set-tojson 
Rejected ESNext Proposal: Set#toJSON https://github.com/DavidBruant/Map-Set.prototype.toJSON
This polyfill is spec-compliant (based on the spec so far). It will work in every engine in which Set exists natively, or where it is polyfilled with the (es6-shim)es6-shim-url
Example
var setToJSON = require('set-tojson');
var assert = require('assert');
var items = ['a', 'b', 'c'];
var entries = [[1, 2], [3, 4]];
assert.deepEqual(setToJSON(new Set()), []);
assert.deepEqual(setToJSON(new Set(items)), items);
assert.deepEqual(setToJSON(new Map()), []);
assert.deepEqual(setToJSON(new Map(entries)), entries);
assert.deepEqual(setToJSON(''), []);
assert.deepEqual(setToJSON('abc'), ['a', 'b', 'c']);
assert.deepEqual(setToJSON([]), []);
assert.deepEqual(setToJSON(items), items);
assert.deepEqual(setToJSON(entries), entries);
setToJSON.shim();
assert.deepEqual(new Set(items).toJSON(), items);
assert.deepEqual(new Set().toJSON(), []);Tests
Simply clone the repo, npm install, and run npm test