numberToEnglish.js

/**
 * Given a non-negative integer as input, returns the number translated into
 * English.
 *
 * Don't worry about inserting hyphens, commas, or "and" anywhere.
 *
 * @example
 * numberToEnglish(100);     // => 'one hundred'
 * numberToEnglish(5491);    // => 'five thousand four hundred ninety one'
 * numberToEnglish(12345);   // => 'twelve thousand three hundred forty five'
 * numberToEnglish(9456012); // => 'nine million four hundred fifty six thousand twelve'
 *
 * @param {number} num - A number to format
 * @returns {string} A string representation of the input number formatted with
 *   commas in the appropriate places.
 */
function numberToEnglish(num) {
  return smallNumberToEnglish(num);
}

function smallNumberToEnglish(num) {
  let onesInEnglish = [
    'zero',
    'one',
    'two',
    'three',
    'four',
    'five',
    'six',
    'seven',
    'eight',
    'nine',
    'ten',
    'eleven',
    'twelve',
    'thirteen',
    'fourteen',
    'fifteen',
    'sixteen',
    'seventeen',
    'eighteen',
    'nineteen',
  ];

  if (num > 19) {
    throw new Error(`Invalid input: 1-digit or 2-digit input required, received: ${num}`);
  }

  return onesInEnglish[num];
}

if (require.main === module) {
  console.log('Running sanity checks for numberToEnglish:');

  console.log(numberToEnglish(0) === 'zero');
  console.log(numberToEnglish(5) === 'five');
  console.log(numberToEnglish(10) === 'ten');
  console.log(numberToEnglish(19) === 'nineteen');

  // Add more sanity checks as you go along.
  // How else will you know if your code does what you expect,
  // or whether you've broken your existing code without realizing?
}

module.exports = numberToEnglish;