JSON Validator

Check whether text is valid JSON per RFC 8259. Errors are pinpointed with line, column and a caret marker — plus warnings for duplicate keys and precision-losing numbers that parsers accept silently.

Validation that points at the problem

Most parsers fail with “unexpected token at position 847” — technically true, practically useless. This validator scans your input against the JSON grammar (RFC 8259) and reports the failure as a line, a column, the offending line printed with a caret under the exact character, and a plain-English reason: a trailing comma, single quotes, an unquoted key, an unescaped line break. It also flags two things that are legal but dangerous: duplicate keys (parsers silently keep only the last value) and numbers long enough to lose precision.

Worked example

Input:

{
  "title": "Report",
  "tags": ["a", "b",],
}

Report:

✖ Invalid JSON

Line 3, column 20:
    "tags": ["a", "b",],
                     ^

Trailing comma before "]" is not allowed in JSON

When to use it

• Debugging a request body an API rejected with 400 Bad Request.
• Checking hand-edited config files before deploying them.
• Verifying JSON produced by string concatenation or templates.
• Auditing documents for duplicate keys that silently lose data.

Limitations to know

• Syntax only: a syntactically valid document can still violate your API contract — that is the JSON Schema Validator’s job.
• One error at a time: the scanner stops at the first violation, since errors after it are usually side effects of the first.

Common errors this catches

• Trailing commas, single quotes, unquoted keys — the classics from hand-editing.
• Unescaped line breaks or control characters inside strings.
• NaN, Infinity, undefined — JavaScript values that are not JSON.
• Multiple top-level values (two objects pasted back to back).

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