JSON Formatter

JSON formatting and common parse errors

The JSON formatter parses your text with the browser JSON.parse rules, then reprints either a pretty-indented document or a compact single-line form depending on the mode you choose. Successful parsing means the value can be represented as ordinary JSON data; it does not preserve original spacing, key order guarantees across all engines, or comments—because standard JSON has no comments.

Useful scenarios

• Pretty-printing an API response before reading nested fields.
• Compacting a config file to compare byte size roughly.
• Validating that a hand-edited snippet still parses.
• Catching trailing commas before a deploy script fails.
• Sharing a cleaned example in documentation.

Errors you will see often

• Single quotes around strings instead of double quotes.
• Trailing commas after the last array or object element.
• Unquoted keys such as {name: "Ada"}.
• JavaScript-only literals like undefined, bare NaN, or comments starting with //.
• Smart quotes copied from a word processor instead of ASCII ".
• Duplicate keys: parsers typically keep one value, so duplicates are a data smell even if parsing succeeds.

Worked example

Input (invalid): {'a': 1,}
Why it fails: single quotes and trailing comma.

Input (valid): {"a": 1, "b": [true, null]}
Pretty output sketch:
{
  "a": 1,
  "b": [
    true,
    null
  ]
}

Large integers beyond JavaScript’s safe integer range may lose exactness when parsed as numbers. If exact digits matter, keep them as JSON strings in the source system.

Common mistakes

• Treating “formatted” as “encrypted” or “safe to publish.”
• Expecting key order and whitespace to round-trip identically.
• Pasting JSON5 or YAML and expecting JSON.parse to accept it.
• Overwriting a production file without keeping the original bytes.

Limits

Local parsing only—no schema validation, no JSON Schema draft checking, and no streaming for multi-gigabyte files. Educational and convenience use inside the browser’s memory limits.