The selectedIndex property of a dropdown select list can be accessed via JavaScript as follows:
select name="SelectBox" onChange="alert(this.selectedIndex);"
The preceding example shows that selectedIndex is a property of the dropdown select list. selectedIndex is 0-based, meaning that the first item in the select list has a selectedIndex of 0; the second item in the select list has a selectedIndex of 1; and so forth. If nothing is selected, the selectedIndex is -1.
Related articles:
JavaScript: How to Get Selected Item in Dropdown Select List
Accessing Form Elements with JavaScript
