Get a listbox's selected items in javascript - asp.net

Question

I have two listboxes in asp.net. On the click of a button I want to load a list box with the elements of the selected items in the other box. The problem is that this has to be done on the client side because when the button is clicked I don't allow it to submit. I want to call a javascript function onselectedindexchange but that is server side. any ideas? Should i be more clear?
Solution
enter code here
function Updatelist() {
var sel = document.getElementById('<%=ListBox1.ClientID%>')
var lst2 = document.getElementById('<%=ListBox2.ClientId %>')
var listLength = sel.options.length;
var list2length = lst2.options.length;
for (var i = 0; i < listLength; i++) {
if (sel.options[i].selected) {
//lst2.options.add(sel.options[i].text);
lst2.options[list2length] = new Option(sel.options[i].text);
list2length++;
}
}
}

Answer 1

Try:
//onclick for button calls this function
function Updatelist() {
var sel = document.getElementbyId("list1");
var listLength = sel.options.length;
for(var i=0;i<listLength;i++){
if(sel.options[i].selected)
document.getElementById("list2").add(new Option(sel.options[i].value));
}

Answer 2

more precisely we can do it like;
function selectedVal(list) {
alert(list.options[list.selectedIndex].text);
}
<select id="listbox" multiple="multiple"
style="height: 300px; width: 200px;"
onclick="javascript:selectedVal(this);">
</select>

Answer 3

Here is a good article on how to do this using Jquery.
You could also stick your drop downs in an Update Panel.

Answer 4

function Updatelist() {
var sel = document.getElementById('<%=ListBox1.ClientID%>')
var lst2 = document.getElementById('<%=ListBox2.ClientId %>')
var listLength = sel.options.length;
var list2length = lst2.options.length;
for (var i = 0; i < listLength; i++) {
if (sel.options[i].selected) {
//lst2.options.add(sel.options[i].text);
lst2.options[list2length] = new Option(sel.options[i].text);
list2length++;
}
}
}