• Validate the JSON as-is to get an exact error location.
• Check the first and last 50 characters for truncation and prefixes.
• Fix one issue at a time and re-validate after each change.

Code example (ruby)

const text = String(input);
console.log(text.slice(0, 200));

const obj = JSON.parse(text);

Fix without uploading data

If the JSON contains sensitive data, validate and fix it locally. No Upload Tools runs 100% in your browser.

• JSON Validator to pinpoint the exact syntax error.
• JSON Repair to remove comments/trailing commas when the source is not strict.
• JSON Formatter to pretty-print and inspect structure.
• JSON String Escape if the issue is inside a string (newlines/tabs/quotes).

Workflow: validate -> fix the first error -> validate again -> export/convert.

FAQ

Does the exact token matter? Yes. The token often hints at the root cause: < is usually HTML, u is often undefined, and / often points to comments.

Should I just regex-fix JSON? Avoid blind regex edits. Validate after each change so you know what you fixed and what broke.