number_to_word.js

// (224) 577 5898

const getTens = tens => ['Ten', 'Twenty', 'Thirty', 'Forty', 'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninetey'][tens - 1]

const below20 = integer => ['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eightteen', 'Nineteen'][integer - 1]

const addWords = (result, words) => result.length === 0 ? words : `${result} ${words}`;

const solution = (integer) => {
  if (integer === 0) return 'Zero';
  
  if (integer < 20) return below20(integer);
  
  const thousands = Math.floor(integer / 1000);
  const hundreds = Math.floor((integer - (thousands * 1000)) / 100);
  const tens = Math.floor((integer - (thousands * 1000) - (hundreds * 100)) / 10);
  const tens2 = integer - (thousands * 1000) - (hundreds * 100);
  const ones = integer - (thousands * 1000) - (hundreds * 100) - (tens * 10);

  let result = '';
  if (thousands) {
    result = addWords(result, `${solution(thousands)} Thousand`);
  }
  
  if (hundreds) {
    result = addWords(result, `${below20(hundreds)} Hundred`);
  }
  
  if (tens2 < 20) {
    result = addWords(result, below20(tens2));
    return result;
  }
  
  if (tens) {
    result = addWords(result, getTens(tens));
  }

  if (ones) {
    result = addWords(result, below20(ones));
  }

  return result;
}


console.log(solution(3542), '3542')
console.log(solution(11), '11')
console.log(solution(123915), '123915')
console.log(solution(99999), '99999')