Pretty straightforward twelve days of Christmas implementation in JS with a few ES6 bits.

twelvedays.js

const gifts = ['partridge in a pear tree', 'turtle doves', 'french hens', 'calling birds', 'GOLDEN RINGS', 'geese-a-laying', 'swans-a-swimming', 'maids-a-milking', 'ladies dancing', 'lords-a-leaping', 'pipers piping', 'drummers drumming'];
const ordinals = ['first','second','third','fourth','fifth','sixth','seventh','eighth','ninth','tenth','eleventh','twelfth'];
const numbersToWords = ['One','Two','Three','Four','FIVE','Six','Seven','Eight','Nine','Ten','Eleven','Twelve'];

function twelveDays() {
  for(var i = 0; i < gifts.length; i++) {
    console.log(`On the ${ordinals[i]} day of Christmas,\nMy true love gave to me,`);
    if(i == 0) {
      console.log(`a ${gifts[0]}.\n`);
    }
    else {
      giftForDays(i);
    }
  }
}

function giftForDays(day) {
  if(day == 0) {
    console.log(`And a ${gifts[0]}.\n`);
  }
  else {
    console.log(`${numbersToWords[day]} ${gifts[day]},`);
    giftForDays(day-1);
  }
}

twelveDays();