numToWordFunctional.js

const ones = ['', 'one ', 'two ', 'three ', 'four ', 'five ', 'six ', 'seven ', 'eight ', 'nine ', 'ten ', 'eleven ', 'twelve ', 'thirteen ', 'fourteen ', 'fifteen ', 'sixteen ', 'seventeen ', 'eighteen ', 'nineteen ']
const tens = ['', 'ten ', 'twenty ', 'thirty ', 'fourty ', 'fifty ', 'sixty ', 'seventy ', 'eighty ', 'ninety ']
const exps = (v, i) => v + (v ? ['quadrillion ', 'trillion ', 'billion ', 'million ', 'thousand ', ''][i] : '')

const subthousands = n => n == 0 ? '' :
    n > 99 ? subthousands(~~(n / 100)) + 'hundred ' + (n % 100 ? 'and ' : '') + subthousands(n % 100) :
    n > 19 ? tens[~~(n / 10)] + subthousands(n % 10) : ones[n]  

const numToWord = n => 
    (n < 0 || !Number.isSafeInteger(n)) ? null : n === 0 ? 'zero ' :
    String(n).padStart(18, '0').match(/.{1,3}/g).map(Number).map(subthousands).map(exps).join('')