.JSON5 File Format – Syntax

JSON5

The JSON5 Data Interchange Format 1.0.0 (March 2018) is a superset of JSON that adds syntax from ECMAScript 5.1; the values are the same (objects, arrays, strings, numbers, Booleans, null). This page follows the JSON5 specification and, for identifier names, the ECMAScript 5.1 specification, read 2026-11-12. The JSON5 specification defines strings as sequences of Unicode characters and does not name a file encoding.

Objects and arrays

  • Keys may be ECMAScript 5.1 IdentifierNames without quotes ({unquoted: 1}), or strings in single or double quotes. In ECMAScript 5.1 an IdentifierName starts with a Unicode letter, $, _ or a \u escape, and reserved words are IdentifierNames too (only Identifier excludes them).
  • A single trailing comma is allowed after the last member of an object or array.
  • Names within an object should be unique; with duplicates, behaviour is unpredictable.

Strings

  • Strings may use single quotes; the other quote character needs no escape. The quote that opens the string, the backslash and line terminators must be escaped.
  • A backslash before a line terminator (LF, CR, CR LF, U+2028 or U+2029) continues the string on the next line; neither is part of the string.
  • U+2028 and U+2029 may appear unescaped, but authors should avoid them, parsers should warn and generators should escape them, since ECMAScript 5.1 does not allow them in strings.
EscapeCharacter
\', \", \\Quote, double quote, backslash
\b, \f, \n, \r, \t, \vBackspace, form feed, line feed, carriage return, tab, vertical tab
\0NUL, when not followed by a digit
\xHHA character from U+0000 to U+00FF, by its 2-digit hex code
\uHHHHA UTF-16 code unit (pairs for characters beyond U+FFFF)
Any other character after \That character itself; \1–\9 are not allowed

Numbers

  • Hexadecimal integers with 0x or 0X: 0xdecaf, -0xC0FFEE.
  • A leading or trailing decimal point: .456, 8675309..
  • An explicit plus sign: +1.
  • The IEEE 754 values Infinity (optionally +Infinity), -Infinity and NaN (optionally +NaN or -NaN).

Comments and white space

  • Single-line comments // … and block comments /* … */ (not nested).
  • White space may also be vertical tab, form feed, no-break space (U+00A0), line and paragraph separators (U+2028, U+2029), the byte order mark (U+FEFF) and any other Unicode space separator (category Zs).
{
  // comments
  unquoted: 'and you can quote me on that',
  singleQuotes: 'I can use "double quotes" here',
  lineBreaks: "Look, Mom! \
No \\n's!",
  hexadecimal: 0xdecaf,
  leadingDecimalPoint: .8675309, andTrailing: 8675309.,
  positiveSign: +1,
  trailingComma: 'in objects', andIn: ['arrays',],
  "backwardsCompatible": "with JSON",
}

The example is the one from the specification. The specification names no media type, and IANA’s register of media types (read 2026-11-12) has no application/ entry for JSON5.