code: function getPos(thumbnail, group) {
      var arr = getByGroup(group);
      for (var i = 0; i < arr.length; i++) {
        // compare elements
        if (getAttr(thumbnail, 'src') === getAttr(arr[i], 'src') &&
          getAttr(thumbnail, _const_dataattr + '-index') === getAttr(arr[i], _const_dataattr + '-index') &&
          getAttr(thumbnail, _const_dataattr) === getAttr(arr[i], _const_dataattr)) {
          
          return i;
        }
      }
    }
docs: Get the position of thumbnail in group-array
  @param  {Object} thumbnail
  @param  {String} group
  @return {number}