Implementing basic set operations - isSuperset, union, intersection and difference

setOperations.js

function isSuperset(set, subset) {
    for (var elem of subset) {
        if (!set.has(elem)) {
            return false;
        }
    }
    return true;
}

function union(setA, setB) {
    var _union = new Set(setA);
    for (var elem of setB) {
        _union.add(elem);
    }
    return _union;
}

function intersection(setA, setB) {
    var _intersection = new Set();
    for (var elem of setB) {
        if (setA.has(elem)) {
            _intersection.add(elem);
        }
    }
    return _intersection;
}

function difference(setA, setB) {
    var _difference = new Set(setA);
    for (var elem of setB) {
        if(setA.has(elem))
          _difference.delete(elem);
        else
          _difference.add(elem);
    }
    return _difference;
}

//Examples
var setA = new Set([1, 2, 3, 4]),
    setB = new Set([2, 3]),
    setC = new Set([3, 4, 5, 6]);

isSuperset(setA, setB); // => true
union(setA, setC); // => Set [1, 2, 3, 4, 5, 6]
intersection(setA, setC); // => Set [3, 4]
difference(setA, setC); // => Set [1, 2, 5, 6]