Javascript - HiddenField Value Doesn't Get Removed

I have a javascript function that moves items between 2 select multiple box, when I move item from the source select box to the destination select box, I add the value to my HiddenField so that I can access in code behind, works fine but when I move item(s) from the destination select box back to the source select box, I try to use : hidMemType.value = ""; to clear the hiddenfield. I thought this works but apparently in the event of a postback, the item still gets stuck in the destination box.

// Move items to and fro select box
function move(sourceFrom, sourceTo) {
var hidOutlet = document.getElementById('<%=hdnOutlet.ClientID%>');

[Code]....