How to fix the most common JSON syntax errors

JSON looks like JavaScript, which is exactly why it breaks: it is much stricter. Almost every “Unexpected token” error comes from one of a handful of habits carried over from code or config files.

Read the error position first

Parsers stop at the first character they cannot accept and report its position, either as a character offset (“at position 42”) or as a line and column. The real mistake is usually just before that point: a missing comma is reported at the start of the next value, not where the comma should have been.

Seven mistakes behind most errors

1. Trailing commas

{ "name": "Ada", "role": "admin", }

Remove the comma after the last item in every object and array.

2. Single quotes

{ 'name': 'Ada' }

Strings and keys must use double quotes.

3. Unquoted keys

{ name: "Ada" }

Valid in JavaScript, invalid in JSON. Every key is a double-quoted string: {"name": "Ada"}.

4. Comments

JSON has no comment syntax. Remove // and /* */ blocks, or move notes into a field such as "_comment" if the consumer ignores unknown keys.

5. Values JavaScript allows but JSON does not

undefined, NaN, Infinity, functions and dates are not JSON values. Use null, a number or an ISO date string such as "2026-10-15".

6. Unescaped characters in strings

A double quote or backslash inside a string must be escaped as \" or \\. A real line break inside a string is not allowed; write \n instead. Windows paths are a classic trap: "C:\\Users\\Ada".

7. Numbers with leading zeros or plus signs

007 and +5 are invalid. If leading zeros matter, as with postal codes or IDs, store the value as a string.

When the input is not really JSON

• JSON Lines / NDJSON: one object per line with no surrounding array. Parse line by line, or wrap the lines in [ ] and separate them with commas.
• JSON5 or JSONC: formats used by some config files that allow comments and trailing commas. A strict JSON parser will reject them, and that is correct behavior.
• Escaped JSON: text that starts with "{\" is a JSON string containing JSON. Parse it twice.

A quick debugging routine

1. Paste the text into a formatter and note the line and column.
2. Look at the character before the reported position for a missing comma, quote or bracket.
3. Fix one error at a time and re-validate; later errors are often side effects of the first.
4. Once it parses, pretty-print it so the structure is easy to review.