A simple function to add an ordinal to a number

ordinalize.js

function ordinalize(num) {
  const lastDigit = (num + '').slice(-1);
  
  if (lastDigit == 1) { return `${num}st`; }
  else if (lastDigit == 2) { return `${num}nd`; }
  else if (lastDigit == 3) { return `${num}rd`; }
  
  return `${num}th`;
}

console.log(ordinalize(1)); // 1st
console.log(ordinalize(19)); // 19th
console.log(ordinalize(10)); // 10th
console.log(ordinalize(33)); // 33rd
console.log(ordinalize(21)); // 21st