JSON Formatter / Validator

About this JSON formatter and validator

JSON (JavaScript Object Notation) is the most common wire format on the modern web. It is defined by RFC 8259 (IETF, 2017) and ECMA-404 (ECMA, second edition 2017), which agree on six value types (object, array, string, number, boolean, null) and a strict whitespace grammar. This page calls the browser's native JSON.parse to validate, then JSON.stringify(value, null, indent) to pretty-print or re-emit minified. Validation errors include the position the engine reports so you can jump to the offending byte.

Pretty-printing matters because minified JSON is unreadable: a 12-key object collapses to one 600-character line with no indentation, no key alignment, no rule boundaries. After formatting, the same object spans 14 lines with a two-space indent and nested objects visibly recurse one level deeper per {. The output parses identically; only whitespace changed.

How the formatter works

The validator and formatter are a thin shell over JSON.parse and JSON.stringify, both shipped natively in every browser since Internet Explorer 8.

try {
  const value = JSON.parse(input);                  // throws SyntaxError on invalid input
  const pretty = JSON.stringify(value, null, 2);    // two-space indent
  const min    = JSON.stringify(value);             // minified, no whitespace
} catch (err) {
  // err.message includes the position: 'Unexpected token } in JSON at position 47'
}

The engine's parser is a strict RFC 8259 implementation: trailing commas, single quotes, unquoted keys, comments, and bareword identifiers all fail. The error message tells you the byte offset of the first invalid character so a simple input.slice(0, position) walks straight to the bug.

Worked example: validating and pretty-printing

Suppose you paste this minified API response (89 bytes):

{"user":{"id":42,"name":"Ada","roles":["admin","editor"],"verified":true,"score":3.14}}

The validator confirms 'Valid JSON' and the formatter emits (162 bytes):

{
  "user": {
    "id": 42,
    "name": "Ada",
    "roles": [
      "admin",
      "editor"
    ],
    "verified": true,
    "score": 3.14
  }
}

JSON value types reference

JSON quick reference

JSON (JavaScript Object Notation) is a text data format defined by ECMA-404 and RFC 8259. It supports six value types and is the default wire format for most modern APIs.

JSON vs JSON5 vs JSONC vs YAML

Common mistakes

• Trailing commas - JSON forbids them. {"a": 1,} is invalid. JavaScript object literals allow them; JSON doesn't.
• Single quotes - JSON requires double quotes. {'a': 1} is invalid.
• Unquoted keys - keys must be quoted strings, even if they look like identifiers.
• NaN, Infinity, undefined - none of these are valid JSON values. Most parsers reject them.
• BigInt - JSON numbers are IEEE 754 doubles. Integers above 2^53 - 1 (~9 quadrillion) lose precision. Use strings for IDs from Twitter, Discord, Snowflake-based systems.
• Date handling - JSON has no date type. The convention is ISO 8601 strings: "2026-05-13T12:00:00Z". JSON.parse won't auto-parse them; you must convert.