docs: function indexOf(arr, item, fromIndex) {
          fromIndex = fromIndex || 0;
          if (arr == null) {
              return -1;
          }
  
          var len = arr.length,
              i = fromIndex < 0 ? len + fromIndex : fromIndex;
          while (i < len) {
              // we iterate over sparse items since there is no way to make it
              // work properly on IE 7-8. see #64
              if (arr[i] === item) {
                  return i;
              }
  
              i++;
          }
  
          return -1;
      }