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JSON5 syntax

We support slighly modified version of JSON5, see https://groups.google.com/forum/#!topic/json5/3DjClVYI6Wg

I started from ES5 specification and added a set of additional restrictions on top of ES5 spec. So I'd expect my implementation to be much closer to javascript. It's no longer an extension of json, but a reduction of ecmascript, which was my original intent.

This JSON5 version is a subset of ES5 language, specification is here:

http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf

This is a language that defines data structures only, so following notes/restrictions are applied:

  • Literals (NullLiteral, BooleanLiteral, NumericLiteral, StringLiteral) are allowed.
  • Compatibility syntax is not supported, which means octal literals are forbidden.
  • ArrayLiterals and allowed, but instead of AssignmentExpressions you can only use other allowed Literals, ArrayLiterals and ObjectLiterals. Elisions are currently not supported.
  • ObjectLiterals and allowed, but instead of AssignmentExpressions you can only use other allowed Literals, ArrayLiterals and ObjectLiterals. Setters and getters are forbidden.
  • All other primary expressions ("this", Identifier, Expression) are forbidden.
  • Two unary expressions ('-' and '+') allowed before NumericLiterals.
  • Any data that has a number type can be represented, including +0, -0, +Infinity, -Infinity and NaN.
  • "undefined" is forbidden, use null instead if applicable.
  • Comments and whitespace are defined according to spec.

Main authority here is ES5 spec, so strict backward JSON compatibility is not guaranteed.

If you're unsure whether a behaviour of this library is a bug or not, you can run this test:

JSON5.parse(String(something))

Should always be equal to:

eval('(function(){return ('+String(something)+'\n)\n})()')

If something meets all rules above. Parens and newlines in the example above are carefully placed so comments and another newlines will work properly, so don't look so impressed about that.

Weirdness of JSON5

These are the parts that I don't particulary like, but see no good way to fix:

  • no elisions, [,,,] -> [null,null,null]
  • [Object], [Circular] aren't parsed
  • no way of nicely representing multiline strings
  • unicode property names are way to hard to implement
  • Date and other custom objects
  • incompatible with YAML (at least comments)