Currently I am using this line of code in my JavaScript
var tabIndex = $(':focus').attr('tabIndex');
However this constantly fails to get the active index.
Here is asp:TabContainer header in case this helps. I have also tried document.GetElementById, however to no avail as well.
<asp:TabContainer ID="AdvOrBasicSearch" runat="server" ActiveTabIndex="0">
I found this method works much better. I created a variable with the tabContainer itself. Then, I just needed to go inside the variable and pull the value from the _activeTabIndex property.