API responses, config files, and log dumps often arrive as one unreadable line of JSON. A JSON formatter parses the text, flags syntax errors with context, and reprints it with indentation—or strips whitespace for smaller payloads.

This guide covers valid JSON rules, common errors, and Toolsle’s free JSON formatter.

Valid JSON rules (RFC 8259)

• Double quotes for strings and keys ("key", not 'key')
• No trailing commas after the last array/object item
• Lowercase true, false, null
• No comments in strict JSON

JavaScript object literals are not JSON until you fix those differences.

Pretty-print vs minify

Formatting does not change values—only whitespace.

Typical workflow

1. Paste response from curl, Postman, or browser DevTools
2. Fix parse errors (trailing comma, unquoted key)
3. Sort keys if you need a stable diff for git
4. Copy formatted output into tickets or documentation

Pair with the regex tester when extracting JSON from mixed logs.

Common mistakes

• Pasting JSON5 or JSONC (comments) into strict parsers
• Assuming NaN or Infinity are valid (they are not in JSON)
• Committing secrets in formatted JSON—rotate keys if leaked

Try the free JSON formatter

Open the JSON formatter, paste your payload, and click Format or Minify. Errors show with line context when parsing fails.