how to consume data from response.write

I am opening an html page with the code below. I am also sending is data to that page with the response.write() function:

fs.readFile('./calc.html', function(error, data){
   if(error){
       //do nothing for now              
   } else if (data){
       resp.writeHead(200, {'Content-Type':'text/html'});
       var sum = 9;
       resp.write(sum);
       resp.end(data);
   }
});        

How do I consume the value from 'sum' in calc.html when the page opens? In the script tag in the , I'm utilizing the Window.onload method to perform an action when the page loads. The number 9 appears in the top left hand corner of the web page when it loads, so I konw it's there, I just dont know how to consume it and use it.

<script type="text/javascript">
    var htmlSum = 0;
    function fetchData() {
        htmlSum = //How to I scrape the 'sum' variable sent into the page?????
    }
    window.onload = fetchData;        
</script>

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