JSON Formatter and Validator

The usual culprits are habits imported from JavaScript: a trailing comma after the last item, single quotes instead of double quotes, unquoted object keys, or NaN and Infinity as values. Copy-paste adds two more: smart quotes from Word or Slack, and invisible characters like non-breaking spaces. The validator above points at the exact line and column, and the Fix common mistakes button repairs comments, quote problems, bare keys, trailing commas, undefined, NaN and Infinity, showing you every change before it touches your text.

Most can, and that is exactly why this one exists. Tools that run your JSON through the JavaScript JSON.parse() and JSON.stringify() functions silently round integers beyond 9007199254740991 (an ID like 9007199254740993 becomes ...992), collapse duplicate keys to a single value, and rewrite tokens like -0, 1e3 or 2.370. This formatter re-emits your original tokens and only changes whitespace, so Format and Minify can never alter a value. Anything risky, like a duplicate key or an unsafe number, is shown as a warning instead of silently changed.

JSON itself allows numbers of any size, but JavaScript stores them as 64-bit floats, which are only exact for integers between -9007199254740991 and 9007199254740991 (2 to the 53rd minus 1). RFC 8259 warns about exactly this interoperability gap. A database ID like 9007199254740993 is perfectly valid JSON, but the moment JavaScript parses it, it silently becomes 9007199254740992. This tool preserves the digits exactly and flags the number, so you know to handle it as a string or with BigInt on the JavaScript side.

The grammar does not forbid them, but RFC 8259 says object names should be unique and that behavior is unpredictable when they are not: some parsers keep the first value, most keep the last, and a few error out. A formatter that quietly keeps one value destroys the evidence of the problem. This tool keeps every pair through Format and Minify, lists each duplicate with its line and JSON Pointer path, and only resolves them when you explicitly choose keep-first or keep-last, with Undo available.

Yes. Everything runs as JavaScript inside your own browser tab. Your JSON is never uploaded, sent to a server, stored or logged, and the page makes no network requests with your data. You can load the page, disconnect from the internet, and the formatter keeps working. That makes it safe for API responses containing tokens, customer records or production configuration.

Minify anything a machine consumes. Whitespace adds 20 to 40 percent to payload size before compression and buys nothing at runtime, since parsers ignore it. Pretty-print anything a human reads: files in version control, documentation examples and debug logs. Two-space indentation is the common convention for committed files because it keeps deep structures readable without excessive line width.

Documents up to 25 MB are formatted and validated; above that the tool refuses rather than freeze your tab, since everything runs on the page itself. Above 3 MB the interactive tree view and syntax coloring switch off to keep the page responsive, and formatting a document near the limit can pause briefly. For multi-hundred-megabyte files, use a streaming command-line tool like jq instead of any browser-based formatter.

Not to the specification: two objects with the same keys in different order are equivalent, and consumers must not rely on ordering. It matters to humans and to diffs. Sorting keys alphabetically makes committed config files deterministic, so version control shows real changes instead of reshuffles. The Sort keys A-Z toggle above reorders keys textually at every level while leaving every value token untouched.