document object in user-extensions.js

How do I access the javascript "document" object for
the current page in the user extensions file. 

e.g. I have defined a new function that makes use of
code such as:

if(document.getElementById('footable')
{
 
tabletds=document.getElementById('footable').getElementsByTagName('td');
  ...
}
else
{
  Assert.fail('document not defined')  
}


        
                
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