JSON Formatter

JSON is the lingua franca of web APIs and configuration files, but raw JSON from APIs or minified production builds is often a single unreadable line. The JSON Formatter transforms messy, compacted JSON into beautifully indented, readable structure, validates syntax to catch errors, and can minify JSON back down for production use.

Web developers debugging API responses use it to make sense of nested data structures, backend engineers validate configuration files before deployment, and frontend developers minify JSON payloads to reduce bandwidth. The tool also provides a tree view for navigating complex objects without scrolling through thousands of lines.

Because all processing happens in your browser using JavaScript, you can paste sensitive API responses, configuration files containing secrets, or internal data structures without worrying about data leaving your device. The validator pinpoints exact error locations with line numbers when syntax is broken.

Formatting runs your text through the browser's own JSON parser and re-serialises it at two-space indentation, so a document that formats successfully is guaranteed valid — the parser is the same one your code will use. Minifying is the same round trip with the whitespace argument dropped.

The repair option handles the mistakes people actually make when writing JSON by hand: keys left unquoted, strings in single quotes, and trailing commas before a closing brace or bracket. It is deliberately string-aware, meaning it only rewrites structure that sits outside a string literal. That distinction matters more than it sounds: a value like "http://example.com" or "12:30" contains a colon inside a string, and a naive repair would mangle both.

What it cannot do is guess intent. A genuinely truncated document, mismatched brackets, or a missing value has no single correct completion, so the parser error and the highlighted line are the useful output there rather than an automatic fix.

JSON is a deliberately small format and much of what people expect it to allow, it does not. Comments are not permitted in any form. Trailing commas are invalid. Keys must be double-quoted strings. Single quotes are never legal for strings or keys. Numbers cannot have leading zeros or a leading plus, and NaN and Infinity are not representable.

Two behaviours surprise people even in valid documents. On duplicate keys RFC 8259 commits only to "the names within an object SHOULD be unique", and is candid that practice diverges: many implementations keep the last pair, others report an error or refuse to parse the object, and some hand back every pair including the duplicates. Relying on any one of those is relying on your parser rather than on JSON — the browser parser behind this tool keeps the last occurrence. Key order is the second: most implementations preserve insertion order in practice, but nothing in the specification requires it.

If you need comments and trailing commas, the format you want is JSON5 or JSONC, both of which are supported by tooling but neither of which is JSON. Configuration files that appear to allow comments are generally being read by a parser that accepts a superset.