C# - Access Controls On ContentPages Via Javascript By MasterPage?

I need my MasterPage to be able to get ControlIDs of Controls on ContentPages, but I cannot use <%= xxx.CLIENTID%> as it would return an error as the control(s) might not be loaded by the contentplaceholder.

Some controls have a so called BehaviourID, which is exactly what I would need as they can be directly accessed with the ID: [Asp.net does always create unique IDs, thus modifies the ID I entered] Unfortunately I need to access

e.g. ASP.NET Control with BehaviouraID="test"

document.getElementById("test")

if I were to use e.g. Label control with ID="asd"

document.getElementById('<%= asd.ClientID%>')

But if the Labelcontrol isn't present on the contentpage, I of course get an error on my masterpage.