code: function arraySibling(arr, item, offset) {
    var index = arr.indexOf(item);
  
    if (index === -1) {
      throw 'item is not in array';
    }
  
    if (isArray(offset)) {
      return offset.map(function (v) {
        return arr[index + v];
      });
    }
  
    return arr[index + offset];
  }
docs: offset can be many