What does the JSON Formatter do?
The KitLumi JSON Formatter turns compact JSON into readable, indented text and checks whether the input follows strict JSON syntax. As of September 2026, it also supports recursive key sorting, minification, clipboard actions, file download, and line-and-column error reporting.
Formatting changes whitespace, not the parsed JSON value. Minifying performs the reverse operation by removing optional whitespace. The parser follows the JSON grammar described in RFC 8259 (opens in a new tab).
Does this JSON Formatter send data to a server?
No. The JSON Formatter uses your browser’s built-in JSON parser and a local Web Worker for input at or above 100 KB. Your input is not submitted to a KitLumi endpoint, written to local storage, added to the page URL, or included in analytics by this tool.
How do you format and validate JSON?
- Paste or type JSON. Validation starts after a short pause. Use Sample if you want a safe example.
- Choose the output. Select two spaces, four spaces, or tabs. Turn on Sort keys or Minify only when needed.
- Fix or export. Invalid JSON shows a parser message with its line and column when available. Valid output can be copied or downloaded as
formatted.json.
Should you format or minify JSON?
Format JSON when a person needs to read, review, or debug it. Minify JSON when whitespace size matters and people do not need to edit the payload directly.
| Mode | Best for | Whitespace | Data value |
|---|---|---|---|
| Format | Code review, debugging, documentation | Adds indentation and line breaks | Unchanged |
| Minify | Compact payloads and storage | Removes optional spaces and line breaks | Unchanged |
What does formatted JSON look like?
A compact object such as {"name":"KitLumi","local":true} becomes the following two-space-indented JSON:
{
"name": "KitLumi",
"local": true
}Both versions represent the same object. The formatted version is easier to scan and produces clearer code-review diffs.
What are the most common JSON syntax errors?
| Invalid pattern | Why it fails | Valid replacement |
|---|---|---|
| {'name': 'Ada'} | JSON strings and property names require double quotes. | {"name": "Ada"} |
| {"a": 1,} | Strict JSON does not allow a trailing comma. | {"a": 1} |
| {name: "Ada"} | Property names cannot be bare identifiers. | {"name": "Ada"} |
| {"a": undefined} | undefined is not a JSON value. | {"a": null} |
| {"a": 1 // note} | JSON does not support comments. | Remove the comment or store it as a string field. |
What should you check before formatting sensitive or numeric data?
The JSON Formatter runs locally, but browser extensions and device policies are outside this page’s control. Use a trusted browser profile for secrets. JavaScript also represents ordinary JSON numbers as IEEE 754 double-precision values, so integers above 9,007,199,254,740,991 can lose precision. Quote large identifiers when every digit must remain exact.
Frequently asked questions about JSON formatting
Does the JSON Formatter upload my data?
No. The JSON Formatter parses and formats input inside your browser. The input is not sent to a KitLumi server, added to the URL, or stored by this page.
What is the difference between formatting and minifying JSON?
Formatting adds line breaks and indentation for people to read. Minifying removes optional whitespace for a smaller payload; both outputs represent the same JSON value.
Why is my JSON invalid?
Common causes are trailing commas, single-quoted strings, comments, unquoted property names, and missing brackets. Strict JSON requires double quotes and does not allow comments or trailing commas.
Can the JSON Formatter sort object keys?
Yes. Enable Sort keys to order keys recursively in every object. Array item order is preserved because changing array order would change the data.
Can I format very large JSON files?
Large input is processed in a Web Worker after it reaches 100 KB, which helps keep the page responsive. The practical limit still depends on the memory available to your browser and device.
Will the formatter preserve very large integers exactly?
Not always. This formatter uses the browser JSON parser, so integers above 9,007,199,254,740,991 can lose precision. Store identifiers and larger integers as quoted strings when exact digits matter.
What changed?
Added local formatting, strict validation, recursive key sorting, minification, Web Worker handling for larger input, copy, download, and parser error locations.