es-shimsset.prototype.union 
ES Proposal spec-compliant shim for Set.prototype.union. Invoke its "shim" method to shim Set.prototype.union if it is unavailable or noncompliant.
This package implements the es-shim API interface. It works in an ES3-supported environment, and complies with the proposed spec. When shimmed, it uses es-set to shim the Set implementation itself if needed.
Most common usage:
var assert = require('assert');
var union = require('set.prototype.union');
var set1 = new Set([1, 2]);
var set2 = new Set([2, 3]);
var result = union(set1, set2);
assert.deepEqual(result, new Set([1, 2, 3]));
union.shim();
var shimmedResult = set1.union(set2);
assert.deepEqual(shimmedResult, new Set([1, 2, 3]));Tests
Simply clone the repo, npm install, and run npm test