Validation for Singapore NRIC and FIN number

is-nric.js

//Based on http://www.samliew.com/icval/
const isNRIC = (str) => {
  if (str.length != 9)
      return false;

  str = str.toUpperCase();

  let i;
  const icArray = [];
  for(i = 0; i < 9; i++) {
      icArray[i] = str.charAt(i);
  }

  icArray[1] = parseInt(icArray[1], 10) * 2;
  icArray[2] = parseInt(icArray[2], 10) * 7;
  icArray[3] = parseInt(icArray[3], 10) * 6;
  icArray[4] = parseInt(icArray[4], 10) * 5;
  icArray[5] = parseInt(icArray[5], 10) * 4;
  icArray[6] = parseInt(icArray[6], 10) * 3;
  icArray[7] = parseInt(icArray[7], 10) * 2;

  let weight = 0;
  for(i = 1; i < 8; i++) {
      weight += icArray[i];
  }

  const offset = (icArray[0] == "T" || icArray[0] == "G") ? 4:0;
  const temp = (offset + weight) % 11;

  const st = ["J","Z","I","H","G","F","E","D","C","B","A"];
  const fg = ["X","W","U","T","R","Q","P","N","M","L","K"];

  let theAlpha;
  if (icArray[0] == "S" || icArray[0] == "T") { theAlpha = st[temp]; }
  else if (icArray[0] == "F" || icArray[0] == "G") { theAlpha = fg[temp]; }

  return (icArray[8] === theAlpha);
}

module.exports = isNRIC