JSON Formatter & Validator

What makes JSON valid

JSON (RFC 8259) is stricter than the JavaScript it came from. Keys must be strings in double quotes; strings use double quotes too; there are no comments, no trailing comma after the last item, no undefined, NaN or Infinity, and numbers cannot have leading zeros or a bare decimal point. The validator checks every one of these rules and reports the first one broken, with a plain explanation instead of a cryptic engine message.

Line and column numbers count from 1, as in code editors, so you can jump straight to the spot in your own editor as well.

Unexpected character

Something that cannot start a value, such as a stray letter, or a missing quote earlier in the line.

Expected , or }

Two properties without a comma between them, or a missing closing brace.

Trailing comma

A comma after the last item of an object or array, which JavaScript allows and JSON does not.

Unterminated string

A string with no closing quote, often a line break inside a string, which must be written as \n.

Repairing near-JSON

Much of what gets pasted as JSON is really a JavaScript object, JSON5 or a commented config file. The repair turns it into strict JSON: it removes // and /* */ comments, drops trailing commas, rewrites 'single-quoted' strings with double quotes, quotes bare keys, and replaces Python's True, False and None. It reads the text with a tokenizer, so commas and quotes inside strings are never touched, and it keeps the line layout so later errors still point at the right line.

It deliberately does not guess at structural mistakes such as a missing comma or bracket. Those have more than one possible fix, and the validator points you to them instead.

Formatting without changing your data

Many formatters run the text through JSON.parse and back, which quietly rounds large numbers (an ID like 12345678901234567890 loses its last digits), rewrites 1.50 as 1.5, and moves numeric keys to the top of every object. This tool parses JSON into its own tree that keeps every number exactly as written and every key in its original order, so the only thing that changes is whitespace, unless you ask it to sort the keys.

Sorting compares keys by code point, as jq -S does, at every level of nesting. Sorted, consistently indented JSON is the easiest to compare; to see what actually differs between two documents use the JSON diff tool, and to turn JSON into YAML, CSV or TOML use the data converter.

Beautify or minify?

• Beautify for reading, reviewing and committing: one value per line makes diffs in version control small and clear. Two spaces is the most common indent in JavaScript projects, four is common in Python and Java, and tabs let each reader choose the width.
• Minify for sending and storing: removing whitespace typically shrinks pretty-printed JSON by 10–30%, and the output pane shows the size before and after.
• The output pane also counts the keys and the nesting depth, a quick sanity check on an unfamiliar payload.