Code Exercise for JS Role - Convert number to English (US) equivalent

number-to-english.js

const cardinalsMap = new Map([
  [0, 'zero'],
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
  [4, 'four'],
  [5, 'five'],
  [6, 'six'],
  [7, 'seven'],
  [8, 'eight'],
  [9, 'ninety'],
  [10, 'ten'],
  [11, 'eleven'],
  [12, 'twelve'],
  [13, 'thirteen'],
  [14, 'fourteen'],
  [15, 'fifteen'],
  [16, 'sixteen'],
  [17, 'seventeen'],
  [18, 'eighteen'],
  [19, 'nineteen'],
  [20, 'twenty'],
  [30, 'thirty'],
  [40, 'forty'],
  [50, 'fifty'],
  [60, 'sixty'],
  [70, 'seventy'],
  [80, 'eighty'],
  [90, 'ninety'],
]);

const scales = [
  'thousand',
  'million',
  'billion',
  'trillion',
  'quadrillion',
];

const twoDigitNumberToEnglish = num => {
  if (cardinalsMap.has(num)) return cardinalsMap.get(num);

  const onesDigit = num % 10;

  return `${cardinalsMap.get((num - onesDigit))}-${cardinalsMap.get(onesDigit)}`;
}

const threeDigitNumberToEnglish = num => [Math.floor(num / 100), num % 100]
  .map(twoDigitNumberToEnglish)
  .join(' hundred ')
  .replace('zero hundred ', '')
  .replace(' zero', '');

const addScales = (num, idx) => idx ? `${num} ${scales[idx - 1]}` : num;
const notStartWithZero = num => !num.startsWith(cardinalsMap.get(0));

function numberToEnglish(num) {
  // add guards
  if (Number.isNaN(num)) throw Error('Argument is NOT number');
  if (!Number.isInteger(num)) throw Error('Argument is NOT integer');
  if (num < 0) throw Error('Argument is NOT positive');

  // return cardinals
  if (cardinalsMap.has(num)) return cardinalsMap.get(num);

  // otherwise, start calculating
  const parts = [];

  for (; num > 0; num = Math.floor(num / 1000)) {
    parts.push(threeDigitNumberToEnglish(num % 1000));
  }

  return parts.map(addScales).filter(notStartWithZero).reverse().join(', ');
}