JSON formatter and validator

What a JSON formatter does — and why it matters

JSON (JavaScript Object Notation) is the lingua franca of web APIs, configuration files and structured logs. It is built from four simple things: objects ({}), arrays ([]), a handful of primitive values (strings, numbers, true, false, null) and the whitespace between them. That whitespace is purely cosmetic — a parser ignores it — which is exactly why JSON moves between two states. Over the wire and in storage it is usually minified to a single dense line to save bytes; in a code editor or a debugger you want it pretty-printed across many indented lines so the structure is obvious. A formatter is the round trip between those two states, plus a validation pass that tells you whether the document is well-formed in the first place.

Strict JSON vs. the lookalikes (JSON5, JSONC)

This tool validates against RFC 8259, the official JSON standard, using the browser's own JSON.parse() engine. That matters because a lot of text that looks like JSON is not actually valid JSON. The standard forbids comments, trailing commas, single-quoted strings, unquoted object keys, hex or octal numbers, leading + signs, and special values like NaN or Infinity. Those conveniences belong to extensions such as JSON5 and JSONC (the "JSON with comments" used by VS Code and tsconfig.json). By holding the line at strict JSON, the formatter guarantees that anything it accepts is something your API, your database column or another language's parser will also accept — no surprises in production because your editor was lenient.

The errors that actually break your JSON

When validation fails, the status line shows the first problem the parser hits. The same handful of mistakes account for the vast majority of "Unexpected token" errors:

• Trailing commas — a comma after the last item in an object or array ([1, 2, 3,]). Legal in JavaScript, illegal in JSON. Delete it.
• Single quotes — JSON strings and keys must use double quotes. {'name': 'x'} is invalid; it must be {"name": "x"}.
• Unquoted keys — {name: "x"} is a JavaScript object literal, not JSON. Every key needs double quotes.
• Comments — // like this or /* this */ are not allowed. Strip them, or use a JSONC-aware tool.
• Wrong primitives — undefined, NaN, function calls and dates have no JSON representation. Use null, a number, or an ISO-8601 string.
• Trailing or duplicate content — two objects pasted back to back, or a stray character after the closing brace, ends parsing early.

Because the error reports the position of the first failure, work top to bottom: fix the earliest error, re-run, and the next one (if any) surfaces. A document with no errors pretty-prints immediately.

Indent, minify, sort — what each option is for

Indent (2 spaces, 4 spaces, tab) controls how deeply nested levels are pushed right. Two spaces is the de-facto web default and what most linters expect; tabs suit teams that configure indent width in the editor. Minify strips every non-essential byte to one line — the form you ship in an API response or bundle into a build, where it commonly removes 10–30% of the size before compression even runs. Sort keys recursively alphabetises every object's keys (leaving array order untouched, since order carries meaning in arrays). Sorting gives you a canonical form: two responses that differ only in key order collapse to identical text, which makes diffs clean, config deduplication trivial, and snapshot tests deterministic.

Everything stays in your browser

Formatting, validation and sorting all run locally through the browser's native JSON engine — nothing is uploaded, logged or sent to a server — so working with configuration that contains secrets, an API response with customer data, or a token you are debugging means none of it leaves the tab. (As a general habit, still treat production secrets carefully in any browser-based tool.) It also means there is no per-request size cap: the only limit is your device's memory, so multi-megabyte documents work, just more slowly than small ones. When you need to compare two JSON documents rather than tidy one, hand off to the JSON diff tool; to reshape JSON into a spreadsheet, use JSON to CSV. Working with JSON in code? Our guide to JSON in JavaScript covers JSON.parse/stringify, pretty-printing and the gotchas behind most parse errors.