[рони]

sovsem-nub,

function excelInd(title)
{  title = title.toUpperCase();
   let up = "A".charCodeAt(0) - 1, index = 0;
   for(letter of title) { 
      index *= 26;
      index += letter.charCodeAt(0) - up
   }
   return --index
}
alert(excelInd('aa'))