It's now easier to do using Set.prototype.symmetricDifference API:

function areEqualSets(a, b) {
    return a.size === b.size && a.symmetricDifference(b).size === 0;
}

areEqualSets(new Set([1, 2, 3]), new Set([1, 2, 4])); // false
areEqualSets(new Set([1, 2, 3]), new Set([1, 2, 3])); // true