es-shimsset.prototype.issubsetof 
ES Proposal spec-compliant shim for Set.prototype.isSubsetOf. Invoke its "shim" method to shim Set.prototype.isSubsetOf 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 isSubsetOf = require('set.prototype.issubsetof');
var set1 = new Set([1, 2]);
var set2 = new Set([2, 3]);
var set3 = new Set([1]);
assert.equal(isSubsetOf(set1, set2), false);
assert.equal(isSubsetOf(set2, set1), false);
assert.equal(isSubsetOf(set3, set1), true);
isSubsetOf.shim();
assert.equal(set1.isSubsetOf(set2), false);
assert.equal(set2.isSubsetOf(set1), false);
assert.equal(set3.isSubsetOf(set1), true);Tests
Simply clone the repo, npm install, and run npm test