Simple JavaScript to convert a numeral to its ordinal. Values less than 10 are written as words. Values equal to 10 or greater are written with leading numerals. The optional second argument is a boolean representing whether the collection of numbers is zero-based, as in an array.

convertToOrdinal.js

function convertToOrdinal (i, zero_based) {
  var number = zero_based ? ((i - 0) + 1) : (i - 0), // bootleg coercion
      one = /\d+1$/,
      two = /\d+2$/,
      three = /\d+3$/,
      zero_and_four_through_nine = /\d+[0456789]$/;

  // For whatever goofy reason, typeof NaN == 'number'
  if (typeof number == 'number' && !isNaN(number)) {
    if (number == 0) {
      console.log('Error: Argument cannot be zero - Zero can\'t be converted to a ranking.');
    }
    else if (number == 1) {
      return 'first';
    }
    else if (number > 1 && number < 10) {
      if (number == 2) {
        return 'second';
      }
      if (number == 3) {
        return 'third';
      }
      if (number == 4) {
        return 'fourth';
      }
      if (number == 5) {
        return 'fifth';
      }
      if (number == 6) {
        return 'sixth';
      }
      if (number == 7) {
        return 'seventh';
      }
      if (number == 8) {
        return 'eighth';
      }
      if (number == 9) {
        return 'ninth';
      }
    }
    else if (number >= 10 && number < 20) {
      return number + 'th';
    }
    else {
      if (one.test(number)) {
        return number + 'st';
      }
      if (two.test(number)) {
        return number + 'nd';
      }
      if (three.test(number)) {
        return number + 'rd';
      }
      if (zero_and_four_through_nine.test(number)) {
        return number + 'th';
      }
    }
  }
  else {
    console.log('Error: Argument is not a number or cannot be converted to a number.');
  }
}

// return "first"
convertToOrdinal(1);
convertToOrdinal('1');

// return "second"
convertToOrdinal(1, true);
convertToOrdinal('1', true);