title: Generate Base64 from ExcelJS WorkBook
body: <p>I am working with ExcelJS library in NodeJS for creating a excel file.
  What I need it's to generate a Base64 string from a Workbook object.</p>
  
  <p>I have this code</p>
  
  <pre><code>let workbook = new Excel.stream.xlsx.WorkbookWriter({});
  let worksheet = workbook.addWorksheet(`CREReport_${origcontractid}`);
  worksheet.getCell('A1').value = 'CRE evolution';
  worksheet.commit(); 
  </code></pre>
  
  <p>but I don't know how to generate a Base64 string based on my workbook. I don't want to create a file in my disk, i want to return in my webservice a base64 string which represent the file. any idea? </p>
  
  <p>Thanks!</p>