I declare a variable to find a control using the "document.getElementById('<%=ContentPlaceHolder1.FindControl("txtBox1").ClientID %>" in a test function that is never called. Even thought this function is never called, I get

I am trying to have functions in a Master page that declare variables to access controls in content pages when the content pages are being viewed.  I haven't figured out a way to dynamically do that on the fly and there was a reccommendation to just declare them and have them available all the time. I am trying to put as much javascript code in my Master page as I can. 
I was having problems, so I created a function that isn't called by anything, but just declares a variable to test.  What I find is that even having the declaration of a variable for a control on a content page throws an error unless I am viewing the page that the control exists on.
For example, I have the following funciton just to test.  This function just exsists in my javascript of my master page, but isn't called by anything.
function test (elem)
{
var txtBox1= document.getElementById('<%=MainC ...