10 common JSON errors and how to fix them
Every JSON syntax error you'll ever see — trailing commas, unquoted keys, bad escapes — with a working repair strategy.
By JSON Formatter Team
1. Trailing commas
JSON forbids a comma after the last item in an object or array. {"a":1,} is invalid — remove the comma.
2. Single quotes
Keys and strings must use double quotes. {'a':1} is not JSON. Convert every ' around a key or value into ".
3. Unquoted keys
{a:1} is JavaScript, not JSON. Every key must be a double-quoted string.
4. Comments
Standard JSON has no comments. Use JSONC only where the consumer expects it (VS Code settings, tsconfig). Strip // and / / before shipping.
5. Unescaped characters in strings
Newlines, tabs and backslashes inside a string must be escaped: \n, \t, \\.
6. Wrong booleans / null
Only lowercase true, false and null are valid. True, None, nil are not.
7. Leading zeros on numbers
007 is invalid. Use 7 or a string.
8. Duplicate keys
Technically valid but universally discouraged — most parsers silently keep only the last value.
9. Encoding issues
Save JSON as UTF-8 without a BOM. The BOM (\uFEFF) trips many parsers.
10. Mismatched brackets
The classic. Use the JSON Formatter — it points at the exact line and column of the first mismatch.
Auto-repair
Our formatter's Repair button applies safe fixes for the first six errors above automatically. Paste, click Repair, then Beautify.