activeIndex Property

Gets sets index of active TabItem as integer. That property has effect only when activation is enabled and control is visible. If the get returns -1, then control does not have focus and there is no active item. If the get returns get_tabs().length, then add-new-tab button is active. If control does not have focus, then the set will set focus to control and activate TabItem. Index of active tab

Syntax

ASP.NET Ajax Javascript (Specification)
function get_activeIndex() : Integer function set_activeIndex(value : Integer)

ASP.NET Ajax Javascript (Usage)
var resultVar = instanceOfWebTab.get_activeIndex();