How to calc CheckSum8 Modulo 256

i need to calc checksum like this site
https://www.scadacore.com/tools/programming-calculators/online-checksum-calculator/

for hex 313233343536373839 the site calc DD

with my function i get 168, how exactly to do it?

function calculateChecksum8Modulo256(data) {
  let checksum = 0;
  for (let i = 0; i < data.length; i++) {
    checksum = (checksum + data.charCodeAt(i)) % 256;
  }
  return checksum;
}

let res = calculateChecksum8Modulo256('313233343536373839')