Integer to English Words

export function numberToWords(num: number): string {
  return num === 0 ? 'Zero' : helper(num).trim();
}
 
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',
  'Forty',
  'Fifty',
  'Sixty',
  'Seventy',
  'Eighty',
  'Ninety',
];
const THOUSANDS = ['', 'Thousand', 'Million', 'Billion'];
 
function helper(num: number): string {
  if (num < 20) {
    return ONES[num];
  }
  if (num < 100) {
    return `${TENS[Math.floor(num / 10)]} ${helper(num % 10)}`.trim();
  }
  if (num < 1000) {
    return `${helper(Math.floor(num / 100))} Hundred ${helper(num % 100)}`.trim();
  }
  const i = Math.floor(Math.log10(num) / 3);
  const unit = 10 ** (3 * i);
  return `${helper(Math.floor(num / unit))} ${THOUSANDS[i]} ${helper(num % unit)}`.trim();
}