The selectedIndex property returns the index number of the selected option in that instance of a select box.
If this property is used to access a multiple select box, it will return the index number of the first selected item.
<html> <form name="myForm"> <select name="mySelect" onChange='alert(mySelect.selectedIndex)'> <option value=A>AA</option> <option value=B>BB</option> <option value=C>CC</option> <option value=D>DD</option> </select> </form> </html>