set4.js

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) {
        _difference.delete(elem);
    }
    return _difference;
}

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

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