JSON Formatter & Validator — Prettify JSON Online

Debugging a 10,000-character minified API response by hand is impossible. Paste your raw or broken JSON here and get a readable, indented, colour-coded output in milliseconds — with the exact error line highlighted if your JSON is invalid.

Format, validate, and prettify raw or minified JSON instantly in your browser. Highlights syntax errors on the exact line, supports nested structures, and outputs clean indented JSON. Zero data upload — your payloads never leave your device.

Valid JSON requires double-quoted keys, no trailing commas, and no comments.

How to Use the json formatter

The tool runs your input through the browser's native JSON.parse() engine, catches syntax errors with line/column positions, and re-serialises valid JSON using JSON.stringify(null, 2) for readable 2-space indentation.

1Paste your raw, minified, or potentially broken JSON into the input panel.
2The formatter validates and prettifies instantly — any syntax errors are shown with the exact line number.
3Copy the formatted output with one click, or use the minify toggle to compress it back for production use.

How JSON Validation & Formatting Works

JSON must follow strict RFC 8259 syntax: all keys must be double-quoted strings, values must be valid types (string, number, boolean, null, array, or object), no trailing commas are allowed, and comments are not permitted.

Valid JSON   = JSON.parse(input) succeeds
Formatted   = JSON.stringify(parsed, null, 2)
Minified    = JSON.stringify(parsed)

Common Reference Values & Comparisons

Refer to this table to study common calculations, reference ratios, or performance profiles for this tool:

Common JSON Error	Root Cause & Fix
Unexpected token "'"	Single quotes used — JSON requires double quotes
Trailing comma	Remove the last comma in the array or object
Unexpected token 'u'	'undefined' is not valid JSON — use null
SyntaxError at pos 0	Input is empty or is not JSON
Circular reference	Object references itself — flatten structure first

Frequently Asked Questions (FAQ)

No. All parsing and formatting is done using the browser's built-in JSON engine. Your payloads, tokens, and API keys never leave your device.

Yes. Since processing is done in browser memory it can handle several megabytes of JSON. Very large files (50MB+) may briefly freeze the tab while JavaScript parses the structure.

Formatting (prettifying) just adds whitespace for readability and requires the JSON to already be valid. Validation checks whether the JSON follows the RFC 8259 specification. This tool does both: it validates first, shows an error with line number if invalid, and formats if valid.

No. This tool validates strict JSON only (RFC 8259). JSON5 and JSONC are supersets of JSON — they allow comments and single quotes — but they are not standard JSON and will correctly show a syntax error here.

Yes. After formatting, use the minify option to produce a compact single-line JSON string optimised for API payloads and configuration files.

A JSON formatter is an essential bookmark for any developer working with APIs, databases, or configuration files. Keep this tool open alongside your API client to instantly debug malformed responses without sending data anywhere.