JSON.parse Unexpected Token — Trailing Commas and Copy-Paste Failures

(Updated: July 16, 2026 ) JSON JavaScript debugging

Stack Overflow answer includes a trailing comma. Works in Chrome console because you pasted into an object literal, not through JSON.parse. Breaks CI on Friday when config hits Node. The error Unexpected token ] in JSON at position 847 is JSON’s way of saying “this is JavaScript cosplay, not JSON.”

  • “JSON is JavaScript” — JSON is a subset. No comments, no trailing commas, no undefined, no NaN, keys must be double-quoted strings.
  • “It works in VS Code” — VS Code JSONC mode allows comments; your production parser doesn’t.
  • “I’ll use eval()” — Please don’t. Use a formatter, fix the syntax, or use JSON5 explicitly if you control the parser.
  • Blaming the API — Half the time the bug is copy-paste from a log line that truncated or escaped badly.

Top offenders (with examples)

1. Trailing comma

{
  "name": "app",
  "version": "1.0.0",
}

2. Single-quoted keys

{ 'id': 1 }  // JS object literal — invalid JSON
{ "id": 1 }   // valid JSON

3. Unescaped control characters in strings

Log copied a multiline stack trace into a "message" field without \n escaping.

4. NaN / Infinity

{ "score": NaN }

Invalid — use null or omit the field.

5. Hex or octal numbers

JSON numbers are decimal only — 0xFF fails.

Reading the error message

Error snippetLikely cause
Unexpected token ]Trailing comma in array
Unexpected token }Trailing comma in object
Unexpected token 'Single quotes
Unexpected token u … undefinedundefined value
Unexpected end of JSON inputTruncated paste, missing closing bracket

Position number is byte offset — open in editor, go to column, or use a json formatter that highlights the line.

Debug workflow

  1. Paste into formatter — Local browser tool shows line/column of first error.
  2. Compare with schema — OpenAPI / Zod / JSON Schema expected shape vs actual.
  3. Minimize — Remove half the file until error disappears — binary search for large configs.
  4. Fix at source — If API returns bad JSON, fix server serializer; don’t patch client with regex forever.
  5. Add CI check — jq . config.json or jsonlint in pipeline before deploy.

JavaScript vs JSON quick reference

FeatureJS object literalJSON
Trailing commaAllowed (ES5+)Not allowed
CommentsAllowed in JSONCNot allowed
KeysUnquoted or quotedDouble-quoted only
undefinedAllowedNot allowed

Config file hygiene

  • Store env-specific JSON in separate files — don’t comment out keys with // in .json.
  • Use .jsonc extension + appropriate editor if you need comments — rename on build or use JSON5 loader explicitly.
  • Generate JSON from typed code (TypeScript, Zod) when possible — hand-edited JSON rots.

API responses that look like JSON but aren’t

Logs sometimes capture SSE lines (data: {...}) or NDJSON streams. Feeding the whole file into JSON.parse fails on line 2. Parse line-by-line or strip data: prefixes first. GraphQL errors wrapped in HTML error pages also produce “Unexpected token <” — that’s your CDN login page, not malformed JSON.

Tool: validate before deploy

The json-formatter tool validates, formats, and minifies JSON in your browser — catch trailing commas before they hit production.

→ Try the json-formatter tool

TL;DR

JSON.parse is strict. Trailing commas and single quotes are the usual suspects. Format locally, read line numbers, fix at source, add CI validation.