Advent of code 2022 day 3

function myFunction() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getDataRange();
  var numRows = range.getNumRows();
  for(var i=1; i<=numRows; i++) {
    var left = range.getCell(i, 2).getValue();
    var right = range.getCell(i, 3).getValue();
    var c = commonChar(left, right);
    range.getCell(i,4).setValue(c);
  } 
}

function commonChar(a,b) {
  for (const x of a.split("")) {
    for(const y of b.split("")) {
      if (x == y) {
        var c = x.charCodeAt() 
        return (c < 97)? c-65+26+1:c-97+1;
      }
    }
  }
  throw "no common char"
}