JSON Formatter & Validator

How it works

The formatter pipes your input through JSON.parse, the browser's native parser. Valid JSON must be UTF-8, use double-quoted strings, and contain only six value types — objects, arrays, strings, numbers, booleans, and null. Property names must be quoted, numbers cannot carry leading zeros, and trailing commas are rejected. On success the tool re-serializes with JSON.stringify: pretty-print mode passes a null replacer plus an indentation argument of 2 or 4 spaces, while minify mode omits the indentation so all whitespace is stripped. On failure the engine's error message is parsed to report the exact line and column — V8 reports “at position N” while SpiderMonkey reports “line X column Y” — so you can jump straight to the fault. The result is validated live as you type, so the Valid or Invalid badge updates on every keystroke.

Use cases

Pretty-print API responses returned by curl or Postman, inspect minified bundles, validate config files before deployment, tidy log entries into readable diffs, or teach JSON syntax. Because nothing leaves the browser, it is also a safe place to reformat tokens and payloads you would rather not paste into a remote service. It also normalizes whitespace before you commit JSON to source control, so diffs stay clean and reviewable.

Troubleshooting

• “Unexpected token” — look for single quotes, unquoted keys, or a trailing comma.
• “Unexpected end of JSON input” — usually a missing closing brace or bracket.
• “Unexpected token” at position 0 — the file may carry a UTF-8 byte order mark (BOM, U+FEFF). Re-save it without a BOM, or strip the leading U+FEFF character before parsing.
• Duplicate keys collapse silently — JSON.parse keeps only the last occurrence, so run a linter if duplicates matter.
• Large inputs freeze — parsing is synchronous, so multi-megabyte documents may stall the tab.
• Precision loss — numbers are IEEE 754 doubles, so integers larger than 2^53 lose precision on re-serialization.