JSON Formatter & Validator

Six data types

• Object — {} with key:value pairs
• Array — [] ordered list
• String — "..." double quotes only, with escapes
• Number — integer or float, no leading zero, no NaN/Infinity
• Boolean — true or false (lowercase)
• Null — null (lowercase)

Strict requirements

✓ Valid JSON
{
  "name": "Alice",
  "age": 30,
  "hobbies": ["coding", "reading"],
  "active": true,
  "address": null
}

✗ Invalid (common mistakes)
{
  name: "Alice",         // unquoted key
  'age': 30,             // single quotes
  "hobbies": [           // trailing comma below
    "coding",
    "reading",
  ],
  "active": True,        // capitalized boolean
  "comment": /* nope */  // comments not allowed
}

Encoding

JSON files must be UTF-8 (RFC 8259 mandates Unicode). String escapes: \", \\, \n, \r, \t, \b, \f, \/, and \uXXXX for arbitrary code points. UTF-8 BOM is explicitly forbidden.

JSON variants

• JSONC — JSON with Comments (used by VSCode, TypeScript). Allows // and /* */. NOT valid JSON.
• JSON5 — looser superset (unquoted keys, trailing commas, single quotes). NOT valid JSON.
• JSON Lines / NDJSON — one JSON value per line. Used for streaming/log files.
• JSON Schema — schema definition language for JSON, used for API validation.

1. Trailing commas

[1, 2, 3,] is invalid JSON (allowed in JS, allowed in JSON5, NOT in standard JSON). Many parsers produce confusing error messages on this.

2. Single quotes

JSON strings must use "double quotes" only. Many developers paste from JavaScript and forget — 'value' throws an error.

3. Numeric precision (Number.MAX_SAFE_INTEGER)

JavaScript represents numbers as 64-bit floats. Integers above 2⁵³ (9,007,199,254,740,992) lose precision — JSON.parse('9999999999999999') becomes 10000000000000000. For large IDs (Twitter snowflakes, PostgreSQL bigints), serialize as strings.

4. NaN and Infinity

JSON doesn't allow NaN, Infinity, or -Infinity. JSON.stringify(NaN) in JavaScript returns "null" silently — easy to miss in tests. Validate before serialization.

5. Date serialization

JSON has no native Date type. JSON.stringify(new Date()) produces an ISO 8601 string, but JSON.parse() gives back a string, not a Date. Use a custom reviver: JSON.parse(json, (k, v) => isoRegex.test(v) ? new Date(v) : v).

6. Comments are not allowed

Standard JSON forbids // and /* */. If you need comments in config files, use JSONC (VSCode pattern) or YAML/TOML instead.

Is my data safe?

Yes. This tool runs entirely in your browser using native JSON.parse and JSON.stringify. No data is transmitted to any server — verify in DevTools Network tab.

What is the maximum JSON size I can format?

Limited by your device memory. Files up to 10 MB process instantly. 100 MB+ may freeze the browser briefly. For multi-GB JSON streams, use jq on the command line or stream parsers (JSONStream, oboe.js).

Can I use this tool offline?

Yes — once the page loads, all formatting works without internet. No external API calls.

What's the difference between JSON and JavaScript objects?

Every valid JSON is valid JavaScript object syntax, but not vice versa. JS objects allow unquoted keys, trailing commas, single quotes, comments, methods, and references — JSON allows none of these. JSON is strictly serializable; JS objects are runtime constructs.

Why does my JSON validator say "Unexpected token"?

Most common causes: trailing comma, single quotes, unquoted keys, missing closing brace/bracket, line breaks inside strings (need \n). Use this tool's Validate feature to get exact line/column.