Common JSON Syntax Errors and How to Fix Them

JSON parsers are strict. A single stray character — an unquoted key, a trailing comma, or a single-quoted string — makes the entire document invalid. This guide walks through the errors you see most often in API responses, log exports, and hand-edited config files, and how to fix them systematically.

Why JSON parsers fail on small mistakes

Unlike JavaScript object literals, JSON (RFC 8259) allows only double-quoted strings, no comments, no trailing commas, and no undefined values. Parsers report the first failure they encounter, so the line number in the error may not be where you actually made the mistake — especially in minified payloads.

When debugging:

• Start from the character position in the error message if your tool provides it.
• Re-format (beautify) only after you have valid JSON, or use a repair tool that pinpoints the break.
• Compare against a known-good sample if the payload was copied from logs or chat.

Unexpected token — causes and fixes

SyntaxError: Unexpected token usually means the parser hit something illegal at the current position.

Unquoted keys

Invalid:

{ name: "Ada" }

Valid:

{ "name": "Ada" }

Object keys must be double-quoted strings. This is the most common error when pasting JavaScript objects into a JSON validator.

Single-quoted strings

Invalid:

{ "role": 'admin' }

JSON does not allow single quotes. Replace with double quotes and escape inner quotes: "He said \"hi\"".

Trailing garbage after valid JSON

Concatenated log lines or copy-paste artifacts often leave extra characters after a closing } or ]. Trim whitespace first; if the error persists, search for a second { or stray text after the document ends.

Trailing commas

In arrays and objects

Invalid:

{ "items": [1, 2, 3,], "ok": true, }

Remove the comma before ] and }. JSON5 and some JavaScript parsers allow trailing commas; standard JSON does not.

JSON5 confusion

Tools and APIs that accept JSON5 (comments, unquoted keys, trailing commas) are not validating strict JSON. If your API contract says JSON, strip JSON5 features before submission.

Unescaped strings and control characters

Newlines inside strings

Line breaks must be escaped as \n, not literal newlines inside "...".

Backslashes and Unicode

• Backslash starts an escape: \", \\, \/, \b, \f, \n, \r, \t, \uXXXX.
• Raw tab characters inside strings can break parsers copied from spreadsheets.
• Invalid \u sequences cause Unexpected token at the escape.

Smart quotes and invisible characters

Word processors replace " with curly quotes " ". Zero-width spaces from web copy break validation with no visible clue — paste into a hex view or re-type quotes if errors persist.

Numbers and literals

• No leading zeros: 01 is invalid.
• No NaN, Infinity, or undefined — use null or omit the key.
• No comments: // and /* */ are not JSON.

Fix workflow in BracketView

1. Paste the raw text into the JSON Validator — errors highlight at the character level.
2. Fix the reported issue; repeat until the status shows valid JSON.
3. Use Format to beautify for reading, or Minify before sending over the wire.
4. For large broken exports, use tree view to confirm structure after repair.

Related reading: JSON escape sequences and debugging malformed API responses.