How to Format and Validate JSON

Formatted, minified, and the error if there is one

Paste JSON and you get it indented and minified together. If it doesn't parse, both boxes show the parser's own message with a position — not a bare "invalid".

Everything runs in your browser. Nothing is uploaded, which matters more than it sounds given what people paste into JSON tools.

{
  "name": "Ada",
  "active": true,
  "score": 9.5,
  "langs": [
    "ts",
    "py"
  ],
  "meta": {
    "id": 42,
    "tags": null
  }
}

{"name":"Ada","active":true,"score":9.5,"langs":["ts","py"],"meta":{"id":42,"tags":null}}

How to read the error

This is the part worth knowing, because parser messages are precise and slightly misleading at the same time.

Paste {"a":1,} and you get:

Invalid JSON — Expected double-quoted property name in JSON at position 7 (line 1 column 8)

Two things to understand:

It tells you what it expected, not what you did wrong. After a comma, the parser is looking for another key — so a trailing comma reports as a missing property name. Once you know that translation, "expected double-quoted property name" means "you left a comma on the end."

The position is where it gave up, not where you erred. Position 7 is the }. The actual mistake is the comma at position 6. Look at the character before the one it names.

The five things that break JSON

In rough order of how often they turn up:

Notice what these have in common: all five are legal JavaScript. That's exactly why they get typed by accident. JSON looks like a JavaScript object literal and is far stricter than one.

Minifying never changes your data

Formatted and minified parse to the identical value, so the minified form is safe anywhere the formatted one would have worked.

It isn't a pure whitespace strip, though — minifying re-serialises through the parser exactly as formatting does, so numbers come back in canonical form (see below). What it never does is change what the data means.

Whitespace inside a string is left alone — that's data, not formatting.

Why your number looks different

Paste {"n":1.50,"e":1e3} and you get back:

{
  "n": 1.5,
  "e": 1000
}

Nothing changed value — only spelling. The parser re-serialises numbers in canonical JSON form.

If the exact original text matters — a version like 1.10, an ID with leading zeros, a value beyond what a double can hold precisely — it should have been a string in the first place. That's not this tool being lossy; that's JSON numbers being numbers.

What JSON doesn't have

Worth keeping straight, because these are the things that come up when generating JSON from another language:

• No undefined, NaN, or Infinity
• No dates — they're strings by convention, usually ISO 8601
• No comments
• Numbers: no leading +, no leading zeros, no hex
• The top level can be any value, not just an object or array

Key order

Keys come back in the order they went in. JSON objects are formally unordered and most parsers preserve insertion order in practice.

If something downstream genuinely depends on key order, that's worth knowing about that thing, because the specification doesn't promise it to you.

A word about pasting

This runs entirely in your browser — no upload, no server, nothing stored.

Not every online JSON tool works that way, and the things that get pasted into them are frequently production API responses, configuration files, and credentials that were sitting inside them. Worth a habit of checking before pasting, wherever you happen to be.

Print the JSON debugging sheet if you're the person on your team who keeps getting handed a broken config file.