How to get index of selected option in Select control

It took me some time to get the index of selected value of <select> html control. I tried to use ‘attr’, but ‘prop’ should be used for this.

    $('#selectControlId').on('change', function () { 
        var selectName = $(this).val(); 
        var i = $(this).prop("selectedIndex"); 
                alert(i); 
                });