How to call Javascript for my Listbox Selected Index change

Hi, <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

Currently I have the following javascript which can find the last selected index of my listbox when I click on a button. But I do not want to invoke the javascript when the button is pressed. I want to invoke the javascript in my listbox's selectedindexchange event. But I can't seem to get it working. Attached is my javascript code and aspx code. Could someone help me out on this. Thank you.

 function Button2_onclick()

{

dowork();

}

function dowork()

 {

var select = document.getElementById('lbxSelectedUnits');

len = document.getElementById("lbxSelectedUnits").length;

for(var j=0;j<len;j++)
{
 if(documen ...