Numarly

JSON Formatter & Validator

Format, minify, and validate JSON, with instant error messages.

Valid JSON

What is it?

This tool validates JSON text, reformats it with readable indentation, and produces a minified single-line version — all updated instantly as you type.

Formula Explanation

JSON is parsed using the browser's built-in JSON.parse(), which strictly enforces the JSON specification (double-quoted keys and strings, no trailing commas, no comments). Formatting re-serializes the parsed data with JSON.stringify() using your chosen indentation, while minifying serializes it with no extra whitespace at all.

Example Calculation

{"a":1,"b":2} formats to a readable multi-line block with each key on its own indented line, or stays as a single compact line when minified.

How to Use

  1. Paste your JSON into the input box.
  2. Valid JSON shows both a Formatted and Minified version instantly.
  3. Invalid JSON shows an error message describing what's wrong.
  4. Choose your preferred indent width, then copy whichever version you need.

Benefits

  • Validates and formats in one step, so you immediately see both the error and the fix.
  • Produces both a readable formatted version and a compact minified version at once.
  • Runs entirely in your browser, so sensitive JSON payloads are never uploaded anywhere.

Use Cases

  • Debugging a malformed API response by seeing exactly what part is invalid.
  • Formatting a minified JSON config file to make it readable for editing.
  • Minifying a formatted JSON payload before sending it over a network to save bytes.

What Your Result Means

The error message reflects exactly where the browser's JSON parser stopped succeeding — often pointing near (but not always exactly at) the character position of a missing comma, quote, or bracket.

Tips

  • JSON requires double quotes for all keys and strings — single quotes are invalid.
  • Trailing commas after the last item in an array or object are invalid JSON, unlike in JavaScript object literals.
  • If validation fails right after pasting, check for accidentally included comments — JSON doesn't support them.

Common Mistakes

  • Using single quotes instead of double quotes around strings and keys.
  • Leaving a trailing comma after the last property or array item.
  • Pasting JavaScript object syntax (like unquoted keys) instead of strict JSON.

FAQs

Does this tool send my JSON data anywhere?

No, all parsing, formatting, and minifying happens locally in your browser — nothing is uploaded to a server.

Why does it say my JSON is invalid when it looks fine?

Common causes are trailing commas, single quotes instead of double quotes, or unquoted object keys — all invalid in strict JSON even though some are valid in JavaScript.

What's the difference between formatted and minified JSON?

Formatted JSON adds line breaks and indentation for readability, while minified JSON removes all unnecessary whitespace to reduce file size for transmission.

Can this tool handle very large JSON files?

It can handle reasonably large JSON, but extremely large files (many megabytes) may slow down your browser tab since parsing happens in memory.

JSON is parsed and formatted locally in your browser and is never sent to or stored on a server.

Last updated: July 27, 2026