title: Exceljs+nodejs to serve excel stream
body: <p>Suppose I want to use exceljs to serve excel to web clients. Also, suppose that I am using streaming (due to the files being potentially large and not wanted to keep them in memory).</p>
  
  <p>Is it possible to find out what the Content-Length header should be? Because without this, I can't see the download progress spinner from chrome. </p>
  
  <p>Example code:</p>
  
  <pre><code>app.get('/some/route', function(req, res) {
    res.writeHead(200, {
      'Content-Disposition': 'attachment; filename="file.xlsx"',
      'Content-Type': 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
      "Content-Length" : inf //this is what I want!!
    })
    var workbook = new Excel.stream.xlsx.WorkbookWriter({ stream: res })
    var worksheet = workbook.addWorksheet('some-worksheet')
    worksheet.addRow(['foo', 'bar']).commit()
    worksheet.commit()
    workbook.commit()
  }
  </code></pre>