access user control in web control via javascript

I have a user control in a web page

<NW:TitleBar id="cntlTitleBarTabs" runat="server"/>

How can I get control of cntlTitleBarTabs  in the javascript of my aspx file of web page.

I tried using the below lines, but they come out to be null.

document.getElementById("cntlTitleBarTabs");

alert (cntlTitleBarTabs)

 Please help me here. Thanks

-Sarah