Add a suffix to a number. "st", "nd", "rd", and "th".

addSuffixToNumber.js

/**
 * Adds suffix to number passed.
 *
 * @method _addSuffixToNumber
 *
 * @param  {number}          number - The number to have a suffix added to.
 *
 * @return {string}                 - Number with suffix added.
 */
function addSuffixToNumber(number) {
  // Get remainder of `number` divided by 10.
  var lastDigit = number % 10;
  // Get remainder of `number` divided by 100.
  var lastTwoDigits = number % 100;
  // If lastDigit is 1 but last two digits not 1, return with added "st".
  if (lastDigit === 1 && lastTwoDigits !== 11) {
    return number + 'st';
  }
  // If lastDigit is 2 but second to last digit is not 1, return with added "nd".
  if (lastDigit === 2 && lastTwoDigits !== 12) {
    return number + 'nd';
  }
  // If lastDigit is 2 but second to last digit is not 1, return with added "rd".
  if (lastDigit === 3 && lastTwoDigits !== 13) {
    return number + 'rd';
  }
  // For all other numbers, return with added "th".
  return number + 'th';
}

addSuffixToNumber.ts

/**
 * Adds suffix to number passed.
 *
 * @method _addSuffixToNumber
 *
 * @param  {number} number - The number to have a suffix added to.
 *
 * @return {string}        - Number with suffix added.
 */
function addSuffixToNumber(number: number): string {
  // Get remainder of `number` divided by 10.
  const lastDigit = number % 10;
  // Get remainder of `number` divided by 100.
  const lastTwoDigits = number % 100;

  // If lastDigit is 1 but last two digits not 1, return with added "st".
  if (lastDigit === 1 && lastTwoDigits !== 11) return number + 'st'

  // If lastDigit is 2 but second to last digit is not 1, return with added "nd".
  if (lastDigit === 2 && lastTwoDigits !== 12) return number + 'nd'

  // If lastDigit is 2 but second to last digit is not 1, return with added "rd".
  if (lastDigit === 3 && lastTwoDigits !== 13) return number + 'rd'

  // For all other numbers, return with added "th".
  return number + 'th';
}