code: function getArrayDifferences(a1, a2) {
                      var a = [ ];
                      var diff = [ ];
                      var i;
                      var k;
                      for (i = 0; i < a1.length; i++)
                          a[a1[i]] = true;
                      for (i = 0; i < a2.length; i++) {
                          if (a[a2[i]])
                              delete a[a2[i]];
                          else
                              a[a2[i]] = true;
                      }
                      for (k in a)
                          diff.push(k);
                      return diff;
                  }
docs: Compares 2 arrays and returns the differences between them as a array.
  @param a1 The first array which shall be compared.
  @param a2 The second array which shall be compared.
  @returns {Array} The differences between the two arrays.