How to validate JSON (and fix common errors)

Valid JSON follows a few strict rules. When parsing fails, it's almost always one of a handful of mistakes — here they are, with the fix.

The usual culprits

• Trailing comma. [1, 2, 3,] → JSON allows no comma after the last item: [1, 2, 3].
• Single quotes. {'a': 1} → strings and keys must use double quotes: {"a": 1}.
• Unquoted keys. {a: 1} → keys must be quoted: {"a": 1}.
• Missing comma between items, or a comma where none belongs.
• Comments. JSON has none — remove // and /* */.
• Wrong/again unmatched brackets — every { needs a }, every [ a ].
• Undefined / NaN / trailing text — only true, false, null, numbers, strings, arrays, objects are allowed.

The fastest way to find the error

Rather than scan by eye, paste the JSON into a validator that points at the exact spot. The parser reports the position, so you jump straight to the broken line instead of hunting.