XSLT - passing xml document as a parameter from javascript

I am currently working on an application that takes two xml files and compares them with XSLT 2.0 called from Javascript.

I am passing in parameters to the XSLT from my javascript like so:

 xsltProcessor.setParameter(null, "testParameter", "I AM A TEST PARAMETER HEAR ME ROAR");
 xsltProcessor.setParameter(null, "InputDirectory", "file:///D:/WORKSPACE/results_comparison_1.1/testng-results.xml");

And I am able to display these in the XSLT like so:

<xsl:value-of select="$testParameter" />    
<xsl:value-of select="$InputDirectory"/>

Which gives me this output when I load the file up in my browser:

I AM A TEST PARAMETER HEAR ME ROARfile:///D:/WORKSPACE/results_comparison_1.1/testng-results.xml

All fine so far.

My problem:

The trouble is that I have a hardcoded variable like this, which I want to be assigned by an external parameter:

<xsl:variable name="reportFile" select="document('file:///D:/WORKSPACE/results_comparison_1.1/testng-results.xml')" />

I have tried something like:

    <xsl:variable name="reportFile" select="document('$InputDirectory')" />

When InputDirectory corresponds to the parameter above. But it does not seem to work. Furthermore when I try to print it using something like this:

<xsl:value-of select="$reportFile" />   

I get no error at all when I would usually get a long complicated error message showing up on the screen, which would seem to imply to me the "types" are different.

I was wondering how exactly I can get an xml file from external parameters in this way? Thanks very much.