jQuery get select dropdown list selected item index

When learning how jQuery gets the selected text and value of the select, I found that I can also get the index of the selected item, so I searched and recorded it.

$('#selectOne').find('option:selected').selectedIndex;

$('#selectOne).find('option:selected').attr('selectedIndex');

The above two methods cannot obtain the index value, and the following four methods can obtain the index value of the selected item after testing.

$("#selectOne").get(0).selectedIndex;

$('#selectOne').prop('selectedIndex');

$('option:selected', '#selectOne').index();

$('#selectOne option').index($('#selectOne option:selected'));