getting tabindex of tabpanel in tabconatiner

function clientActiveTabChanged(sender, args)
{
    var tabindex=sender.get_activeTabIndex();
}

i hve above to get the tabindex of the tabpanel in a tabconatiner

but by defualt as the firts tab is always active i dont get the tabindex of first tab when the conatiner is first loaded

how do i get it