JSON5 is an actively used metalanguage created in 2012.

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Example code from the web:

{
  // 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",
} 

Example code from Linguist:

/*
 * The following is a contrived example, but it illustrates most of the features:
 */

{
    foo: 'bar',
    while: true,

    this: 'is a \
multi-line string',

    // this is an inline comment
    here: 'is another', // inline comment

    /* this is a block comment
       that continues on another line */

    hex: 0xDEADbeef,
    half: .5,
    delta: +10,
    to: Infinity,   // and beyond!

    finally: 'a trailing comma',
    oh: [
        "we shouldn't forget",
        'arrays can have',
        'trailing commas too',
    ],
}
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Last updated February 11th, 2019