How to Validate JSON (and Fix Common Errors)

What 'valid JSON' actually means

JSON has a strict, small grammar: object keys and string values are double-quoted, numbers have no leading zeros or trailing decimal points, and there is no trailing comma after the last item in an array or object. A single stray character breaks the whole document — unlike HTML, JSON parsers do not guess at your intent.

The most common errors

• Trailing commas — a comma after the last item in an array or object, often left over from adding or removing an item.
• Single quotes instead of double quotes — valid in JavaScript object literals, invalid in JSON.
• Unquoted or badly quoted keys — {name: "value"} instead of {"name": "value"}.
• Unescaped special characters inside strings, such as a literal quote or backslash.
• Mismatched brackets or braces — an extra or missing } or ] in a nested structure.
• Comments — JSON has no comment syntax; // or /* */ left over from JavaScript will break it.

How to find the actual line

Most validators only tell you "unexpected token" and a position — paste the JSON into the JSON Formatter and use Validate; it points at the exact character, which is usually faster than scanning by eye in a deeply nested payload.

A quick workflow

• Paste the raw response into JSON Formatter and click Format — this alone reveals many bracket-mismatch errors by making the indentation visibly wrong.
• Run Validate and jump to the reported error.
• Fix one error at a time; a syntax error can sometimes mask a second one further down.
• Once it is clean, open it in JSON Viewer if you need to browse a large or deeply nested structure.