toWords.js

const arr = x => Array.from(x);
const num = x => Number(x) || 0;
const str = x => String(x);
const isEmpty = xs => xs.length === 0;
const take = n => xs => xs.slice(0,n);
const drop = n => xs => xs.slice(n);
const reverse = xs => xs.slice(0).reverse();
const comp = f => g => x => f (g (x));
const chunk = n => xs =>
  isEmpty(xs) ? [] : [take(n)(xs), ...chunk (n) (drop (n) (xs))];

let numToWords = n => {
  
  let onesGroup = [
    '', 'one', 'two', 'three', 'four',
    'five', 'six', 'seven', 'eight', 'nine',
    'ten', 'eleven', 'twelve', 'thirteen', 'fourteen',
    'fifteen', 'sixteen', 'seventeen', 'eighteen', 'nineteen'
  ];
  
  let tensGroup = [
    '', '', 'twenty', 'thirty', 'forty',
    'fifty', 'sixty', 'seventy', 'eighty', 'ninety'
  ];
  
  let thousandGroup = [
    '', 'thousand', 'million', 'billion', 'trillion', 'quadrillion',
    'quintillion', 'sextillion', 'septillion', 'octillion', 'nonillion'
  ];
  
  let makeGroup = ([ones,tens,huns]) => {
    return [
      num(huns) === 0 ? '' : onesGroup[huns] + ' hundred ',
      num(ones) === 0 ? tensGroup[tens] : tensGroup[tens] && tensGroup[tens] + '-' || '',
      onesGroup[tens+ones] || onesGroup[ones]
    ].join('');
  };
  
  let thousand = (group,i) => group === '' ? group : `${group} ${thousandGroup[i]}`;
  
  if (typeof n === 'number')
    return numToWords(String(n));
  else if (n === '0')
    return 'zero';
  else
    return comp (chunk(3)) (reverse) (arr(n))
      .map(makeGroup)
      .map(thousand)
      .filter(!isEmpty)
      .reverse()
      .join(' ');
};