JSON formatter and validator

Finding the error, not just reporting one

The useful part of a JSON validator is not "invalid JSON" — it is where. Browsers report a character offset, which is useless in a 4,000-character payload, so that offset is converted here into a line and column number you can actually navigate to.

The overwhelming majority of JSON errors are one of four things: a trailing comma after the last item, single quotes instead of double, an unquoted key, or an unescaped quote inside a string. If the reported position looks fine, check the line above it — a missing comma is usually reported at the start of the next token.

Formatting options

Pretty-printing with two or four spaces, or tabs, is the readable form for editing and for diffs. Minifying strips every unnecessary byte, which is what you want in a payload or a config that will be read by machines rather than people.

Sorting keys alphabetically is the underrated one. Two JSON objects with identical content but different key order produce a completely unreadable diff; sorting both first makes the real differences visible. It is the standard trick for comparing API responses.

What is not JSON

Comments, trailing commas, single-quoted strings, and unquoted keys are all JavaScript object syntax, not JSON. They are the most common reason a config file that "looks fine" is rejected. JSON5 and JSONC allow them; strict JSON parsers, including this one, do not.

It runs in your browser

Nothing is uploaded — which matters, because JSON pasted into a formatter routinely contains API keys, tokens and customer records. Any formatter that posts to a server has seen all of it.