code: function union(xs, ys) {
    var obj = {};
    for (var i = 0; i < xs.length; i++) {
      obj[xs[i]] = true;
    }
    for (var j = 0; j < ys.length; j++) {
      obj[ys[j]] = true;
    }
    var keys = [];
    for (var k in obj) {
      if ({}.hasOwnProperty.call(obj, k)) {
        keys.push(k);
      }
    }
    keys.sort();
    return keys;
  }
docs: Returns the sorted set union of two arrays of strings