title: how to set formula for cell data for (export to .xlsx) SheetJS js-xlsx: https://github.com/SheetJS/js-xlsx
body: <p>Refering this example <a href="https://github.com/SheetJS/js-xlsx/blob/master/tests/write.js" rel="noreferrer">https://github.com/SheetJS/js-xlsx/blob/master/tests/write.js</a>  , it doesn't have any implementation for cell formula in xlsx spreadsheet
  I tried using cell.f = "=SUM(A1+B1)" for the cell C1 and cell.v as the summation value which was 3.
  But i didn't succeed. With the exported file, when opened in MS excel, the cell contained just the data and when selected, didn't show any formula which i assigned in f(x) field. </p>
  
  <p>Can someone post me a example which actually uses the functions/property '.f' and 'cellFormula'</p>
  
  <p>Will be very helpful. I just need a working example with static values. </p>