JSON Formatter & Validator

Format JSON with 2-space indentation, or minify it to a single line. Invalid input produces a specific error message instead of a blank result. Everything runs in your browser.

How formatting and minifying work

Both operations parse the input into a data structure and then serialize it again. Formatting writes it back with 2-space indentation and line breaks, minifying writes it with no whitespace at all.

A useful side effect: because the input must parse successfully before anything is written back, this doubles as a validator. If the output appears, the JSON is syntactically valid.

Minifying matters for transport. Whitespace in a JSON payload is meaningless to a parser but still costs bandwidth, so APIs typically send minified JSON and formatting is applied only for human reading.

JSON is stricter than JavaScript

Nearly every JSON error comes from assuming JSON accepts what JavaScript object literals accept. It does not. JSON is a deliberately minimal subset.

Valid in JavaScriptValid in JSONWhy
{name: "x"}NoKeys must be in double quotes
{'name': 'x'}NoSingle quotes are not allowed anywhere
{"a": 1,}NoTrailing commas are forbidden
// commentNoJSON has no comment syntax
{"a": undefined}NoOnly null exists, not undefined
{"a": 0x1F}NoNumbers must be decimal
{"a": .5}NoA leading digit is required, use 0.5
{"a": NaN}NoNaN and Infinity are not JSON values

The restrictions are intentional. JSON was designed to be trivially parseable in any language, so anything ambiguous or language-specific was left out.

What JSON does allow

The complete set of JSON value types is short:

Notably absent: dates, binary data, and comments. Dates are conventionally sent as ISO 8601 strings such as "2026-07-28T14:30:00Z", binary data as Base64 strings.

Examples

InputFormatted
{"a":1,"b":[1,2]} 
{
  "a": 1,
  "b": [
    1,
    2
  ]
}

Common errors and their causes

Broken inputProblemFix
{name: "Ada"}Unquoted key{"name": "Ada"}
{"a": 1,}Trailing comma{"a": 1}
{'a': 1}Single quotes{"a": 1}
{"a": "he said "hi""}Unescaped quote{"a": "he said \"hi\""}
{"a": True}Capitalized boolean{"a": true}

Practical notes

Large numbers lose precision

JSON numbers are parsed as IEEE 754 doubles in JavaScript, which safely represent integers only up to 253. A 64-bit database ID such as 9007199254740993 silently loses precision. The standard workaround is to transmit such IDs as strings.

Key order is not guaranteed

The JSON specification defines objects as unordered. Most implementations preserve insertion order in practice, but relying on it is unsafe. If order matters, use an array.

Escaping inside strings

Within a JSON string, the backslash, double quote, and control characters must be escaped. Newlines become \n, tabs \t, and a literal backslash \\. Forgetting to escape backslashes in Windows file paths is a frequent cause of parse failures.

JSON versus JSONC and JSON5

Some tools accept extended dialects. JSONC allows comments and is used by VS Code configuration files, JSON5 adds unquoted keys, trailing commas, and more. Neither is standard JSON, and generic parsers will reject them.

Frequently asked questions

Why does my input get rejected?

Most often because of a trailing comma, single quotes, or unquoted keys. All three are legal in JavaScript object literals but invalid in JSON. The error message names the position where parsing failed.

Can JSON contain comments?

No. The format has no comment syntax. If you need annotations, add a regular field such as "_comment", or use a dialect like JSONC if your parser supports it.

Does this handle very large JSON?

Yes, within your browser's memory limits. There is an input length guard to prevent the page freezing on an accidental multi-megabyte paste.

How should dates be represented in JSON?

JSON has no date type. The convention is an ISO 8601 string such as "2026-07-28T14:30:00Z", which sorts correctly as text and parses reliably in every language.

Why do large IDs come back wrong?

JavaScript parses JSON numbers as doubles, which lose precision above 253. Send large 64-bit identifiers as strings to preserve them exactly.

Is my JSON sent to a server?

No. Parsing and formatting happen entirely in your browser, which means you can safely format payloads containing production data or tokens.

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