How the JSON Formatter Validates and Pretty-Prints JSON

A JSON formatter looks like it just adds newlines and spaces, but the interesting part happens before any indentation is applied. The pipeline is only three steps: trim the input, parse it, and if the parse succeeds reserialize the parsed value. Parse failure never reaches the formatting step — you get an error with a precise location instead.

Two passes under the hood

The first pass is validation, and it uses the browser's own JSON.parse. Because JSON.parse implements the JSON grammar (RFC 8259), it is deliberately strict: keys must be double-quoted, strings must use double quotes, there are no comments, no single quotes, no trailing commas, and bare words like NaN, Infinity or undefined are rejected. The formatter never tries to 'fix' the input — invalid JSON fails loudly rather than being silently interpreted.

{                      // ← no comments allowed
  "valid": true,
  "trailing": 1,          // ← no trailing commas
  'singles': "no",        // ← single quotes are not strings
}

Pinpointing the exact error

When JSON.parse throws, its message tells you where parsing stopped — for example 'Unexpected token } in JSON at position 14'. The formatter reads that message with a small parser of its own: it matches /position (\d+)/ to get the character offset, then slices the input up to that offset and counts newlines to convert it into a 1-based line number and a column. For single-line inputs it also understands the 'line X column Y' form directly. The noisy 'in JSON at position N' suffix is stripped from the message so you see the actual problem.

{"name": "first",
 "age": }

Unexpected token } in JSON at position 17
→ line 2, column 9

The format pass

Once the input is valid, formatting is a single call: JSON.stringify(parsedValue, null, 2). The third argument (2) is the indentation level — two spaces per nesting level, which is the web convention you see in API docs and most linters. Because the pretty output is produced from the parsed value rather than by editing the raw string, the two spaces are applied consistently even after numbers, big strings or deeply nested arrays.

What is and is not valid JSON

• Object keys must be quoted strings: { "ok": true }, never { ok: true }.
• Strings use double quotes only; escapes like \n and \uXXXX work.
• No trailing commas, no comments, and no control characters inside strings.
• Numbers can be negative, exponential and fractional, but leading zeros are invalid.
• A document may be any single JSON value: object, array, string, number, true, false or null.

Format, validate or minify?

Try it

Paste any JSON into the JSON Formatter and the position of the first error, if any, is reported with line and column. Formatting, validating and minifying all run in your browser.