code: function elementAt(index, defaultValue) {
    let i = 0;
  
    for (let item of this) {
      if (i === index) return item;
      i++;
    }
  
    return defaultValue;
  }
docs: Returns the element at a specified index in a sequence.
  
  @param positon The zero-base index of element which you want.
  @param defaultValue The default value if the index is out of range.
  @return The element at a specified index or default value