JSON is an actively used metalanguage created in 2001.

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Example code from Linguist:

{
        "id": 1,
        "name": "Foo",
        "price": 123,
        "tags": ["Bar","Eek"],
        "stock": { "warehouse":300, "retail":20 }
}

Example code from Wikipedia:

var y = {a: undefined};
 var ys = JSON.stringify(y,
  function (k, v){return (v === undefined) ? "UNDEFINED" : v});
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Last updated February 11th, 2019