JSON Formatter & Validator

Pure client-side processing. Your data never leaves your computer.

Drop a .json file or click to browse

Input JSON

Indent

Sort KeysAuto-Format

Formatted Output

What is a JSON Formatter?

JSON (JavaScript Object Notation) is the standard data format used for APIs and web configuration. Often, JSON data is minified into a single, unreadable line to save space. Our JSON Formatter instantly parses, indents, and color-codes your raw JSON, making it perfectly readable for developers debugging API responses.

Is formatting JSON here secure?

Absolutely. ZeroServer.tools is completely client-side. Unlike other online formatters that send your sensitive API payloads to a backend server, all parsing happens securely inside your own browser. Your JSON never leaves your device.

Built and maintained by Meet Shah · Last updated 24 July 2026

What this tool is used for

Checking whether a webhook or API response you captured is valid JSON at all, before feeding it to a parser whose error message is worse than this one's.
Making a single-line minified payload readable — logs and browser network panels both emit JSON with no line breaks, which is unreadable past about eighty characters.
Finding the one field you need in a deeply nested response by collapsing the tree instead of scrolling several hundred lines.
Shrinking a config or fixture before committing it, where whitespace is pure diff noise, or before shipping it over the wire.
Comparing two responses that should be identical by pretty-printing both with the same key order, so a real difference stops hiding behind formatting.
Sanity-checking JSON that contains secrets — an access token, a customer record — without pasting it into a tool that posts it to a server.

How it works in practice

A worked example

You have lifted a config block straight out of a JavaScript file and need it as real JSON for a service that will reject anything else.

Input

{
  // service account, rotated 2026-01
  name: 'billing-worker',
  retries: 3,
  tags: ['prod', 'eu-west',],
}

Output

{
  "name": "billing-worker",
  "retries": 3,
  "tags": [
    "prod",
    "eu-west"
  ]
}

Four things changed. The comment is gone, the single quotes became double quotes, the bare name key acquired quotes, and the trailing comma after 'eu-west' disappeared — every one of them legal in a JavaScript object literal and none of them legal in JSON. Strict parsing gives up at line 2, column 3, on the comment, so the tool falls back to a real JSON5 parser and re-serialises what it gets. That fallback is a parser rather than a set of repair patterns, which is why it cannot mangle the contents of a string that happens to contain a brace or a comma of its own.

The edge case that catches people

The lenient fallback is a convenience with a sharp edge: a payload that formats cleanly here can still be refused by the service you are sending it to. Strict parsing runs first and only hands over to JSON5 when it fails, so a clean result tells you the tool understood your input, not that the input was valid. Copy what is in the output panel, which is the strict re-serialised version; the thing you pasted may not parse anywhere else.

When not to use this tool

Reformatting is not querying. To pull one field out of a large response, or to reshape it, jq does in a single expression what a tree view makes you scroll for. Annotations do not survive either, because the document is parsed and written out fresh — a JSONC or JSON5 config keeps its data and loses every comment, so edit those in a text editor. And the whole document is held in memory at once, which makes a multi-gigabyte log a job for a streaming parser rather than a browser tab.

Frequently Asked Questions

Does pasting JSON here send my data to a server?: No. All formatting runs in your browser via JavaScript. Your JSON never leaves your device — not even for error checking. The address bar hash contains only your local state.
Why is my JSON showing an error?: Common causes: trailing commas (not valid in JSON), single quotes instead of double quotes, unquoted object keys, and undefined or NaN values. The error indicator shows the exact line and column of the issue.
What's the difference between pretty-print and minify?: Pretty-print adds indentation and newlines for readability. Minify removes all whitespace to produce the smallest possible string — useful for reducing payload size when sending JSON over a network.
Can I format very large JSON files?: Yes. Since formatting runs in your browser, the limit is your device's memory, not a server timeout. Files over 10 MB may take a few seconds but won't fail mid-process.
What does the JSON tree view show?: Tree view renders your JSON as a collapsible, expandable hierarchy. Click any node to expand or collapse its children — useful for navigating deeply nested objects without reading raw text.
Are duplicate keys allowed?: The specification does not forbid them, and parsers differ — most JavaScript implementations keep the last, some libraries keep the first, and a few error. Anything relying on that behaviour is relying on an implementation detail.
What happens to a number bigger than JavaScript can hold?: It loses precision silently. Integers above 2^53 round on parse, so a 19-digit id comes back with its last digits changed. Transporting such ids as strings is the standard fix, and the reason many APIs do.

Common errors and gotchas

"Unexpected token ’ in JSON" — a word processor or chat client replaced your straight quotes with typographic ones. JSON only accepts U+0022. Retype the quotes rather than hunting for the invisible character.
A trailing comma after the last element. JavaScript object literals allow it, JSON does not, and this is the single most common paste-from-code failure.
Single-quoted keys or values. Valid JavaScript, invalid JSON — if it came out of a JS file rather than off the wire, it is probably an object literal, not JSON.
Copying a payload out of a terminal that wrapped it: the wrap inserts real newlines mid-string, which is a parse error rather than a formatting one.
A leading byte-order mark from a file saved as UTF-8-with-BOM. Invisible in every editor, and it makes the very first character illegal.
Expecting the formatter to fix the document. It reports the first structural error it finds and stops, so a payload with three problems takes three passes.
Long integer ids that survive the round trip looking different. That is not a formatter bug — anything above 2^53 loses precision in any JavaScript parser.