/**
 * @param {number} num
 * @return {string}
 */
const thousand = 1000;
const million = thousand * thousand;
const billion = million * thousand;
const trillion = billion * thousand;
const quadrillion = trillion * thousand
const quintillion = quadrillion * thousand;

const units = [undefined, "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"]
const teens = ["Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"]
const tens = [undefined, "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"]

function subHundred(num){
  const trimmed = num % 100
  const unit = trimmed % 10;
  const ten = Math.floor(trimmed / 10);

  if(ten >= 2){
    return [tens[ten], units[unit]]
      .filter(part => part)
      .join(' ');
  }
  if(ten >= 1){
    return teens[unit];
  }
  return units[unit];
}

function subThousand(num){
  const trimmed = num % 1000;
  const parts = [trimmed >= 100 && `${units[Math.floor(trimmed / 100)]} Hundred`, subHundred(trimmed)];
  return parts.filter(part => part).join(' ') || undefined;
}

function getThousandPower(min, word) {
  return function(num){
    const numOfMins = subThousand(Math.floor((num % (min * thousand)) / min ));
    return numOfMins && `${numOfMins} ${word}`;
  }
}
var numberToWords = function(num) {
  return [
    getThousandPower(quadrillion, "Quadrillion"),
    getThousandPower(trillion, "Trillion"),
    getThousandPower(billion, "Billion"),
    getThousandPower(million, "Million"),
    getThousandPower(thousand, "Thousand"),
    subThousand
  ].map(proc => proc(num))
  .filter(part => part)
  .join(" ") || "Zero";
};

[000, 12345, 1234567, 1234567891].forEach(num => {
  console.log(numberToWords(num));
});