code: function (a, b) {
                  var result = [], inB = {}, i, n;
                  for (i = 0, n = b.length; i < n; i++) {
                      inB[b[i]] = true;
                  }
                  for (i = 0, n = a.length; i < n; i++) {
                      if (!inB[a[i]]) {
                          result.push(a[i]);
                      }
                  }
                  return result;
              }
docs: returns elements in that are in a but not in b subtractAsSets([4, 5, 6], [4, 5, 7]) => [6]