how to know select index value of option select in javascript code example

Example 1: javascript get selected option

var e = document.getElementById("selectElementID");
var value=e.selectElement.options[e.selectedIndex].value;// get selected option value
var text=e.options[e.selectedIndex].text;//get the selected option text

Example 2: get index of selected option javascript

$("#sel").attr("selectedIndex")
// or
$("#sel")[0].selectedIndex
//or
$(this).prop('selectedIndex')