Convert a non-negative integer num to its English words representation.

Script
/**
 * @param {number} num
 * @return {string}
 */
var numberToWords = function(num) {
  if (num === 0) {
    return 'Zero';
  }
  const units = ['', '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 '];
  let result;
  for (let i = 0; i < thousands.length; i++) {
    if (num % 1000 !== 0) {
      result = helper(num % 1000) + thousands[i] + (result || '');
    }
    num = parseInt(num