write your JSON in Javascript

JSON.stringify(eval(...)())

Unquoted object keys and inline arithmetic can be handy. So can Array.prototype.map and friends.

json.js is a node script that reads a file or standard input, evaluates its contents as Javascript in an empty environment (except for that guaranteed by the language, e.g. JSON and Object), invokes the evaluated function, and prints to standard output the returned value as JSON.

$ ./json.js <<'END_JS' () => ({ foo: 'bar', ['hello' + 'there']: [1, 2, 3].map(n => `number${n}`) }) END_JS {"foo":"bar","hellothere":["number1","number2","number3"]}